This is bison.info, produced by makeinfo version 6.6 from bison.texi.
This manual (22 May 2019) is for GNU Bison (version 3.4.1), the GNU
parser generator.
Copyright © 1988-1993, 1995, 1998-2015, 2018-2019 Free Software
Foundation, Inc.
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INFO-DIR-SECTION Software development
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* bison: (bison). GNU parser generator (Yacc replacement).
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Bison
*****
This manual (22 May 2019) is for GNU Bison (version 3.4.1), the GNU
parser generator.
Copyright © 1988-1993, 1995, 1998-2015, 2018-2019 Free Software
Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with no Invariant Sections, with the Front-Cover texts
being “A GNU Manual,” and with the Back-Cover Texts as in (a)
below. A copy of the license is included in the section entitled
“GNU Free Documentation License.”
(a) The FSF’s Back-Cover Text is: “You have the freedom to copy and
modify this GNU manual. Buying copies from the FSF supports it in
developing GNU and promoting software freedom.”
* Menu:
* Introduction:: What GNU Bison is.
* Conditions:: Conditions for using Bison and its output.
* Copying:: The GNU General Public License says
how you can copy and share Bison.
Tutorial sections:
* Concepts:: Basic concepts for understanding Bison.
* Examples:: Three simple explained examples of using Bison.
Reference sections:
* Grammar File:: Writing Bison declarations and rules.
* Interface:: C-language interface to the parser function ‘yyparse’.
* Algorithm:: How the Bison parser works at run-time.
* Error Recovery:: Writing rules for error recovery.
* Context Dependency:: What to do if your language syntax is too
messy for Bison to handle straightforwardly.
* Debugging:: Understanding or debugging Bison parsers.
* Invocation:: How to run Bison (to produce the parser implementation).
* Other Languages:: Creating C++ and Java parsers.
* History:: How Bison came to be
* FAQ:: Frequently Asked Questions
* Table of Symbols:: All the keywords of the Bison language are explained.
* Glossary:: Basic concepts are explained.
* GNU Free Documentation License:: Copying and sharing this manual
* Bibliography:: Publications cited in this manual.
* Index of Terms:: Cross-references to the text.
— The Detailed Node Listing —
The Concepts of Bison
* Language and Grammar:: Languages and context-free grammars,
as mathematical ideas.
* Grammar in Bison:: How we represent grammars for Bison’s sake.
* Semantic Values:: Each token or syntactic grouping can have
a semantic value (the value of an integer,
the name of an identifier, etc.).
* Semantic Actions:: Each rule can have an action containing C code.
* GLR Parsers:: Writing parsers for general context-free languages.
* Locations:: Overview of location tracking.
* Bison Parser:: What are Bison’s input and output,
how is the output used?
* Stages:: Stages in writing and running Bison grammars.
* Grammar Layout:: Overall structure of a Bison grammar file.
Writing GLR Parsers
* Simple GLR Parsers:: Using GLR parsers on unambiguous grammars.
* Merging GLR Parses:: Using GLR parsers to resolve ambiguities.
* GLR Semantic Actions:: Considerations for semantic values and deferred actions.
* Semantic Predicates:: Controlling a parse with arbitrary computations.
* Compiler Requirements for GLR:: GLR parsers require a modern C compiler.
Examples
* RPN Calc:: Reverse Polish Notation Calculator;
a first example with no operator precedence.
* Infix Calc:: Infix (algebraic) notation calculator.
Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @N and @$.
* Multi-function Calc:: Calculator with memory and trig functions.
It uses multiple data-types for semantic values.
* Exercises:: Ideas for improving the multi-function calculator.
Reverse Polish Notation Calculator
* Rpcalc Declarations:: Prologue (declarations) for rpcalc.
* Rpcalc Rules:: Grammar Rules for rpcalc, with explanation.
* Rpcalc Lexer:: The lexical analyzer.
* Rpcalc Main:: The controlling function.
* Rpcalc Error:: The error reporting function.
* Rpcalc Generate:: Running Bison on the grammar file.
* Rpcalc Compile:: Run the C compiler on the output code.
Grammar Rules for ‘rpcalc’
* Rpcalc Input:: Explanation of the ‘input’ nonterminal
* Rpcalc Line:: Explanation of the ‘line’ nonterminal
* Rpcalc Expr:: Explanation of the ‘expr’ nonterminal
Location Tracking Calculator: ‘ltcalc’
* Ltcalc Declarations:: Bison and C declarations for ltcalc.
* Ltcalc Rules:: Grammar rules for ltcalc, with explanations.
* Ltcalc Lexer:: The lexical analyzer.
Multi-Function Calculator: ‘mfcalc’
* Mfcalc Declarations:: Bison declarations for multi-function calculator.
* Mfcalc Rules:: Grammar rules for the calculator.
* Mfcalc Symbol Table:: Symbol table management subroutines.
* Mfcalc Lexer:: The lexical analyzer.
* Mfcalc Main:: The controlling function.
Bison Grammar Files
* Grammar Outline:: Overall layout of the grammar file.
* Symbols:: Terminal and nonterminal symbols.
* Rules:: How to write grammar rules.
* Semantics:: Semantic values and actions.
* Tracking Locations:: Locations and actions.
* Named References:: Using named references in actions.
* Declarations:: All kinds of Bison declarations are described here.
* Multiple Parsers:: Putting more than one Bison parser in one program.
Outline of a Bison Grammar
* Prologue:: Syntax and usage of the prologue.
* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations:: Syntax and usage of the Bison declarations section.
* Grammar Rules:: Syntax and usage of the grammar rules section.
* Epilogue:: Syntax and usage of the epilogue.
Grammar Rules
* Rules Syntax:: Syntax of the rules.
* Empty Rules:: Symbols that can match the empty string.
* Recursion:: Writing recursive rules.
Defining Language Semantics
* Value Type:: Specifying one data type for all semantic values.
* Multiple Types:: Specifying several alternative data types.
* Type Generation:: Generating the semantic value type.
* Union Decl:: Declaring the set of all semantic value types.
* Structured Value Type:: Providing a structured semantic value type.
* Actions:: An action is the semantic definition of a grammar rule.
* Action Types:: Specifying data types for actions to operate on.
* Midrule Actions:: Most actions go at the end of a rule.
This says when, why and how to use the exceptional
action in the middle of a rule.
Actions in Midrule
* Using Midrule Actions:: Putting an action in the middle of a rule.
* Typed Midrule Actions:: Specifying the semantic type of their values.
* Midrule Action Translation:: How midrule actions are actually processed.
* Midrule Conflicts:: Midrule actions can cause conflicts.
Tracking Locations
* Location Type:: Specifying a data type for locations.
* Actions and Locations:: Using locations in actions.
* Location Default Action:: Defining a general way to compute locations.
Bison Declarations
* Require Decl:: Requiring a Bison version.
* Token Decl:: Declaring terminal symbols.
* Precedence Decl:: Declaring terminals with precedence and associativity.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
* Symbol Decls:: Summary of the Syntax of Symbol Declarations.
* Initial Action Decl:: Code run before parsing starts.
* Destructor Decl:: Declaring how symbols are freed.
* Printer Decl:: Declaring how symbol values are displayed.
* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
* Push Decl:: Requesting a push parser.
* Decl Summary:: Table of all Bison declarations.
* %define Summary:: Defining variables to adjust Bison’s behavior.
* %code Summary:: Inserting code into the parser source.
Parser C-Language Interface
* Parser Function:: How to call ‘yyparse’ and what it returns.
* Push Parser Function:: How to call ‘yypush_parse’ and what it returns.
* Pull Parser Function:: How to call ‘yypull_parse’ and what it returns.
* Parser Create Function:: How to call ‘yypstate_new’ and what it returns.
* Parser Delete Function:: How to call ‘yypstate_delete’ and what it returns.
* Lexical:: You must supply a function ‘yylex’
which reads tokens.
* Error Reporting:: You must supply a function ‘yyerror’.
* Action Features:: Special features for use in actions.
* Internationalization:: How to let the parser speak in the user’s
native language.
The Lexical Analyzer Function ‘yylex’
* Calling Convention:: How ‘yyparse’ calls ‘yylex’.
* Token Values:: How ‘yylex’ must return the semantic value
of the token it has read.
* Token Locations:: How ‘yylex’ must return the text location
(line number, etc.) of the token, if the
actions want that.
* Pure Calling:: How the calling convention differs in a pure parser
(*note A Pure (Reentrant) Parser: Pure Decl.).
The Bison Parser Algorithm
* Lookahead:: Parser looks one token ahead when deciding what to do.
* Shift/Reduce:: Conflicts: when either shifting or reduction is valid.
* Precedence:: Operator precedence works by resolving conflicts.
* Contextual Precedence:: When an operator’s precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
* Mysterious Conflicts:: Conflicts that look unjustified.
* Tuning LR:: How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted. How to avoid it.
Operator Precedence
* Why Precedence:: An example showing why precedence is needed.
* Using Precedence:: How to specify precedence and associativity.
* Precedence Only:: How to specify precedence only.
* Precedence Examples:: How these features are used in the previous example.
* How Precedence:: How they work.
* Non Operators:: Using precedence for general conflicts.
Tuning LR
* LR Table Construction:: Choose a different construction algorithm.
* Default Reductions:: Disable default reductions.
* LAC:: Correct lookahead sets in the parser states.
* Unreachable States:: Keep unreachable parser states for debugging.
Handling Context Dependencies
* Semantic Tokens:: Token parsing can depend on the semantic context.
* Lexical Tie-ins:: Token parsing can depend on the syntactic context.
* Tie-in Recovery:: Lexical tie-ins have implications for how
error recovery rules must be written.
Debugging Your Parser
* Understanding:: Understanding the structure of your parser.
* Graphviz:: Getting a visual representation of the parser.
* Xml:: Getting a markup representation of the parser.
* Tracing:: Tracing the execution of your parser.
Tracing Your Parser
* Enabling Traces:: Activating run-time trace support
* Mfcalc Traces:: Extending ‘mfcalc’ to support traces
* The YYPRINT Macro:: Obsolete interface for semantic value reports
Invoking Bison
* Bison Options:: All the options described in detail,
in alphabetical order by short options.
* Option Cross Key:: Alphabetical list of long options.
* Yacc Library:: Yacc-compatible ‘yylex’ and ‘main’.
Parsers Written In Other Languages
* C++ Parsers:: The interface to generate C++ parser classes
* Java Parsers:: The interface to generate Java parser classes
C++ Parsers
* A Simple C++ Example:: A short introduction to C++ parsers
* C++ Bison Interface:: Asking for C++ parser generation
* C++ Parser Interface:: Instantiating and running the parser
* C++ Semantic Values:: %union vs. C++
* C++ Location Values:: The position and location classes
* C++ Scanner Interface:: Exchanges between yylex and parse
* A Complete C++ Example:: Demonstrating their use
C++ Location Values
* C++ position:: One point in the source file
* C++ location:: Two points in the source file
* Exposing the Location Classes:: Using the Bison location class in your
project
* User Defined Location Type:: Required interface for locations
A Complete C++ Example
* Calc++ --- C++ Calculator:: The specifications
* Calc++ Parsing Driver:: An active parsing context
* Calc++ Parser:: A parser class
* Calc++ Scanner:: A pure C++ Flex scanner
* Calc++ Top Level:: Conducting the band
Java Parsers
* Java Bison Interface:: Asking for Java parser generation
* Java Semantic Values:: %type and %token vs. Java
* Java Location Values:: The position and location classes
* Java Parser Interface:: Instantiating and running the parser
* Java Scanner Interface:: Specifying the scanner for the parser
* Java Action Features:: Special features for use in actions
* Java Push Parser Interface:: Instantiating and running the a push parser
* Java Differences:: Differences between C/C++ and Java Grammars
* Java Declarations Summary:: List of Bison declarations used with Java
A Brief History of the Greater Ungulates
* Yacc:: The original Yacc
* yacchack:: An obscure early implementation of reentrancy
* Byacc:: Berkeley Yacc
* Bison:: This program
* Other Ungulates:: Similar programs
Frequently Asked Questions
* Memory Exhausted:: Breaking the Stack Limits
* How Can I Reset the Parser:: ‘yyparse’ Keeps some State
* Strings are Destroyed:: ‘yylval’ Loses Track of Strings
* Implementing Gotos/Loops:: Control Flow in the Calculator
* Multiple start-symbols:: Factoring closely related grammars
* Enabling Relocatability:: Moving Bison/using it through network shares
* Secure? Conform?:: Is Bison POSIX safe?
* I can't build Bison:: Troubleshooting
* Where can I find help?:: Troubleshouting
* Bug Reports:: Troublereporting
* More Languages:: Parsers in C++, Java, and so on
* Beta Testing:: Experimenting development versions
* Mailing Lists:: Meeting other Bison users
Copying This Manual
* GNU Free Documentation License:: Copying and sharing this manual
File: bison.info, Node: Introduction, Next: Conditions, Prev: Top, Up: Top
Introduction
************
“Bison” is a general-purpose parser generator that converts an annotated
context-free grammar into a deterministic LR or generalized LR (GLR)
parser employing LALR(1) parser tables. As an experimental feature,
Bison can also generate IELR(1) or canonical LR(1) parser tables. Once
you are proficient with Bison, you can use it to develop a wide range of
language parsers, from those used in simple desk calculators to complex
programming languages.
Bison is upward compatible with Yacc: all properly-written Yacc
grammars ought to work with Bison with no change. Anyone familiar with
Yacc should be able to use Bison with little trouble. You need to be
fluent in C or C++ programming in order to use Bison or to understand
this manual. Java is also supported as an experimental feature.
We begin with tutorial chapters that explain the basic concepts of
using Bison and show three explained examples, each building on the
last. If you don’t know Bison or Yacc, start by reading these chapters.
Reference chapters follow, which describe specific aspects of Bison in
detail.
Bison was written originally by Robert Corbett. Richard Stallman
made it Yacc-compatible. Wilfred Hansen of Carnegie Mellon University
added multi-character string literals and other features. Since then,
Bison has grown more robust and evolved many other new features thanks
to the hard work of a long list of volunteers. For details, see the
‘THANKS’ and ‘ChangeLog’ files included in the Bison distribution.
This edition corresponds to version 3.4.1 of Bison.
File: bison.info, Node: Conditions, Next: Copying, Prev: Introduction, Up: Top
Conditions for Using Bison
**************************
The distribution terms for Bison-generated parsers permit using the
parsers in nonfree programs. Before Bison version 2.2, these extra
permissions applied only when Bison was generating LALR(1) parsers in C.
And before Bison version 1.24, Bison-generated parsers could be used
only in programs that were free software.
The other GNU programming tools, such as the GNU C compiler, have
never had such a requirement. They could always be used for nonfree
software. The reason Bison was different was not due to a special
policy decision; it resulted from applying the usual General Public
License to all of the Bison source code.
The main output of the Bison utility—the Bison parser implementation
file—contains a verbatim copy of a sizable piece of Bison, which is the
code for the parser’s implementation. (The actions from your grammar
are inserted into this implementation at one point, but most of the rest
of the implementation is not changed.) When we applied the GPL terms to
the skeleton code for the parser’s implementation, the effect was to
restrict the use of Bison output to free software.
We didn’t change the terms because of sympathy for people who want to
make software proprietary. *Software should be free.* But we concluded
that limiting Bison’s use to free software was doing little to encourage
people to make other software free. So we decided to make the practical
conditions for using Bison match the practical conditions for using the
other GNU tools.
This exception applies when Bison is generating code for a parser.
You can tell whether the exception applies to a Bison output file by
inspecting the file for text beginning with “As a special exception...”.
The text spells out the exact terms of the exception.
File: bison.info, Node: Copying, Next: Concepts, Prev: Conditions, Up: Top
GNU GENERAL PUBLIC LICENSE
**************************
Version 3, 29 June 2007
Copyright © 2007 Free Software Foundation, Inc.
Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.
Preamble
========
The GNU General Public License is a free, copyleft license for software
and other kinds of works.
The licenses for most software and other practical works are designed
to take away your freedom to share and change the works. By contrast,
the GNU General Public License is intended to guarantee your freedom to
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When we speak of free software, we are referring to freedom, not
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TERMS AND CONDITIONS
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File: bison.info, Node: Concepts, Next: Examples, Prev: Copying, Up: Top
1 The Concepts of Bison
***********************
This chapter introduces many of the basic concepts without which the
details of Bison will not make sense. If you do not already know how to
use Bison or Yacc, we suggest you start by reading this chapter
carefully.
* Menu:
* Language and Grammar:: Languages and context-free grammars,
as mathematical ideas.
* Grammar in Bison:: How we represent grammars for Bison’s sake.
* Semantic Values:: Each token or syntactic grouping can have
a semantic value (the value of an integer,
the name of an identifier, etc.).
* Semantic Actions:: Each rule can have an action containing C code.
* GLR Parsers:: Writing parsers for general context-free languages.
* Locations:: Overview of location tracking.
* Bison Parser:: What are Bison’s input and output,
how is the output used?
* Stages:: Stages in writing and running Bison grammars.
* Grammar Layout:: Overall structure of a Bison grammar file.
File: bison.info, Node: Language and Grammar, Next: Grammar in Bison, Up: Concepts
1.1 Languages and Context-Free Grammars
=======================================
In order for Bison to parse a language, it must be described by a
“context-free grammar”. This means that you specify one or more
“syntactic groupings” and give rules for constructing them from their
parts. For example, in the C language, one kind of grouping is called
an ‘expression’. One rule for making an expression might be, “An
expression can be made of a minus sign and another expression”. Another
would be, “An expression can be an integer”. As you can see, rules are
often recursive, but there must be at least one rule which leads out of
the recursion.
The most common formal system for presenting such rules for humans to
read is “Backus-Naur Form” or “BNF”, which was developed in order to
specify the language Algol 60. Any grammar expressed in BNF is a
context-free grammar. The input to Bison is essentially
machine-readable BNF.
There are various important subclasses of context-free grammars.
Although it can handle almost all context-free grammars, Bison is
optimized for what are called LR(1) grammars. In brief, in these
grammars, it must be possible to tell how to parse any portion of an
input string with just a single token of lookahead. For historical
reasons, Bison by default is limited by the additional restrictions of
LALR(1), which is hard to explain simply. *Note Mysterious Conflicts::,
for more information on this. As an experimental feature, you can
escape these additional restrictions by requesting IELR(1) or canonical
LR(1) parser tables. *Note LR Table Construction::, to learn how.
Parsers for LR(1) grammars are “deterministic”, meaning roughly that
the next grammar rule to apply at any point in the input is uniquely
determined by the preceding input and a fixed, finite portion (called a
“lookahead”) of the remaining input. A context-free grammar can be
“ambiguous”, meaning that there are multiple ways to apply the grammar
rules to get the same inputs. Even unambiguous grammars can be
“nondeterministic”, meaning that no fixed lookahead always suffices to
determine the next grammar rule to apply. With the proper declarations,
Bison is also able to parse these more general context-free grammars,
using a technique known as GLR parsing (for Generalized LR). Bison’s GLR
parsers are able to handle any context-free grammar for which the number
of possible parses of any given string is finite.
In the formal grammatical rules for a language, each kind of
syntactic unit or grouping is named by a “symbol”. Those which are
built by grouping smaller constructs according to grammatical rules are
called “nonterminal symbols”; those which can’t be subdivided are called
“terminal symbols” or “token types”. We call a piece of input
corresponding to a single terminal symbol a “token”, and a piece
corresponding to a single nonterminal symbol a “grouping”.
We can use the C language as an example of what symbols, terminal and
nonterminal, mean. The tokens of C are identifiers, constants (numeric
and string), and the various keywords, arithmetic operators and
punctuation marks. So the terminal symbols of a grammar for C include
‘identifier’, ‘number’, ‘string’, plus one symbol for each keyword,
operator or punctuation mark: ‘if’, ‘return’, ‘const’, ‘static’, ‘int’,
‘char’, ‘plus-sign’, ‘open-brace’, ‘close-brace’, ‘comma’ and many more.
(These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)
Here is a simple C function subdivided into tokens:
int /* keyword ‘int’ */
square (int x) /* identifier, open-paren, keyword ‘int’,
identifier, close-paren */
{ /* open-brace */
return x * x; /* keyword ‘return’, identifier, asterisk,
identifier, semicolon */
} /* close-brace */
The syntactic groupings of C include the expression, the statement,
the declaration, and the function definition. These are represented in
the grammar of C by nonterminal symbols ‘expression’, ‘statement’,
‘declaration’ and ‘function definition’. The full grammar uses dozens
of additional language constructs, each with its own nonterminal symbol,
in order to express the meanings of these four. The example above is a
function definition; it contains one declaration, and one statement. In
the statement, each ‘x’ is an expression and so is ‘x * x’.
Each nonterminal symbol must have grammatical rules showing how it is
made out of simpler constructs. For example, one kind of C statement is
the ‘return’ statement; this would be described with a grammar rule
which reads informally as follows:
A ‘statement’ can be made of a ‘return’ keyword, an ‘expression’
and a ‘semicolon’.
There would be many other rules for ‘statement’, one for each kind of
statement in C.
One nonterminal symbol must be distinguished as the special one which
defines a complete utterance in the language. It is called the “start
symbol”. In a compiler, this means a complete input program. In the C
language, the nonterminal symbol ‘sequence of definitions and
declarations’ plays this role.
For example, ‘1 + 2’ is a valid C expression—a valid part of a C
program—but it is not valid as an _entire_ C program. In the
context-free grammar of C, this follows from the fact that ‘expression’
is not the start symbol.
The Bison parser reads a sequence of tokens as its input, and groups
the tokens using the grammar rules. If the input is valid, the end
result is that the entire token sequence reduces to a single grouping
whose symbol is the grammar’s start symbol. If we use a grammar for C,
the entire input must be a ‘sequence of definitions and declarations’.
If not, the parser reports a syntax error.
File: bison.info, Node: Grammar in Bison, Next: Semantic Values, Prev: Language and Grammar, Up: Concepts
1.2 From Formal Rules to Bison Input
====================================
A formal grammar is a mathematical construct. To define the language
for Bison, you must write a file expressing the grammar in Bison syntax:
a “Bison grammar” file. *Note Bison Grammar Files: Grammar File.
A nonterminal symbol in the formal grammar is represented in Bison
input as an identifier, like an identifier in C. By convention, it
should be in lower case, such as ‘expr’, ‘stmt’ or ‘declaration’.
The Bison representation for a terminal symbol is also called a
“token type”. Token types as well can be represented as C-like
identifiers. By convention, these identifiers should be upper case to
distinguish them from nonterminals: for example, ‘INTEGER’,
‘IDENTIFIER’, ‘IF’ or ‘RETURN’. A terminal symbol that stands for a
particular keyword in the language should be named after that keyword
converted to upper case. The terminal symbol ‘error’ is reserved for
error recovery. *Note Symbols::.
A terminal symbol can also be represented as a character literal,
just like a C character constant. You should do this whenever a token
is just a single character (parenthesis, plus-sign, etc.): use that same
character in a literal as the terminal symbol for that token.
A third way to represent a terminal symbol is with a C string
constant containing several characters. *Note Symbols::, for more
information.
The grammar rules also have an expression in Bison syntax. For
example, here is the Bison rule for a C ‘return’ statement. The
semicolon in quotes is a literal character token, representing part of
the C syntax for the statement; the naked semicolon, and the colon, are
Bison punctuation used in every rule.
stmt: RETURN expr ';' ;
*Note Syntax of Grammar Rules: Rules.
File: bison.info, Node: Semantic Values, Next: Semantic Actions, Prev: Grammar in Bison, Up: Concepts
1.3 Semantic Values
===================
A formal grammar selects tokens only by their classifications: for
example, if a rule mentions the terminal symbol ‘integer constant’, it
means that _any_ integer constant is grammatically valid in that
position. The precise value of the constant is irrelevant to how to
parse the input: if ‘x+4’ is grammatical then ‘x+1’ or ‘x+3989’ is
equally grammatical.
But the precise value is very important for what the input means once
it is parsed. A compiler is useless if it fails to distinguish between
4, 1 and 3989 as constants in the program! Therefore, each token in a
Bison grammar has both a token type and a “semantic value”. *Note
Defining Language Semantics: Semantics, for details.
The token type is a terminal symbol defined in the grammar, such as
‘INTEGER’, ‘IDENTIFIER’ or ‘','’. It tells everything you need to know
to decide where the token may validly appear and how to group it with
other tokens. The grammar rules know nothing about tokens except their
types.
The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
identifier. (A token such as ‘','’ which is just punctuation doesn’t
need to have any semantic value.)
For example, an input token might be classified as token type
‘INTEGER’ and have the semantic value 4. Another input token might have
the same token type ‘INTEGER’ but value 3989. When a grammar rule says
that ‘INTEGER’ is allowed, either of these tokens is acceptable because
each is an ‘INTEGER’. When the parser accepts the token, it keeps track
of the token’s semantic value.
Each grouping can also have a semantic value as well as its
nonterminal symbol. For example, in a calculator, an expression
typically has a semantic value that is a number. In a compiler for a
programming language, an expression typically has a semantic value that
is a tree structure describing the meaning of the expression.
File: bison.info, Node: Semantic Actions, Next: GLR Parsers, Prev: Semantic Values, Up: Concepts
1.4 Semantic Actions
====================
In order to be useful, a program must do more than parse input; it must
also produce some output based on the input. In a Bison grammar, a
grammar rule can have an “action” made up of C statements. Each time
the parser recognizes a match for that rule, the action is executed.
*Note Actions::.
Most of the time, the purpose of an action is to compute the semantic
value of the whole construct from the semantic values of its parts. For
example, suppose we have a rule which says an expression can be the sum
of two expressions. When the parser recognizes such a sum, each of the
subexpressions has a semantic value which describes how it was built up.
The action for this rule should create a similar sort of value for the
newly recognized larger expression.
For example, here is a rule that says an expression can be the sum of
two subexpressions:
expr: expr '+' expr { $$ = $1 + $3; } ;
The action says how to produce the semantic value of the sum expression
from the values of the two subexpressions.
File: bison.info, Node: GLR Parsers, Next: Locations, Prev: Semantic Actions, Up: Concepts
1.5 Writing GLR Parsers
=======================
In some grammars, Bison’s deterministic LR(1) parsing algorithm cannot
decide whether to apply a certain grammar rule at a given point. That
is, it may not be able to decide (on the basis of the input read so far)
which of two possible reductions (applications of a grammar rule)
applies, or whether to apply a reduction or read more of the input and
apply a reduction later in the input. These are known respectively as
“reduce/reduce” conflicts (*note Reduce/Reduce::), and “shift/reduce”
conflicts (*note Shift/Reduce::).
To use a grammar that is not easily modified to be LR(1), a more
general parsing algorithm is sometimes necessary. If you include
‘%glr-parser’ among the Bison declarations in your file (*note Grammar
Outline::), the result is a Generalized LR (GLR) parser. These parsers
handle Bison grammars that contain no unresolved conflicts (i.e., after
applying precedence declarations) identically to deterministic parsers.
However, when faced with unresolved shift/reduce and reduce/reduce
conflicts, GLR parsers use the simple expedient of doing both,
effectively cloning the parser to follow both possibilities. Each of
the resulting parsers can again split, so that at any given time, there
can be any number of possible parses being explored. The parsers
proceed in lockstep; that is, all of them consume (shift) a given input
symbol before any of them proceed to the next. Each of the cloned
parsers eventually meets one of two possible fates: either it runs into
a parsing error, in which case it simply vanishes, or it merges with
another parser, because the two of them have reduced the input to an
identical set of symbols.
During the time that there are multiple parsers, semantic actions are
recorded, but not performed. When a parser disappears, its recorded
semantic actions disappear as well, and are never performed. When a
reduction makes two parsers identical, causing them to merge, Bison
records both sets of semantic actions. Whenever the last two parsers
merge, reverting to the single-parser case, Bison resolves all the
outstanding actions either by precedences given to the grammar rules
involved, or by performing both actions, and then calling a designated
user-defined function on the resulting values to produce an arbitrary
merged result.
* Menu:
* Simple GLR Parsers:: Using GLR parsers on unambiguous grammars.
* Merging GLR Parses:: Using GLR parsers to resolve ambiguities.
* GLR Semantic Actions:: Considerations for semantic values and deferred actions.
* Semantic Predicates:: Controlling a parse with arbitrary computations.
* Compiler Requirements for GLR:: GLR parsers require a modern C compiler.
File: bison.info, Node: Simple GLR Parsers, Next: Merging GLR Parses, Up: GLR Parsers
1.5.1 Using GLR on Unambiguous Grammars
---------------------------------------
In the simplest cases, you can use the GLR algorithm to parse grammars
that are unambiguous but fail to be LR(1). Such grammars typically
require more than one symbol of lookahead.
Consider a problem that arises in the declaration of enumerated and
subrange types in the programming language Pascal. Here are some
examples:
type subrange = lo .. hi;
type enum = (a, b, c);
The original language standard allows only numeric literals and constant
identifiers for the subrange bounds (‘lo’ and ‘hi’), but Extended Pascal
(ISO/IEC 10206) and many other Pascal implementations allow arbitrary
expressions there. This gives rise to the following situation,
containing a superfluous pair of parentheses:
type subrange = (a) .. b;
Compare this to the following declaration of an enumerated type with
only one value:
type enum = (a);
(These declarations are contrived, but they are syntactically valid, and
more-complicated cases can come up in practical programs.)
These two declarations look identical until the ‘..’ token. With
normal LR(1) one-token lookahead it is not possible to decide between
the two forms when the identifier ‘a’ is parsed. It is, however,
desirable for a parser to decide this, since in the latter case ‘a’ must
become a new identifier to represent the enumeration value, while in the
former case ‘a’ must be evaluated with its current meaning, which may be
a constant or even a function call.
You could parse ‘(a)’ as an “unspecified identifier in parentheses”,
to be resolved later, but this typically requires substantial
contortions in both semantic actions and large parts of the grammar,
where the parentheses are nested in the recursive rules for expressions.
You might think of using the lexer to distinguish between the two
forms by returning different tokens for currently defined and undefined
identifiers. But if these declarations occur in a local scope, and ‘a’
is defined in an outer scope, then both forms are possible—either
locally redefining ‘a’, or using the value of ‘a’ from the outer scope.
So this approach cannot work.
A simple solution to this problem is to declare the parser to use the
GLR algorithm. When the GLR parser reaches the critical state, it
merely splits into two branches and pursues both syntax rules
simultaneously. Sooner or later, one of them runs into a parsing error.
If there is a ‘..’ token before the next ‘;’, the rule for enumerated
types fails since it cannot accept ‘..’ anywhere; otherwise, the
subrange type rule fails since it requires a ‘..’ token. So one of the
branches fails silently, and the other one continues normally,
performing all the intermediate actions that were postponed during the
split.
If the input is syntactically incorrect, both branches fail and the
parser reports a syntax error as usual.
The effect of all this is that the parser seems to “guess” the
correct branch to take, or in other words, it seems to use more
lookahead than the underlying LR(1) algorithm actually allows for. In
this example, LR(2) would suffice, but also some cases that are not
LR(k) for any k can be handled this way.
In general, a GLR parser can take quadratic or cubic worst-case time,
and the current Bison parser even takes exponential time and space for
some grammars. In practice, this rarely happens, and for many grammars
it is possible to prove that it cannot happen. The present example
contains only one conflict between two rules, and the type-declaration
context containing the conflict cannot be nested. So the number of
branches that can exist at any time is limited by the constant 2, and
the parsing time is still linear.
Here is a Bison grammar corresponding to the example above. It
parses a vastly simplified form of Pascal type declarations.
%token TYPE DOTDOT ID
%left '+' '-'
%left '*' '/'
%%
type_decl: TYPE ID '=' type ';' ;
type:
'(' id_list ')'
| expr DOTDOT expr
;
id_list:
ID
| id_list ',' ID
;
expr:
'(' expr ')'
| expr '+' expr
| expr '-' expr
| expr '*' expr
| expr '/' expr
| ID
;
When used as a normal LR(1) grammar, Bison correctly complains about
one reduce/reduce conflict. In the conflicting situation the parser
chooses one of the alternatives, arbitrarily the one declared first.
Therefore the following correct input is not recognized:
type t = (a) .. b;
The parser can be turned into a GLR parser, while also telling Bison
to be silent about the one known reduce/reduce conflict, by adding these
two declarations to the Bison grammar file (before the first ‘%%’):
%glr-parser
%expect-rr 1
No change in the grammar itself is required. Now the parser recognizes
all valid declarations, according to the limited syntax above,
transparently. In fact, the user does not even notice when the parser
splits.
So here we have a case where we can use the benefits of GLR, almost
without disadvantages. Even in simple cases like this, however, there
are at least two potential problems to beware. First, always analyze
the conflicts reported by Bison to make sure that GLR splitting is only
done where it is intended. A GLR parser splitting inadvertently may
cause problems less obvious than an LR parser statically choosing the
wrong alternative in a conflict. Second, consider interactions with the
lexer (*note Semantic Tokens::) with great care. Since a split parser
consumes tokens without performing any actions during the split, the
lexer cannot obtain information via parser actions. Some cases of lexer
interactions can be eliminated by using GLR to shift the complications
from the lexer to the parser. You must check the remaining cases for
correctness.
In our example, it would be safe for the lexer to return tokens based
on their current meanings in some symbol table, because no new symbols
are defined in the middle of a type declaration. Though it is possible
for a parser to define the enumeration constants as they are parsed,
before the type declaration is completed, it actually makes no
difference since they cannot be used within the same enumerated type
declaration.
File: bison.info, Node: Merging GLR Parses, Next: GLR Semantic Actions, Prev: Simple GLR Parsers, Up: GLR Parsers
1.5.2 Using GLR to Resolve Ambiguities
--------------------------------------
Let’s consider an example, vastly simplified from a C++ grammar.
%{
#include
#define YYSTYPE char const *
int yylex (void);
void yyerror (char const *);
%}
%token TYPENAME ID
%right '='
%left '+'
%glr-parser
%%
prog:
%empty
| prog stmt { printf ("\n"); }
;
stmt:
expr ';' %dprec 1
| decl %dprec 2
;
expr:
ID { printf ("%s ", $$); }
| TYPENAME '(' expr ')'
{ printf ("%s ", $1); }
| expr '+' expr { printf ("+ "); }
| expr '=' expr { printf ("= "); }
;
decl:
TYPENAME declarator ';'
{ printf ("%s ", $1); }
| TYPENAME declarator '=' expr ';'
{ printf ("%s ", $1); }
;
declarator:
ID { printf ("\"%s\" ", $1); }
| '(' declarator ')'
;
This models a problematic part of the C++ grammar—the ambiguity between
certain declarations and statements. For example,
T (x) = y+z;
parses as either an ‘expr’ or a ‘stmt’ (assuming that ‘T’ is recognized
as a ‘TYPENAME’ and ‘x’ as an ‘ID’). Bison detects this as a
reduce/reduce conflict between the rules ‘expr : ID’ and ‘declarator :
ID’, which it cannot resolve at the time it encounters ‘x’ in the
example above. Since this is a GLR parser, it therefore splits the
problem into two parses, one for each choice of resolving the
reduce/reduce conflict. Unlike the example from the previous section
(*note Simple GLR Parsers::), however, neither of these parses “dies,”
because the grammar as it stands is ambiguous. One of the parsers
eventually reduces ‘stmt : expr ';'’ and the other reduces ‘stmt :
decl’, after which both parsers are in an identical state: they’ve seen
‘prog stmt’ and have the same unprocessed input remaining. We say that
these parses have “merged.”
At this point, the GLR parser requires a specification in the grammar
of how to choose between the competing parses. In the example above,
the two ‘%dprec’ declarations specify that Bison is to give precedence
to the parse that interprets the example as a ‘decl’, which implies that
‘x’ is a declarator. The parser therefore prints
"x" y z + T
The ‘%dprec’ declarations only come into play when more than one
parse survives. Consider a different input string for this parser:
T (x) + y;
This is another example of using GLR to parse an unambiguous construct,
as shown in the previous section (*note Simple GLR Parsers::). Here,
there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters ‘x’, it does not have
enough information to resolve the reduce/reduce conflict (again, between
‘x’ as an ‘expr’ or a ‘declarator’). In this case, no precedence
declaration is used. Again, the parser splits into two, one assuming
that ‘x’ is an ‘expr’, and the other assuming ‘x’ is a ‘declarator’.
The second of these parsers then vanishes when it sees ‘+’, and the
parser prints
x T y +
Suppose that instead of resolving the ambiguity, you wanted to see
all the possibilities. For this purpose, you must merge the semantic
actions of the two possible parsers, rather than choosing one over the
other. To do so, you could change the declaration of ‘stmt’ as follows:
stmt:
expr ';' %merge
| decl %merge
;
and define the ‘stmtMerge’ function as:
static YYSTYPE
stmtMerge (YYSTYPE x0, YYSTYPE x1)
{
printf (" ");
return "";
}
with an accompanying forward declaration in the C declarations at the
beginning of the file:
%{
#define YYSTYPE char const *
static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
%}
With these declarations, the resulting parser parses the first example
as both an ‘expr’ and a ‘decl’, and prints
"x" y z + T x T y z + =
Bison requires that all of the productions that participate in any
particular merge have identical ‘%merge’ clauses. Otherwise, the
ambiguity would be unresolvable, and the parser will report an error
during any parse that results in the offending merge.
File: bison.info, Node: GLR Semantic Actions, Next: Semantic Predicates, Prev: Merging GLR Parses, Up: GLR Parsers
1.5.3 GLR Semantic Actions
--------------------------
The nature of GLR parsing and the structure of the generated parsers
give rise to certain restrictions on semantic values and actions.
1.5.3.1 Deferred semantic actions
.................................
By definition, a deferred semantic action is not performed at the same
time as the associated reduction. This raises caveats for several Bison
features you might use in a semantic action in a GLR parser.
In any semantic action, you can examine ‘yychar’ to determine the
type of the lookahead token present at the time of the associated
reduction. After checking that ‘yychar’ is not set to ‘YYEMPTY’ or
‘YYEOF’, you can then examine ‘yylval’ and ‘yylloc’ to determine the
lookahead token’s semantic value and location, if any. In a nondeferred
semantic action, you can also modify any of these variables to influence
syntax analysis. *Note Lookahead Tokens: Lookahead.
In a deferred semantic action, it’s too late to influence syntax
analysis. In this case, ‘yychar’, ‘yylval’, and ‘yylloc’ are set to
shallow copies of the values they had at the time of the associated
reduction. For this reason alone, modifying them is dangerous.
Moreover, the result of modifying them is undefined and subject to
change with future versions of Bison. For example, if a semantic action
might be deferred, you should never write it to invoke ‘yyclearin’
(*note Action Features::) or to attempt to free memory referenced by
‘yylval’.
1.5.3.2 YYERROR
...............
Another Bison feature requiring special consideration is ‘YYERROR’
(*note Action Features::), which you can invoke in a semantic action to
initiate error recovery. During deterministic GLR operation, the effect
of ‘YYERROR’ is the same as its effect in a deterministic parser. The
effect in a deferred action is similar, but the precise point of the
error is undefined; instead, the parser reverts to deterministic
operation, selecting an unspecified stack on which to continue with a
syntax error. In a semantic predicate (see *note Semantic Predicates::)
during nondeterministic parsing, ‘YYERROR’ silently prunes the parse
that invoked the test.
1.5.3.3 Restrictions on semantic values and locations
.....................................................
GLR parsers require that you use POD (Plain Old Data) types for semantic
values and location types when using the generated parsers as C++ code.
File: bison.info, Node: Semantic Predicates, Next: Compiler Requirements for GLR, Prev: GLR Semantic Actions, Up: GLR Parsers
1.5.4 Controlling a Parse with Arbitrary Predicates
---------------------------------------------------
In addition to the ‘%dprec’ and ‘%merge’ directives, GLR parsers allow
you to reject parses on the basis of arbitrary computations executed in
user code, without having Bison treat this rejection as an error if
there are alternative parses. (This feature is experimental and may
evolve. We welcome user feedback.) For example,
widget:
%?{ new_syntax } "widget" id new_args { $$ = f($3, $4); }
| %?{ !new_syntax } "widget" id old_args { $$ = f($3, $4); }
;
is one way to allow the same parser to handle two different syntaxes for
widgets. The clause preceded by ‘%?’ is treated like an ordinary
action, except that its text is treated as an expression and is always
evaluated immediately (even when in nondeterministic mode). If the
expression yields 0 (false), the clause is treated as a syntax error,
which, in a nondeterministic parser, causes the stack in which it is
reduced to die. In a deterministic parser, it acts like YYERROR.
As the example shows, predicates otherwise look like semantic
actions, and therefore you must be take them into account when
determining the numbers to use for denoting the semantic values of
right-hand side symbols. Predicate actions, however, have no defined
value, and may not be given labels.
There is a subtle difference between semantic predicates and ordinary
actions in nondeterministic mode, since the latter are deferred. For
example, we could try to rewrite the previous example as
widget:
{ if (!new_syntax) YYERROR; }
"widget" id new_args { $$ = f($3, $4); }
| { if (new_syntax) YYERROR; }
"widget" id old_args { $$ = f($3, $4); }
;
(reversing the sense of the predicate tests to cause an error when they
are false). However, this does _not_ have the same effect if ‘new_args’
and ‘old_args’ have overlapping syntax. Since the midrule actions
testing ‘new_syntax’ are deferred, a GLR parser first encounters the
unresolved ambiguous reduction for cases where ‘new_args’ and ‘old_args’
recognize the same string _before_ performing the tests of ‘new_syntax’.
It therefore reports an error.
Finally, be careful in writing predicates: deferred actions have not
been evaluated, so that using them in a predicate will have undefined
effects.
File: bison.info, Node: Compiler Requirements for GLR, Prev: Semantic Predicates, Up: GLR Parsers
1.5.5 Considerations when Compiling GLR Parsers
-----------------------------------------------
The GLR parsers require a compiler for ISO C89 or later. In addition,
they use the ‘inline’ keyword, which is not C89, but is C99 and is a
common extension in pre-C99 compilers. It is up to the user of these
parsers to handle portability issues. For instance, if using Autoconf
and the Autoconf macro ‘AC_C_INLINE’, a mere
%{
#include
%}
will suffice. Otherwise, we suggest
%{
#if (__STDC_VERSION__ < 199901 && ! defined __GNUC__ \
&& ! defined inline)
# define inline
#endif
%}
File: bison.info, Node: Locations, Next: Bison Parser, Prev: GLR Parsers, Up: Concepts
1.6 Locations
=============
Many applications, like interpreters or compilers, have to produce
verbose and useful error messages. To achieve this, one must be able to
keep track of the “textual location”, or “location”, of each syntactic
construct. Bison provides a mechanism for handling these locations.
Each token has a semantic value. In a similar fashion, each token
has an associated location, but the type of locations is the same for
all tokens and groupings. Moreover, the output parser is equipped with
a default data structure for storing locations (*note Tracking
Locations::, for more details).
Like semantic values, locations can be reached in actions using a
dedicated set of constructs. In the example above, the location of the
whole grouping is ‘@$’, while the locations of the subexpressions are
‘@1’ and ‘@3’.
When a rule is matched, a default action is used to compute the
semantic value of its left hand side (*note Actions::). In the same
way, another default action is used for locations. However, the action
for locations is general enough for most cases, meaning there is usually
no need to describe for each rule how ‘@$’ should be formed. When
building a new location for a given grouping, the default behavior of
the output parser is to take the beginning of the first symbol, and the
end of the last symbol.
File: bison.info, Node: Bison Parser, Next: Stages, Prev: Locations, Up: Concepts
1.7 Bison Output: the Parser Implementation File
================================================
When you run Bison, you give it a Bison grammar file as input. The most
important output is a C source file that implements a parser for the
language described by the grammar. This parser is called a “Bison
parser”, and this file is called a “Bison parser implementation file”.
Keep in mind that the Bison utility and the Bison parser are two
distinct programs: the Bison utility is a program whose output is the
Bison parser implementation file that becomes part of your program.
The job of the Bison parser is to group tokens into groupings
according to the grammar rules—for example, to build identifiers and
operators into expressions. As it does this, it runs the actions for
the grammar rules it uses.
The tokens come from a function called the “lexical analyzer” that
you must supply in some fashion (such as by writing it in C). The Bison
parser calls the lexical analyzer each time it wants a new token. It
doesn’t know what is “inside” the tokens (though their semantic values
may reflect this). Typically the lexical analyzer makes the tokens by
parsing characters of text, but Bison does not depend on this. *Note
The Lexical Analyzer Function ‘yylex’: Lexical.
The Bison parser implementation file is C code which defines a
function named ‘yyparse’ which implements that grammar. This function
does not make a complete C program: you must supply some additional
functions. One is the lexical analyzer. Another is an error-reporting
function which the parser calls to report an error. In addition, a
complete C program must start with a function called ‘main’; you have to
provide this, and arrange for it to call ‘yyparse’ or the parser will
never run. *Note Parser C-Language Interface: Interface.
Aside from the token type names and the symbols in the actions you
write, all symbols defined in the Bison parser implementation file
itself begin with ‘yy’ or ‘YY’. This includes interface functions such
as the lexical analyzer function ‘yylex’, the error reporting function
‘yyerror’ and the parser function ‘yyparse’ itself. This also includes
numerous identifiers used for internal purposes. Therefore, you should
avoid using C identifiers starting with ‘yy’ or ‘YY’ in the Bison
grammar file except for the ones defined in this manual. Also, you
should avoid using the C identifiers ‘malloc’ and ‘free’ for anything
other than their usual meanings.
In some cases the Bison parser implementation file includes system
headers, and in those cases your code should respect the identifiers
reserved by those headers. On some non-GNU hosts, ‘’,
‘’, ‘’, and ‘’ are included as needed to
declare memory allocators and related types. ‘’ is included
if message translation is in use (*note Internationalization::). Other
system headers may be included if you define ‘YYDEBUG’ to a nonzero
value (*note Tracing Your Parser: Tracing.).
File: bison.info, Node: Stages, Next: Grammar Layout, Prev: Bison Parser, Up: Concepts
1.8 Stages in Using Bison
=========================
The actual language-design process using Bison, from grammar
specification to a working compiler or interpreter, has these parts:
1. Formally specify the grammar in a form recognized by Bison (*note
Bison Grammar Files: Grammar File.). For each grammatical rule in
the language, describe the action that is to be taken when an
instance of that rule is recognized. The action is described by a
sequence of C statements.
2. Write a lexical analyzer to process input and pass tokens to the
parser. The lexical analyzer may be written by hand in C (*note
The Lexical Analyzer Function ‘yylex’: Lexical.). It could also be
produced using Lex, but the use of Lex is not discussed in this
manual.
3. Write a controlling function that calls the Bison-produced parser.
4. Write error-reporting routines.
To turn this source code as written into a runnable program, you must
follow these steps:
1. Run Bison on the grammar to produce the parser.
2. Compile the code output by Bison, as well as any other source
files.
3. Link the object files to produce the finished product.
File: bison.info, Node: Grammar Layout, Prev: Stages, Up: Concepts
1.9 The Overall Layout of a Bison Grammar
=========================================
The input file for the Bison utility is a “Bison grammar file”. The
general form of a Bison grammar file is as follows:
%{
PROLOGUE
%}
BISON DECLARATIONS
%%
GRAMMAR RULES
%%
EPILOGUE
The ‘%%’, ‘%{’ and ‘%}’ are punctuation that appears in every Bison
grammar file to separate the sections.
The prologue may define types and variables used in the actions. You
can also use preprocessor commands to define macros used there, and use
‘#include’ to include header files that do any of these things. You
need to declare the lexical analyzer ‘yylex’ and the error printer
‘yyerror’ here, along with any other global identifiers used by the
actions in the grammar rules.
The Bison declarations declare the names of the terminal and
nonterminal symbols, and may also describe operator precedence and the
data types of semantic values of various symbols.
The grammar rules define how to construct each nonterminal symbol
from its parts.
The epilogue can contain any code you want to use. Often the
definitions of functions declared in the prologue go here. In a simple
program, all the rest of the program can go here.
File: bison.info, Node: Examples, Next: Grammar File, Prev: Concepts, Up: Top
2 Examples
**********
Now we show and explain several sample programs written using Bison: a
Reverse Polish Notation calculator, an algebraic (infix) notation
calculator — later extended to track “locations” — and a multi-function
calculator. All produce usable, though limited, interactive desk-top
calculators.
These examples are simple, but Bison grammars for real programming
languages are written the same way. You can copy these examples into a
source file to try them.
* Menu:
* RPN Calc:: Reverse Polish Notation Calculator;
a first example with no operator precedence.
* Infix Calc:: Infix (algebraic) notation calculator.
Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @N and @$.
* Multi-function Calc:: Calculator with memory and trig functions.
It uses multiple data-types for semantic values.
* Exercises:: Ideas for improving the multi-function calculator.
File: bison.info, Node: RPN Calc, Next: Infix Calc, Up: Examples
2.1 Reverse Polish Notation Calculator
======================================
The first example is that of a simple double-precision “Reverse Polish
Notation” calculator (a calculator using postfix operators). This
example provides a good starting point, since operator precedence is not
an issue. The second example will illustrate how operator precedence is
handled.
The source code for this calculator is named ‘rpcalc.y’. The ‘.y’
extension is a convention used for Bison grammar files.
* Menu:
* Rpcalc Declarations:: Prologue (declarations) for rpcalc.
* Rpcalc Rules:: Grammar Rules for rpcalc, with explanation.
* Rpcalc Lexer:: The lexical analyzer.
* Rpcalc Main:: The controlling function.
* Rpcalc Error:: The error reporting function.
* Rpcalc Generate:: Running Bison on the grammar file.
* Rpcalc Compile:: Run the C compiler on the output code.
File: bison.info, Node: Rpcalc Declarations, Next: Rpcalc Rules, Up: RPN Calc
2.1.1 Declarations for ‘rpcalc’
-------------------------------
Here are the C and Bison declarations for the Reverse Polish Notation
calculator. As in C, comments are placed between ‘/*...*/’ or after
‘//’.
/* Reverse Polish Notation calculator. */
%{
#include
#include
int yylex (void);
void yyerror (char const *);
%}
%define api.value.type {double}
%token NUM
%% /* Grammar rules and actions follow. */
The declarations section (*note The prologue: Prologue.) contains two
preprocessor directives and two forward declarations.
The ‘#include’ directive is used to declare the exponentiation
function ‘pow’.
The forward declarations for ‘yylex’ and ‘yyerror’ are needed because
the C language requires that functions be declared before they are used.
These functions will be defined in the epilogue, but the parser calls
them so they must be declared in the prologue.
The second section, Bison declarations, provides information to Bison
about the tokens and their types (*note The Bison Declarations Section:
Bison Declarations.).
The ‘%define’ directive defines the variable ‘api.value.type’, thus
specifying the C data type for semantic values of both tokens and
groupings (*note Data Types of Semantic Values: Value Type.). The Bison
parser will use whatever type ‘api.value.type’ is defined as; if you
don’t define it, ‘int’ is the default. Because we specify ‘{double}’,
each token and each expression has an associated value, which is a
floating point number. C code can use ‘YYSTYPE’ to refer to the value
‘api.value.type’.
Each terminal symbol that is not a single-character literal must be
declared. (Single-character literals normally don’t need to be
declared.) In this example, all the arithmetic operators are designated
by single-character literals, so the only terminal symbol that needs to
be declared is ‘NUM’, the token type for numeric constants.
File: bison.info, Node: Rpcalc Rules, Next: Rpcalc Lexer, Prev: Rpcalc Declarations, Up: RPN Calc
2.1.2 Grammar Rules for ‘rpcalc’
--------------------------------
Here are the grammar rules for the Reverse Polish Notation calculator.
input:
%empty
| input line
;
line:
'\n'
| exp '\n' { printf ("%.10g\n", $1); }
;
exp:
NUM
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
| exp exp '*' { $$ = $1 * $2; }
| exp exp '/' { $$ = $1 / $2; }
| exp exp '^' { $$ = pow ($1, $2); } /* Exponentiation */
| exp 'n' { $$ = -$1; } /* Unary minus */
;
%%
The groupings of the rpcalc “language” defined here are the
expression (given the name ‘exp’), the line of input (‘line’), and the
complete input transcript (‘input’). Each of these nonterminal symbols
has several alternate rules, joined by the vertical bar ‘|’ which is
read as “or”. The following sections explain what these rules mean.
The semantics of the language is determined by the actions taken when
a grouping is recognized. The actions are the C code that appears
inside braces. *Note Actions::.
You must specify these actions in C, but Bison provides the means for
passing semantic values between the rules. In each action, the
pseudo-variable ‘$$’ stands for the semantic value for the grouping that
the rule is going to construct. Assigning a value to ‘$$’ is the main
job of most actions. The semantic values of the components of the rule
are referred to as ‘$1’, ‘$2’, and so on.
* Menu:
* Rpcalc Input:: Explanation of the ‘input’ nonterminal
* Rpcalc Line:: Explanation of the ‘line’ nonterminal
* Rpcalc Expr:: Explanation of the ‘expr’ nonterminal
File: bison.info, Node: Rpcalc Input, Next: Rpcalc Line, Up: Rpcalc Rules
2.1.2.1 Explanation of ‘input’
..............................
Consider the definition of ‘input’:
input:
%empty
| input line
;
This definition reads as follows: “A complete input is either an
empty string, or a complete input followed by an input line”. Notice
that “complete input” is defined in terms of itself. This definition is
said to be “left recursive” since ‘input’ appears always as the leftmost
symbol in the sequence. *Note Recursive Rules: Recursion.
The first alternative is empty because there are no symbols between
the colon and the first ‘|’; this means that ‘input’ can match an empty
string of input (no tokens). We write the rules this way because it is
legitimate to type ‘Ctrl-d’ right after you start the calculator. It’s
conventional to put an empty alternative first and to use the (optional)
‘%empty’ directive, or to write the comment ‘/* empty */’ in it (*note
Empty Rules::).
The second alternate rule (‘input line’) handles all nontrivial
input. It means, “After reading any number of lines, read one more line
if possible.” The left recursion makes this rule into a loop. Since the
first alternative matches empty input, the loop can be executed zero or
more times.
The parser function ‘yyparse’ continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end-of-input.
File: bison.info, Node: Rpcalc Line, Next: Rpcalc Expr, Prev: Rpcalc Input, Up: Rpcalc Rules
2.1.2.2 Explanation of ‘line’
.............................
Now consider the definition of ‘line’:
line:
'\n'
| exp '\n' { printf ("%.10g\n", $1); }
;
The first alternative is a token which is a newline character; this
means that rpcalc accepts a blank line (and ignores it, since there is
no action). The second alternative is an expression followed by a
newline. This is the alternative that makes rpcalc useful. The
semantic value of the ‘exp’ grouping is the value of ‘$1’ because the
‘exp’ in question is the first symbol in the alternative. The action
prints this value, which is the result of the computation the user asked
for.
This action is unusual because it does not assign a value to ‘$$’.
As a consequence, the semantic value associated with the ‘line’ is
uninitialized (its value will be unpredictable). This would be a bug if
that value were ever used, but we don’t use it: once rpcalc has printed
the value of the user’s input line, that value is no longer needed.
File: bison.info, Node: Rpcalc Expr, Prev: Rpcalc Line, Up: Rpcalc Rules
2.1.2.3 Explanation of ‘expr’
.............................
The ‘exp’ grouping has several rules, one for each kind of expression.
The first rule handles the simplest expressions: those that are just
numbers. The second handles an addition-expression, which looks like
two expressions followed by a plus-sign. The third handles subtraction,
and so on.
exp:
NUM
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
...
;
We have used ‘|’ to join all the rules for ‘exp’, but we could
equally well have written them separately:
exp: NUM;
exp: exp exp '+' { $$ = $1 + $2; };
exp: exp exp '-' { $$ = $1 - $2; };
...
Most of the rules have actions that compute the value of the
expression in terms of the value of its parts. For example, in the rule
for addition, ‘$1’ refers to the first component ‘exp’ and ‘$2’ refers
to the second one. The third component, ‘'+'’, has no meaningful
associated semantic value, but if it had one you could refer to it as
‘$3’. The first rule relies on the implicit default action: ‘{ $$ = $1;
}’.
When ‘yyparse’ recognizes a sum expression using this rule, the sum
of the two subexpressions’ values is produced as the value of the entire
expression. *Note Actions::.
You don’t have to give an action for every rule. When a rule has no
action, Bison by default copies the value of ‘$1’ into ‘$$’. This is
what happens in the first rule (the one that uses ‘NUM’).
The formatting shown here is the recommended convention, but Bison
does not require it. You can add or change white space as much as you
wish. For example, this:
exp: NUM | exp exp '+' {$$ = $1 + $2; } | ... ;
means the same thing as this:
exp:
NUM
| exp exp '+' { $$ = $1 + $2; }
| ...
;
The latter, however, is much more readable.
File: bison.info, Node: Rpcalc Lexer, Next: Rpcalc Main, Prev: Rpcalc Rules, Up: RPN Calc
2.1.3 The ‘rpcalc’ Lexical Analyzer
-----------------------------------
The lexical analyzer’s job is low-level parsing: converting characters
or sequences of characters into tokens. The Bison parser gets its
tokens by calling the lexical analyzer. *Note The Lexical Analyzer
Function ‘yylex’: Lexical.
Only a simple lexical analyzer is needed for the RPN calculator.
This lexical analyzer skips blanks and tabs, then reads in numbers as
‘double’ and returns them as ‘NUM’ tokens. Any other character that
isn’t part of a number is a separate token. Note that the token-code
for such a single-character token is the character itself.
The return value of the lexical analyzer function is a numeric code
which represents a token type. The same text used in Bison rules to
stand for this token type is also a C expression for the numeric code
for the type. This works in two ways. If the token type is a character
literal, then its numeric code is that of the character; you can use the
same character literal in the lexical analyzer to express the number.
If the token type is an identifier, that identifier is defined by Bison
as a C macro whose definition is the appropriate number. In this
example, therefore, ‘NUM’ becomes a macro for ‘yylex’ to use.
The semantic value of the token (if it has one) is stored into the
global variable ‘yylval’, which is where the Bison parser will look for
it. (The C data type of ‘yylval’ is ‘YYSTYPE’, whose value was defined
at the beginning of the grammar via ‘%define api.value.type {double}’;
*note Declarations for ‘rpcalc’: Rpcalc Declarations.)
A token type code of zero is returned if the end-of-input is
encountered. (Bison recognizes any nonpositive value as indicating
end-of-input.)
Here is the code for the lexical analyzer:
/* The lexical analyzer returns a double floating point
number on the stack and the token NUM, or the numeric code
of the character read if not a number. It skips all blanks
and tabs, and returns 0 for end-of-input. */
#include
int
yylex (void)
{
int c = getchar ();
/* Skip white space. */
while (c == ' ' || c == '\t')
c = getchar ();
/* Process numbers. */
if (c == '.' || isdigit (c))
{
ungetc (c, stdin);
scanf ("%lf", &yylval);
return NUM;
}
/* Return end-of-input. */
else if (c == EOF)
return 0;
/* Return a single char. */
else
return c;
}
File: bison.info, Node: Rpcalc Main, Next: Rpcalc Error, Prev: Rpcalc Lexer, Up: RPN Calc
2.1.4 The Controlling Function
------------------------------
In keeping with the spirit of this example, the controlling function is
kept to the bare minimum. The only requirement is that it call
‘yyparse’ to start the process of parsing.
int
main (void)
{
return yyparse ();
}
File: bison.info, Node: Rpcalc Error, Next: Rpcalc Generate, Prev: Rpcalc Main, Up: RPN Calc
2.1.5 The Error Reporting Routine
---------------------------------
When ‘yyparse’ detects a syntax error, it calls the error reporting
function ‘yyerror’ to print an error message (usually but not always
‘"syntax error"’). It is up to the programmer to supply ‘yyerror’
(*note Parser C-Language Interface: Interface.), so here is the
definition we will use:
#include
/* Called by yyparse on error. */
void
yyerror (char const *s)
{
fprintf (stderr, "%s\n", s);
}
After ‘yyerror’ returns, the Bison parser may recover from the error
and continue parsing if the grammar contains a suitable error rule
(*note Error Recovery::). Otherwise, ‘yyparse’ returns nonzero. We
have not written any error rules in this example, so any invalid input
will cause the calculator program to exit. This is not clean behavior
for a real calculator, but it is adequate for the first example.
File: bison.info, Node: Rpcalc Generate, Next: Rpcalc Compile, Prev: Rpcalc Error, Up: RPN Calc
2.1.6 Running Bison to Make the Parser
--------------------------------------
Before running Bison to produce a parser, we need to decide how to
arrange all the source code in one or more source files. For such a
simple example, the easiest thing is to put everything in one file, the
grammar file. The definitions of ‘yylex’, ‘yyerror’ and ‘main’ go at
the end, in the epilogue of the grammar file (*note The Overall Layout
of a Bison Grammar: Grammar Layout.).
For a large project, you would probably have several source files,
and use ‘make’ to arrange to recompile them.
With all the source in the grammar file, you use the following
command to convert it into a parser implementation file:
$ bison FILE.y
In this example, the grammar file is called ‘rpcalc.y’ (for “Reverse
Polish CALCulator”). Bison produces a parser implementation file named
‘FILE.tab.c’, removing the ‘.y’ from the grammar file name. The parser
implementation file contains the source code for ‘yyparse’. The
additional functions in the grammar file (‘yylex’, ‘yyerror’ and ‘main’)
are copied verbatim to the parser implementation file.
File: bison.info, Node: Rpcalc Compile, Prev: Rpcalc Generate, Up: RPN Calc
2.1.7 Compiling the Parser Implementation File
----------------------------------------------
Here is how to compile and run the parser implementation file:
# List files in current directory.
$ ls
rpcalc.tab.c rpcalc.y
# Compile the Bison parser.
# ‘-lm’ tells compiler to search math library for ‘pow’.
$ cc -lm -o rpcalc rpcalc.tab.c
# List files again.
$ ls
rpcalc rpcalc.tab.c rpcalc.y
The file ‘rpcalc’ now contains the executable code. Here is an
example session using ‘rpcalc’.
$ rpcalc
4 9 +
⇒ 13
3 7 + 3 4 5 *+-
⇒ -13
3 7 + 3 4 5 * + - n Note the unary minus, ‘n’
⇒ 13
5 6 / 4 n +
⇒ -3.166666667
3 4 ^ Exponentiation
⇒ 81
^D End-of-file indicator
$
File: bison.info, Node: Infix Calc, Next: Simple Error Recovery, Prev: RPN Calc, Up: Examples
2.2 Infix Notation Calculator: ‘calc’
=====================================
We now modify rpcalc to handle infix operators instead of postfix.
Infix notation involves the concept of operator precedence and the need
for parentheses nested to arbitrary depth. Here is the Bison code for
‘calc.y’, an infix desk-top calculator.
/* Infix notation calculator. */
%{
#include
#include
int yylex (void);
void yyerror (char const *);
%}
/* Bison declarations. */
%define api.value.type {double}
%token NUM
%left '-' '+'
%left '*' '/'
%precedence NEG /* negation--unary minus */
%right '^' /* exponentiation */
%% /* The grammar follows. */
input:
%empty
| input line
;
line:
'\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
;
exp:
NUM
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp { $$ = $1 / $3; }
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
;
%%
The functions ‘yylex’, ‘yyerror’ and ‘main’ can be the same as before.
There are two important new features shown in this code.
In the second section (Bison declarations), ‘%left’ declares token
types and says they are left-associative operators. The declarations
‘%left’ and ‘%right’ (right associativity) take the place of ‘%token’
which is used to declare a token type name without
associativity/precedence. (These tokens are single-character literals,
which ordinarily don’t need to be declared. We declare them here to
specify the associativity/precedence.)
Operator precedence is determined by the line ordering of the
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence. Hence, exponentiation
has the highest precedence, unary minus (‘NEG’) is next, followed by ‘*’
and ‘/’, and so on. Unary minus is not associative, only precedence
matters (‘%precedence’. *Note Operator Precedence: Precedence.
The other important new feature is the ‘%prec’ in the grammar section
for the unary minus operator. The ‘%prec’ simply instructs Bison that
the rule ‘| '-' exp’ has the same precedence as ‘NEG’—in this case the
next-to-highest. *Note Context-Dependent Precedence: Contextual
Precedence.
Here is a sample run of ‘calc.y’:
$ calc
4 + 4.5 - (34/(8*3+-3))
6.880952381
-56 + 2
-54
3 ^ 2
9
File: bison.info, Node: Simple Error Recovery, Next: Location Tracking Calc, Prev: Infix Calc, Up: Examples
2.3 Simple Error Recovery
=========================
Up to this point, this manual has not addressed the issue of “error
recovery”—how to continue parsing after the parser detects a syntax
error. All we have handled is error reporting with ‘yyerror’. Recall
that by default ‘yyparse’ returns after calling ‘yyerror’. This means
that an erroneous input line causes the calculator program to exit. Now
we show how to rectify this deficiency.
The Bison language itself includes the reserved word ‘error’, which
may be included in the grammar rules. In the example below it has been
added to one of the alternatives for ‘line’:
line:
'\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
| error '\n' { yyerrok; }
;
This addition to the grammar allows for simple error recovery in the
event of a syntax error. If an expression that cannot be evaluated is
read, the error will be recognized by the third rule for ‘line’, and
parsing will continue. (The ‘yyerror’ function is still called upon to
print its message as well.) The action executes the statement
‘yyerrok’, a macro defined automatically by Bison; its meaning is that
error recovery is complete (*note Error Recovery::). Note the
difference between ‘yyerrok’ and ‘yyerror’; neither one is a misprint.
This form of error recovery deals with syntax errors. There are
other kinds of errors; for example, division by zero, which raises an
exception signal that is normally fatal. A real calculator program must
handle this signal and use ‘longjmp’ to return to ‘main’ and resume
parsing input lines; it would also have to discard the rest of the
current line of input. We won’t discuss this issue further because it
is not specific to Bison programs.
File: bison.info, Node: Location Tracking Calc, Next: Multi-function Calc, Prev: Simple Error Recovery, Up: Examples
2.4 Location Tracking Calculator: ‘ltcalc’
==========================================
This example extends the infix notation calculator with location
tracking. This feature will be used to improve the error messages. For
the sake of clarity, this example is a simple integer calculator, since
most of the work needed to use locations will be done in the lexical
analyzer.
* Menu:
* Ltcalc Declarations:: Bison and C declarations for ltcalc.
* Ltcalc Rules:: Grammar rules for ltcalc, with explanations.
* Ltcalc Lexer:: The lexical analyzer.
File: bison.info, Node: Ltcalc Declarations, Next: Ltcalc Rules, Up: Location Tracking Calc
2.4.1 Declarations for ‘ltcalc’
-------------------------------
The C and Bison declarations for the location tracking calculator are
the same as the declarations for the infix notation calculator.
/* Location tracking calculator. */
%{
#include
int yylex (void);
void yyerror (char const *);
%}
/* Bison declarations. */
%define api.value.type {int}
%token NUM
%left '-' '+'
%left '*' '/'
%precedence NEG
%right '^'
%% /* The grammar follows. */
Note there are no declarations specific to locations. Defining a data
type for storing locations is not needed: we will use the type provided
by default (*note Data Types of Locations: Location Type.), which is a
four member structure with the following integer fields: ‘first_line’,
‘first_column’, ‘last_line’ and ‘last_column’. By conventions, and in
accordance with the GNU Coding Standards and common practice, the line
and column count both start at 1.
File: bison.info, Node: Ltcalc Rules, Next: Ltcalc Lexer, Prev: Ltcalc Declarations, Up: Location Tracking Calc
2.4.2 Grammar Rules for ‘ltcalc’
--------------------------------
Whether handling locations or not has no effect on the syntax of your
language. Therefore, grammar rules for this example will be very close
to those of the previous example: we will only modify them to benefit
from the new information.
Here, we will use locations to report divisions by zero, and locate
the wrong expressions or subexpressions.
input:
%empty
| input line
;
line:
'\n'
| exp '\n' { printf ("%d\n", $1); }
;
exp:
NUM
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp
{
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
fprintf (stderr, "%d.%d-%d.%d: division by zero",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
This code shows how to reach locations inside of semantic actions, by
using the pseudo-variables ‘@N’ for rule components, and the
pseudo-variable ‘@$’ for groupings.
We don’t need to assign a value to ‘@$’: the output parser does it
automatically. By default, before executing the C code of each action,
‘@$’ is set to range from the beginning of ‘@1’ to the end of ‘@N’, for
a rule with N components. This behavior can be redefined (*note Default
Action for Locations: Location Default Action.), and for very specific
rules, ‘@$’ can be computed by hand.
File: bison.info, Node: Ltcalc Lexer, Prev: Ltcalc Rules, Up: Location Tracking Calc
2.4.3 The ‘ltcalc’ Lexical Analyzer.
------------------------------------
Until now, we relied on Bison’s defaults to enable location tracking.
The next step is to rewrite the lexical analyzer, and make it able to
feed the parser with the token locations, as it already does for
semantic values.
To this end, we must take into account every single character of the
input text, to avoid the computed locations of being fuzzy or wrong:
int
yylex (void)
{
int c;
/* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
++yylloc.last_column;
/* Step. */
yylloc.first_line = yylloc.last_line;
yylloc.first_column = yylloc.last_column;
/* Process numbers. */
if (isdigit (c))
{
yylval = c - '0';
++yylloc.last_column;
while (isdigit (c = getchar ()))
{
++yylloc.last_column;
yylval = yylval * 10 + c - '0';
}
ungetc (c, stdin);
return NUM;
}
/* Return end-of-input. */
if (c == EOF)
return 0;
/* Return a single char, and update location. */
if (c == '\n')
{
++yylloc.last_line;
yylloc.last_column = 0;
}
else
++yylloc.last_column;
return c;
}
Basically, the lexical analyzer performs the same processing as
before: it skips blanks and tabs, and reads numbers or single-character
tokens. In addition, it updates ‘yylloc’, the global variable (of type
‘YYLTYPE’) containing the token’s location.
Now, each time this function returns a token, the parser has its
number as well as its semantic value, and its location in the text. The
last needed change is to initialize ‘yylloc’, for example in the
controlling function:
int
main (void)
{
yylloc.first_line = yylloc.last_line = 1;
yylloc.first_column = yylloc.last_column = 0;
return yyparse ();
}
Remember that computing locations is not a matter of syntax. Every
character must be associated to a location update, whether it is in
valid input, in comments, in literal strings, and so on.
File: bison.info, Node: Multi-function Calc, Next: Exercises, Prev: Location Tracking Calc, Up: Examples
2.5 Multi-Function Calculator: ‘mfcalc’
=======================================
Now that the basics of Bison have been discussed, it is time to move on
to a more advanced problem. The above calculators provided only five
functions, ‘+’, ‘-’, ‘*’, ‘/’ and ‘^’. It would be nice to have a
calculator that provides other mathematical functions such as ‘sin’,
‘cos’, etc.
It is easy to add new operators to the infix calculator as long as
they are only single-character literals. The lexical analyzer ‘yylex’
passes back all nonnumeric characters as tokens, so new grammar rules
suffice for adding a new operator. But we want something more flexible:
built-in functions whose syntax has this form:
FUNCTION_NAME (ARGUMENT)
At the same time, we will add memory to the calculator, by allowing you
to create named variables, store values in them, and use them later.
Here is a sample session with the multi-function calculator:
$ mfcalc
pi = 3.141592653589
⇒ 3.1415926536
sin(pi)
⇒ 0.0000000000
alpha = beta1 = 2.3
⇒ 2.3000000000
alpha
⇒ 2.3000000000
ln(alpha)
⇒ 0.8329091229
exp(ln(beta1))
⇒ 2.3000000000
$
Note that multiple assignment and nested function calls are
permitted.
* Menu:
* Mfcalc Declarations:: Bison declarations for multi-function calculator.
* Mfcalc Rules:: Grammar rules for the calculator.
* Mfcalc Symbol Table:: Symbol table management subroutines.
* Mfcalc Lexer:: The lexical analyzer.
* Mfcalc Main:: The controlling function.
File: bison.info, Node: Mfcalc Declarations, Next: Mfcalc Rules, Up: Multi-function Calc
2.5.1 Declarations for ‘mfcalc’
-------------------------------
Here are the C and Bison declarations for the multi-function calculator.
%{
#include /* For printf, etc. */
#include /* For pow, used in the grammar. */
#include "calc.h" /* Contains definition of 'symrec'. */
int yylex (void);
void yyerror (char const *);
%}
%define api.value.type union /* Generate YYSTYPE from these types: */
%token NUM /* Double precision number. */
%token VAR FUN /* Symbol table pointer: variable/function. */
%type exp
%precedence '='
%left '-' '+'
%left '*' '/'
%precedence NEG /* negation--unary minus */
%right '^' /* exponentiation */
The above grammar introduces only two new features of the Bison
language. These features allow semantic values to have various data
types (*note More Than One Value Type: Multiple Types.).
The special ‘union’ value assigned to the ‘%define’ variable
‘api.value.type’ specifies that the symbols are defined with their data
types. Bison will generate an appropriate definition of ‘YYSTYPE’ to
store these values.
Since values can now have various types, it is necessary to associate
a type with each grammar symbol whose semantic value is used. These
symbols are ‘NUM’, ‘VAR’, ‘FUN’, and ‘exp’. Their declarations are
augmented with their data type (placed between angle brackets). For
instance, values of ‘NUM’ are stored in ‘double’.
The Bison construct ‘%type’ is used for declaring nonterminal
symbols, just as ‘%token’ is used for declaring token types. Previously
we did not use ‘%type’ before because nonterminal symbols are normally
declared implicitly by the rules that define them. But ‘exp’ must be
declared explicitly so we can specify its value type. *Note Nonterminal
Symbols: Type Decl.
File: bison.info, Node: Mfcalc Rules, Next: Mfcalc Symbol Table, Prev: Mfcalc Declarations, Up: Multi-function Calc
2.5.2 Grammar Rules for ‘mfcalc’
--------------------------------
Here are the grammar rules for the multi-function calculator. Most of
them are copied directly from ‘calc’; three rules, those which mention
‘VAR’ or ‘FUN’, are new.
%% /* The grammar follows. */
input:
%empty
| input line
;
line:
'\n'
| exp '\n' { printf ("%.10g\n", $1); }
| error '\n' { yyerrok; }
;
exp:
NUM
| VAR { $$ = $1->value.var; }
| VAR '=' exp { $$ = $3; $1->value.var = $3; }
| FUN '(' exp ')' { $$ = $1->value.fun ($3); }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp { $$ = $1 / $3; }
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
;
/* End of grammar. */
%%
File: bison.info, Node: Mfcalc Symbol Table, Next: Mfcalc Lexer, Prev: Mfcalc Rules, Up: Multi-function Calc
2.5.3 The ‘mfcalc’ Symbol Table
-------------------------------
The multi-function calculator requires a symbol table to keep track of
the names and meanings of variables and functions. This doesn’t affect
the grammar rules (except for the actions) or the Bison declarations,
but it requires some additional C functions for support.
The symbol table itself consists of a linked list of records. Its
definition, which is kept in the header ‘calc.h’, is as follows. It
provides for either functions or variables to be placed in the table.
/* Function type. */
typedef double (func_t) (double);
/* Data type for links in the chain of symbols. */
struct symrec
{
char *name; /* name of symbol */
int type; /* type of symbol: either VAR or FUN */
union
{
double var; /* value of a VAR */
func_t *fun; /* value of a FUN */
} value;
struct symrec *next; /* link field */
};
typedef struct symrec symrec;
/* The symbol table: a chain of 'struct symrec'. */
extern symrec *sym_table;
symrec *putsym (char const *name, int sym_type);
symrec *getsym (char const *name);
The new version of ‘main’ will call ‘init_table’ to initialize the
symbol table:
struct init
{
char const *name;
func_t *fun;
};
struct init const arith_funs[] =
{
{ "atan", atan },
{ "cos", cos },
{ "exp", exp },
{ "ln", log },
{ "sin", sin },
{ "sqrt", sqrt },
{ 0, 0 },
};
/* The symbol table: a chain of 'struct symrec'. */
symrec *sym_table;
/* Put arithmetic functions in table. */
static void
init_table (void)
{
for (int i = 0; arith_funs[i].name; i++)
{
symrec *ptr = putsym (arith_funs[i].name, FUN);
ptr->value.fun = arith_funs[i].fun;
}
}
By simply editing the initialization list and adding the necessary
include files, you can add additional functions to the calculator.
Two important functions allow look-up and installation of symbols in
the symbol table. The function ‘putsym’ is passed a name and the type
(‘VAR’ or ‘FUN’) of the object to be installed. The object is linked to
the front of the list, and a pointer to the object is returned. The
function ‘getsym’ is passed the name of the symbol to look up. If
found, a pointer to that symbol is returned; otherwise zero is returned.
#include /* malloc. */
#include /* strlen. */
symrec *
putsym (char const *name, int sym_type)
{
symrec *res = (symrec *) malloc (sizeof (symrec));
res->name = strdup (name);
res->type = sym_type;
res->value.var = 0; /* Set value to 0 even if fun. */
res->next = sym_table;
sym_table = res;
return res;
}
symrec *
getsym (char const *name)
{
for (symrec *p = sym_table; p; p = p->next)
if (strcmp (p->name, name) == 0)
return p;
return NULL;
}
File: bison.info, Node: Mfcalc Lexer, Next: Mfcalc Main, Prev: Mfcalc Symbol Table, Up: Multi-function Calc
2.5.4 The ‘mfcalc’ Lexer
------------------------
The function ‘yylex’ must now recognize variables, numeric values, and
the single-character arithmetic operators. Strings of alphanumeric
characters with a leading letter are recognized as either variables or
functions depending on what the symbol table says about them.
The string is passed to ‘getsym’ for look up in the symbol table. If
the name appears in the table, a pointer to its location and its type
(‘VAR’ or ‘FUN’) is returned to ‘yyparse’. If it is not already in the
table, then it is installed as a ‘VAR’ using ‘putsym’. Again, a pointer
and its type (which must be ‘VAR’) is returned to ‘yyparse’.
No change is needed in the handling of numeric values and arithmetic
operators in ‘yylex’.
#include
int
yylex (void)
{
int c;
/* Ignore white space, get first nonwhite character. */
while ((c = getchar ()) == ' ' || c == '\t')
continue;
if (c == EOF)
return 0;
/* Char starts a number => parse the number. */
if (c == '.' || isdigit (c))
{
ungetc (c, stdin);
scanf ("%lf", &yylval.NUM);
return NUM;
}
Bison generated a definition of ‘YYSTYPE’ with a member named ‘NUM’ to
store value of ‘NUM’ symbols.
/* Char starts an identifier => read the name. */
if (isalpha (c))
{
/* Initially make the buffer long enough
for a 40-character symbol name. */
static size_t length = 40;
static char *symbuf = 0;
if (!symbuf)
symbuf = malloc (length + 1);
int i = 0;
do
{
/* If buffer is full, make it bigger. */
if (i == length)
{
length *= 2;
symbuf = realloc (symbuf, length + 1);
}
/* Add this character to the buffer. */
symbuf[i++] = c;
/* Get another character. */
c = getchar ();
}
while (isalnum (c));
ungetc (c, stdin);
symbuf[i] = '\0';
symrec *s = getsym (symbuf);
if (!s)
s = putsym (symbuf, VAR);
yylval.VAR = s; /* or yylval.FUN = s. */
return s->type;
}
/* Any other character is a token by itself. */
return c;
}
File: bison.info, Node: Mfcalc Main, Prev: Mfcalc Lexer, Up: Multi-function Calc
2.5.5 The ‘mfcalc’ Main
-----------------------
The error reporting function is unchanged, and the new version of ‘main’
includes a call to ‘init_table’ and sets the ‘yydebug’ on user demand
(*Note Tracing Your Parser: Tracing, for details):
/* Called by yyparse on error. */
void yyerror (char const *s)
{
fprintf (stderr, "%s\n", s);
}
int main (int argc, char const* argv[])
{
/* Enable parse traces on option -p. */
if (argc == 2 && strcmp(argv[1], "-p") == 0)
yydebug = 1;
init_table ();
return yyparse ();
}
This program is both powerful and flexible. You may easily add new
functions, and it is a simple job to modify this code to install
predefined variables such as ‘pi’ or ‘e’ as well.
File: bison.info, Node: Exercises, Prev: Multi-function Calc, Up: Examples
2.6 Exercises
=============
1. Add some new functions from ‘math.h’ to the initialization list.
2. Add another array that contains constants and their values. Then
modify ‘init_table’ to add these constants to the symbol table. It
will be easiest to give the constants type ‘VAR’.
3. Make the program report an error if the user refers to an
uninitialized variable in any way except to store a value in it.
File: bison.info, Node: Grammar File, Next: Interface, Prev: Examples, Up: Top
3 Bison Grammar Files
*********************
Bison takes as input a context-free grammar specification and produces a
C-language function that recognizes correct instances of the grammar.
The Bison grammar file conventionally has a name ending in ‘.y’.
*Note Invoking Bison: Invocation.
* Menu:
* Grammar Outline:: Overall layout of the grammar file.
* Symbols:: Terminal and nonterminal symbols.
* Rules:: How to write grammar rules.
* Semantics:: Semantic values and actions.
* Tracking Locations:: Locations and actions.
* Named References:: Using named references in actions.
* Declarations:: All kinds of Bison declarations are described here.
* Multiple Parsers:: Putting more than one Bison parser in one program.
File: bison.info, Node: Grammar Outline, Next: Symbols, Up: Grammar File
3.1 Outline of a Bison Grammar
==============================
A Bison grammar file has four main sections, shown here with the
appropriate delimiters:
%{
PROLOGUE
%}
BISON DECLARATIONS
%%
GRAMMAR RULES
%%
EPILOGUE
Comments enclosed in ‘/* ... */’ may appear in any of the sections.
As a GNU extension, ‘//’ introduces a comment that continues until end
of line.
* Menu:
* Prologue:: Syntax and usage of the prologue.
* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations:: Syntax and usage of the Bison declarations section.
* Grammar Rules:: Syntax and usage of the grammar rules section.
* Epilogue:: Syntax and usage of the epilogue.
File: bison.info, Node: Prologue, Next: Prologue Alternatives, Up: Grammar Outline
3.1.1 The prologue
------------------
The PROLOGUE section contains macro definitions and declarations of
functions and variables that are used in the actions in the grammar
rules. These are copied to the beginning of the parser implementation
file so that they precede the definition of ‘yyparse’. You can use
‘#include’ to get the declarations from a header file. If you don’t
need any C declarations, you may omit the ‘%{’ and ‘%}’ delimiters that
bracket this section.
The PROLOGUE section is terminated by the first occurrence of ‘%}’
that is outside a comment, a string literal, or a character constant.
You may have more than one PROLOGUE section, intermixed with the
BISON DECLARATIONS. This allows you to have C and Bison declarations
that refer to each other. For example, the ‘%union’ declaration may use
types defined in a header file, and you may wish to prototype functions
that take arguments of type ‘YYSTYPE’. This can be done with two
PROLOGUE blocks, one before and one after the ‘%union’ declaration.
%{
#define _GNU_SOURCE
#include
#include "ptypes.h"
%}
%union {
long n;
tree t; /* ‘tree’ is defined in ‘ptypes.h’. */
}
%{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
%}
...
When in doubt, it is usually safer to put prologue code before all
Bison declarations, rather than after. For example, any definitions of
feature test macros like ‘_GNU_SOURCE’ or ‘_POSIX_C_SOURCE’ should
appear before all Bison declarations, as feature test macros can affect
the behavior of Bison-generated ‘#include’ directives.
File: bison.info, Node: Prologue Alternatives, Next: Bison Declarations, Prev: Prologue, Up: Grammar Outline
3.1.2 Prologue Alternatives
---------------------------
The functionality of PROLOGUE sections can often be subtle and
inflexible. As an alternative, Bison provides a ‘%code’ directive with
an explicit qualifier field, which identifies the purpose of the code
and thus the location(s) where Bison should generate it. For C/C++, the
qualifier can be omitted for the default location, or it can be one of
‘requires’, ‘provides’, ‘top’. *Note %code Summary::.
Look again at the example of the previous section:
%{
#define _GNU_SOURCE
#include
#include "ptypes.h"
%}
%union {
long n;
tree t; /* ‘tree’ is defined in ‘ptypes.h’. */
}
%{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
%}
...
Notice that there are two PROLOGUE sections here, but there’s a subtle
distinction between their functionality. For example, if you decide to
override Bison’s default definition for ‘YYLTYPE’, in which PROLOGUE
section should you write your new definition? You should write it in
the first since Bison will insert that code into the parser
implementation file _before_ the default ‘YYLTYPE’ definition. In which
PROLOGUE section should you prototype an internal function,
‘trace_token’, that accepts ‘YYLTYPE’ and ‘yytokentype’ as arguments?
You should prototype it in the second since Bison will insert that code
_after_ the ‘YYLTYPE’ and ‘yytokentype’ definitions.
This distinction in functionality between the two PROLOGUE sections
is established by the appearance of the ‘%union’ between them. This
behavior raises a few questions. First, why should the position of a
‘%union’ affect definitions related to ‘YYLTYPE’ and ‘yytokentype’?
Second, what if there is no ‘%union’? In that case, the second kind of
PROLOGUE section is not available. This behavior is not intuitive.
To avoid this subtle ‘%union’ dependency, rewrite the example using a
‘%code top’ and an unqualified ‘%code’. Let’s go ahead and add the new
‘YYLTYPE’ definition and the ‘trace_token’ prototype at the same time:
%code top {
#define _GNU_SOURCE
#include
/* WARNING: The following code really belongs
* in a '%code requires'; see below. */
#include "ptypes.h"
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
{
int first_line;
int first_column;
int last_line;
int last_column;
char *filename;
} YYLTYPE;
}
%union {
long n;
tree t; /* ‘tree’ is defined in ‘ptypes.h’. */
}
%code {
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
static void trace_token (enum yytokentype token, YYLTYPE loc);
}
...
In this way, ‘%code top’ and the unqualified ‘%code’ achieve the same
functionality as the two kinds of PROLOGUE sections, but it’s always
explicit which kind you intend. Moreover, both kinds are always
available even in the absence of ‘%union’.
The ‘%code top’ block above logically contains two parts. The first
two lines before the warning need to appear near the top of the parser
implementation file. The first line after the warning is required by
‘YYSTYPE’ and thus also needs to appear in the parser implementation
file. However, if you’ve instructed Bison to generate a parser header
file (*note %defines: Decl Summary.), you probably want that line to
appear before the ‘YYSTYPE’ definition in that header file as well. The
‘YYLTYPE’ definition should also appear in the parser header file to
override the default ‘YYLTYPE’ definition there.
In other words, in the ‘%code top’ block above, all but the first two
lines are dependency code required by the ‘YYSTYPE’ and ‘YYLTYPE’
definitions. Thus, they belong in one or more ‘%code requires’:
%code top {
#define _GNU_SOURCE
#include
}
%code requires {
#include "ptypes.h"
}
%union {
long n;
tree t; /* ‘tree’ is defined in ‘ptypes.h’. */
}
%code requires {
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
{
int first_line;
int first_column;
int last_line;
int last_column;
char *filename;
} YYLTYPE;
}
%code {
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
static void trace_token (enum yytokentype token, YYLTYPE loc);
}
...
Now Bison will insert ‘#include "ptypes.h"’ and the new ‘YYLTYPE’
definition before the Bison-generated ‘YYSTYPE’ and ‘YYLTYPE’
definitions in both the parser implementation file and the parser header
file. (By the same reasoning, ‘%code requires’ would also be the
appropriate place to write your own definition for ‘YYSTYPE’.)
When you are writing dependency code for ‘YYSTYPE’ and ‘YYLTYPE’, you
should prefer ‘%code requires’ over ‘%code top’ regardless of whether
you instruct Bison to generate a parser header file. When you are
writing code that you need Bison to insert only into the parser
implementation file and that has no special need to appear at the top of
that file, you should prefer the unqualified ‘%code’ over ‘%code top’.
These practices will make the purpose of each block of your code
explicit to Bison and to other developers reading your grammar file.
Following these practices, we expect the unqualified ‘%code’ and ‘%code
requires’ to be the most important of the four PROLOGUE alternatives.
At some point while developing your parser, you might decide to
provide ‘trace_token’ to modules that are external to your parser.
Thus, you might wish for Bison to insert the prototype into both the
parser header file and the parser implementation file. Since this
function is not a dependency required by ‘YYSTYPE’ or ‘YYLTYPE’, it
doesn’t make sense to move its prototype to a ‘%code requires’. More
importantly, since it depends upon ‘YYLTYPE’ and ‘yytokentype’, ‘%code
requires’ is not sufficient. Instead, move its prototype from the
unqualified ‘%code’ to a ‘%code provides’:
%code top {
#define _GNU_SOURCE
#include
}
%code requires {
#include "ptypes.h"
}
%union {
long n;
tree t; /* ‘tree’ is defined in ‘ptypes.h’. */
}
%code requires {
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
{
int first_line;
int first_column;
int last_line;
int last_column;
char *filename;
} YYLTYPE;
}
%code provides {
void trace_token (enum yytokentype token, YYLTYPE loc);
}
%code {
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
}
...
Bison will insert the ‘trace_token’ prototype into both the parser
header file and the parser implementation file after the definitions for
‘yytokentype’, ‘YYLTYPE’, and ‘YYSTYPE’.
The above examples are careful to write directives in an order that
reflects the layout of the generated parser implementation and header
files: ‘%code top’, ‘%code requires’, ‘%code provides’, and then
‘%code’. While your grammar files may generally be easier to read if
you also follow this order, Bison does not require it. Instead, Bison
lets you choose an organization that makes sense to you.
You may declare any of these directives multiple times in the grammar
file. In that case, Bison concatenates the contained code in
declaration order. This is the only way in which the position of one of
these directives within the grammar file affects its functionality.
The result of the previous two properties is greater flexibility in
how you may organize your grammar file. For example, you may organize
semantic-type-related directives by semantic type:
%code requires { #include "type1.h" }
%union { type1 field1; }
%destructor { type1_free ($$); }
%printer { type1_print (yyo, $$); }
%code requires { #include "type2.h" }
%union { type2 field2; }
%destructor { type2_free ($$); }
%printer { type2_print (yyo, $$); }
You could even place each of the above directive groups in the rules
section of the grammar file next to the set of rules that uses the
associated semantic type. (In the rules section, you must terminate
each of those directives with a semicolon.) And you don’t have to worry
that some directive (like a ‘%union’) in the definitions section is
going to adversely affect their functionality in some counter-intuitive
manner just because it comes first. Such an organization is not
possible using PROLOGUE sections.
This section has been concerned with explaining the advantages of the
four PROLOGUE alternatives over the original Yacc PROLOGUE. However, in
most cases when using these directives, you shouldn’t need to think
about all the low-level ordering issues discussed here. Instead, you
should simply use these directives to label each block of your code
according to its purpose and let Bison handle the ordering. ‘%code’ is
the most generic label. Move code to ‘%code requires’, ‘%code
provides’, or ‘%code top’ as needed.
File: bison.info, Node: Bison Declarations, Next: Grammar Rules, Prev: Prologue Alternatives, Up: Grammar Outline
3.1.3 The Bison Declarations Section
------------------------------------
The BISON DECLARATIONS section contains declarations that define
terminal and nonterminal symbols, specify precedence, and so on. In
some simple grammars you may not need any declarations. *Note Bison
Declarations: Declarations.
File: bison.info, Node: Grammar Rules, Next: Epilogue, Prev: Bison Declarations, Up: Grammar Outline
3.1.4 The Grammar Rules Section
-------------------------------
The “grammar rules” section contains one or more Bison grammar rules,
and nothing else. *Note Syntax of Grammar Rules: Rules.
There must always be at least one grammar rule, and the first ‘%%’
(which precedes the grammar rules) may never be omitted even if it is
the first thing in the file.
File: bison.info, Node: Epilogue, Prev: Grammar Rules, Up: Grammar Outline
3.1.5 The epilogue
------------------
The EPILOGUE is copied verbatim to the end of the parser implementation
file, just as the PROLOGUE is copied to the beginning. This is the most
convenient place to put anything that you want to have in the parser
implementation file but which need not come before the definition of
‘yyparse’. For example, the definitions of ‘yylex’ and ‘yyerror’ often
go here. Because C requires functions to be declared before being used,
you often need to declare functions like ‘yylex’ and ‘yyerror’ in the
Prologue, even if you define them in the Epilogue. *Note Parser
C-Language Interface: Interface.
If the last section is empty, you may omit the ‘%%’ that separates it
from the grammar rules.
The Bison parser itself contains many macros and identifiers whose
names start with ‘yy’ or ‘YY’, so it is a good idea to avoid using any
such names (except those documented in this manual) in the epilogue of
the grammar file.
File: bison.info, Node: Symbols, Next: Rules, Prev: Grammar Outline, Up: Grammar File
3.2 Symbols, Terminal and Nonterminal
=====================================
“Symbols” in Bison grammars represent the grammatical classifications of
the language.
A “terminal symbol” (also known as a “token type”) represents a class
of syntactically equivalent tokens. You use the symbol in grammar rules
to mean that a token in that class is allowed. The symbol is
represented in the Bison parser by a numeric code, and the ‘yylex’
function returns a token type code to indicate what kind of token has
been read. You don’t need to know what the code value is; you can use
the symbol to stand for it.
A “nonterminal symbol” stands for a class of syntactically equivalent
groupings. The symbol name is used in writing grammar rules. By
convention, it should be all lower case.
Symbol names can contain letters, underscores, periods, and
non-initial digits and dashes. Dashes in symbol names are a GNU
extension, incompatible with POSIX Yacc. Periods and dashes make symbol
names less convenient to use with named references, which require
brackets around such names (*note Named References::). Terminal symbols
that contain periods or dashes make little sense: since they are not
valid symbols (in most programming languages) they are not exported as
token names.
There are three ways of writing terminal symbols in the grammar:
• A “named token type” is written with an identifier, like an
identifier in C. By convention, it should be all upper case. Each
such name must be defined with a Bison declaration such as
‘%token’. *Note Token Type Names: Token Decl.
• A “character token type” (or “literal character token”) is written
in the grammar using the same syntax used in C for character
constants; for example, ‘'+'’ is a character token type. A
character token type doesn’t need to be declared unless you need to
specify its semantic value data type (*note Data Types of Semantic
Values: Value Type.), associativity, or precedence (*note Operator
Precedence: Precedence.).
By convention, a character token type is used only to represent a
token that consists of that particular character. Thus, the token
type ‘'+'’ is used to represent the character ‘+’ as a token.
Nothing enforces this convention, but if you depart from it, your
program will confuse other readers.
All the usual escape sequences used in character literals in C can
be used in Bison as well, but you must not use the null character
as a character literal because its numeric code, zero, signifies
end-of-input (*note Calling Convention for ‘yylex’: Calling
Convention.). Also, unlike standard C, trigraphs have no special
meaning in Bison character literals, nor is backslash-newline
allowed.
• A “literal string token” is written like a C string constant; for
example, ‘"<="’ is a literal string token. A literal string token
doesn’t need to be declared unless you need to specify its semantic
value data type (*note Value Type::), associativity, or precedence
(*note Precedence::).
You can associate the literal string token with a symbolic name as
an alias, using the ‘%token’ declaration (*note Token Declarations:
Token Decl.). If you don’t do that, the lexical analyzer has to
retrieve the token number for the literal string token from the
‘yytname’ table (*note Calling Convention::).
*Warning*: literal string tokens do not work in Yacc.
By convention, a literal string token is used only to represent a
token that consists of that particular string. Thus, you should
use the token type ‘"<="’ to represent the string ‘<=’ as a token.
Bison does not enforce this convention, but if you depart from it,
people who read your program will be confused.
All the escape sequences used in string literals in C can be used
in Bison as well, except that you must not use a null character
within a string literal. Also, unlike Standard C, trigraphs have
no special meaning in Bison string literals, nor is
backslash-newline allowed. A literal string token must contain two
or more characters; for a token containing just one character, use
a character token (see above).
How you choose to write a terminal symbol has no effect on its
grammatical meaning. That depends only on where it appears in rules and
on when the parser function returns that symbol.
The value returned by ‘yylex’ is always one of the terminal symbols,
except that a zero or negative value signifies end-of-input. Whichever
way you write the token type in the grammar rules, you write it the same
way in the definition of ‘yylex’. The numeric code for a character
token type is simply the positive numeric code of the character, so
‘yylex’ can use the identical value to generate the requisite code,
though you may need to convert it to ‘unsigned char’ to avoid
sign-extension on hosts where ‘char’ is signed. Each named token type
becomes a C macro in the parser implementation file, so ‘yylex’ can use
the name to stand for the code. (This is why periods don’t make sense
in terminal symbols.) *Note Calling Convention for ‘yylex’: Calling
Convention.
If ‘yylex’ is defined in a separate file, you need to arrange for the
token-type macro definitions to be available there. Use the ‘-d’ option
when you run Bison, so that it will write these macro definitions into a
separate header file ‘NAME.tab.h’ which you can include in the other
source files that need it. *Note Invoking Bison: Invocation.
If you want to write a grammar that is portable to any Standard C
host, you must use only nonnull character tokens taken from the basic
execution character set of Standard C. This set consists of the ten
digits, the 52 lower- and upper-case English letters, and the characters
in the following C-language string:
"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_{|}~"
The ‘yylex’ function and Bison must use a consistent character set
and encoding for character tokens. For example, if you run Bison in an
ASCII environment, but then compile and run the resulting program in an
environment that uses an incompatible character set like EBCDIC, the
resulting program may not work because the tables generated by Bison
will assume ASCII numeric values for character tokens. It is standard
practice for software distributions to contain C source files that were
generated by Bison in an ASCII environment, so installers on platforms
that are incompatible with ASCII must rebuild those files before
compiling them.
The symbol ‘error’ is a terminal symbol reserved for error recovery
(*note Error Recovery::); you shouldn’t use it for any other purpose.
In particular, ‘yylex’ should never return this value. The default
value of the error token is 256, unless you explicitly assigned 256 to
one of your tokens with a ‘%token’ declaration.
File: bison.info, Node: Rules, Next: Semantics, Prev: Symbols, Up: Grammar File
3.3 Grammar Rules
=================
A Bison grammar is a list of rules.
* Menu:
* Rules Syntax:: Syntax of the rules.
* Empty Rules:: Symbols that can match the empty string.
* Recursion:: Writing recursive rules.
File: bison.info, Node: Rules Syntax, Next: Empty Rules, Up: Rules
3.3.1 Syntax of Grammar Rules
-----------------------------
A Bison grammar rule has the following general form:
RESULT: COMPONENTS...;
where RESULT is the nonterminal symbol that this rule describes, and
COMPONENTS are various terminal and nonterminal symbols that are put
together by this rule (*note Symbols::).
For example,
exp: exp '+' exp;
says that two groupings of type ‘exp’, with a ‘+’ token in between, can
be combined into a larger grouping of type ‘exp’.
White space in rules is significant only to separate symbols. You
can add extra white space as you wish.
Scattered among the components can be ACTIONS that determine the
semantics of the rule. An action looks like this:
{C STATEMENTS}
This is an example of “braced code”, that is, C code surrounded by
braces, much like a compound statement in C. Braced code can contain
any sequence of C tokens, so long as its braces are balanced. Bison
does not check the braced code for correctness directly; it merely
copies the code to the parser implementation file, where the C compiler
can check it.
Within braced code, the balanced-brace count is not affected by
braces within comments, string literals, or character constants, but it
is affected by the C digraphs ‘<%’ and ‘%>’ that represent braces. At
the top level braced code must be terminated by ‘}’ and not by a
digraph. Bison does not look for trigraphs, so if braced code uses
trigraphs you should ensure that they do not affect the nesting of
braces or the boundaries of comments, string literals, or character
constants.
Usually there is only one action and it follows the components.
*Note Actions::.
Multiple rules for the same RESULT can be written separately or can
be joined with the vertical-bar character ‘|’ as follows:
RESULT:
RULE1-COMPONENTS...
| RULE2-COMPONENTS...
...
;
They are still considered distinct rules even when joined in this way.
File: bison.info, Node: Empty Rules, Next: Recursion, Prev: Rules Syntax, Up: Rules
3.3.2 Empty Rules
-----------------
A rule is said to be “empty” if its right-hand side (COMPONENTS) is
empty. It means that RESULT can match the empty string. For example,
here is how to define an optional semicolon:
semicolon.opt: | ";";
It is easy not to see an empty rule, especially when ‘|’ is used. The
‘%empty’ directive allows to make explicit that a rule is empty on
purpose:
semicolon.opt:
%empty
| ";"
;
Flagging a non-empty rule with ‘%empty’ is an error. If run with
‘-Wempty-rule’, ‘bison’ will report empty rules without ‘%empty’. Using
‘%empty’ enables this warning, unless ‘-Wno-empty-rule’ was specified.
The ‘%empty’ directive is a Bison extension, it does not work with
Yacc. To remain compatible with POSIX Yacc, it is customary to write a
comment ‘/* empty */’ in each rule with no components:
semicolon.opt:
/* empty */
| ";"
;
File: bison.info, Node: Recursion, Prev: Empty Rules, Up: Rules
3.3.3 Recursive Rules
---------------------
A rule is called “recursive” when its RESULT nonterminal appears also on
its right hand side. Nearly all Bison grammars need to use recursion,
because that is the only way to define a sequence of any number of a
particular thing. Consider this recursive definition of a
comma-separated sequence of one or more expressions:
expseq1:
exp
| expseq1 ',' exp
;
Since the recursive use of ‘expseq1’ is the leftmost symbol in the right
hand side, we call this “left recursion”. By contrast, here the same
construct is defined using “right recursion”:
expseq1:
exp
| exp ',' expseq1
;
Any kind of sequence can be defined using either left recursion or right
recursion, but you should always use left recursion, because it can
parse a sequence of any number of elements with bounded stack space.
Right recursion uses up space on the Bison stack in proportion to the
number of elements in the sequence, because all the elements must be
shifted onto the stack before the rule can be applied even once. *Note
The Bison Parser Algorithm: Algorithm, for further explanation of this.
“Indirect” or “mutual” recursion occurs when the result of the rule
does not appear directly on its right hand side, but does appear in
rules for other nonterminals which do appear on its right hand side.
For example:
expr:
primary
| primary '+' primary
;
primary:
constant
| '(' expr ')'
;
defines two mutually-recursive nonterminals, since each refers to the
other.
File: bison.info, Node: Semantics, Next: Tracking Locations, Prev: Rules, Up: Grammar File
3.4 Defining Language Semantics
===============================
The grammar rules for a language determine only the syntax. The
semantics are determined by the semantic values associated with various
tokens and groupings, and by the actions taken when various groupings
are recognized.
For example, the calculator calculates properly because the value
associated with each expression is the proper number; it adds properly
because the action for the grouping ‘X + Y’ is to add the numbers
associated with X and Y.
* Menu:
* Value Type:: Specifying one data type for all semantic values.
* Multiple Types:: Specifying several alternative data types.
* Type Generation:: Generating the semantic value type.
* Union Decl:: Declaring the set of all semantic value types.
* Structured Value Type:: Providing a structured semantic value type.
* Actions:: An action is the semantic definition of a grammar rule.
* Action Types:: Specifying data types for actions to operate on.
* Midrule Actions:: Most actions go at the end of a rule.
This says when, why and how to use the exceptional
action in the middle of a rule.
File: bison.info, Node: Value Type, Next: Multiple Types, Up: Semantics
3.4.1 Data Types of Semantic Values
-----------------------------------
In a simple program it may be sufficient to use the same data type for
the semantic values of all language constructs. This was true in the
RPN and infix calculator examples (*note Reverse Polish Notation
Calculator: RPN Calc.).
Bison normally uses the type ‘int’ for semantic values if your
program uses the same data type for all language constructs. To specify
some other type, define the ‘%define’ variable ‘api.value.type’ like
this:
%define api.value.type {double}
or
%define api.value.type {struct semantic_type}
The value of ‘api.value.type’ should be a type name that does not
contain parentheses or square brackets.
Alternatively, instead of relying of Bison’s ‘%define’ support, you
may rely on the C/C++ preprocessor and define ‘YYSTYPE’ as a macro, like
this:
#define YYSTYPE double
This macro definition must go in the prologue of the grammar file (*note
Outline of a Bison Grammar: Grammar Outline.). If compatibility with
POSIX Yacc matters to you, use this. Note however that Bison cannot
know ‘YYSTYPE’’s value, not even whether it is defined, so there are
services it cannot provide. Besides this works only for languages that
have a preprocessor.
File: bison.info, Node: Multiple Types, Next: Type Generation, Prev: Value Type, Up: Semantics
3.4.2 More Than One Value Type
------------------------------
In most programs, you will need different data types for different kinds
of tokens and groupings. For example, a numeric constant may need type
‘int’ or ‘long’, while a string constant needs type ‘char *’, and an
identifier might need a pointer to an entry in the symbol table.
To use more than one data type for semantic values in one parser,
Bison requires you to do two things:
• Specify the entire collection of possible data types. There are
several options:
• let Bison compute the union type from the tags you assign to
symbols;
• use the ‘%union’ Bison declaration (*note The Union
Declaration: Union Decl.);
• define the ‘%define’ variable ‘api.value.type’ to be a union
type whose members are the type tags (*note Providing a
Structured Semantic Value Type: Structured Value Type.);
• use a ‘typedef’ or a ‘#define’ to define ‘YYSTYPE’ to be a
union type whose member names are the type tags.
• Choose one of those types for each symbol (terminal or nonterminal)
for which semantic values are used. This is done for tokens with
the ‘%token’ Bison declaration (*note Token Type Names: Token
Decl.) and for groupings with the ‘%type’ Bison declaration (*note
Nonterminal Symbols: Type Decl.).
File: bison.info, Node: Type Generation, Next: Union Decl, Prev: Multiple Types, Up: Semantics
3.4.3 Generating the Semantic Value Type
----------------------------------------
The special value ‘union’ of the ‘%define’ variable ‘api.value.type’
instructs Bison that the tags used with the ‘%token’ and ‘%type’
directives are genuine types, not names of members of ‘YYSTYPE’.
For example:
%define api.value.type union
%token INT "integer"
%token 'n'
%type expr
%token ID "identifier"
generates an appropriate value of ‘YYSTYPE’ to support each symbol type.
The name of the member of ‘YYSTYPE’ for tokens than have a declared
identifier ID (such as ‘INT’ and ‘ID’ above, but not ‘'n'’) is ‘ID’.
The other symbols have unspecified names on which you should not depend;
instead, relying on C casts to access the semantic value with the
appropriate type:
/* For an "integer". */
yylval.INT = 42;
return INT;
/* For an 'n', also declared as int. */
*((int*)&yylval) = 42;
return 'n';
/* For an "identifier". */
yylval.ID = "42";
return ID;
If the ‘%define’ variable ‘api.token.prefix’ is defined (*note
api.token.prefix: %define Summary.), then it is also used to prefix the
union member names. For instance, with ‘%define api.token.prefix
{TOK_}’:
/* For an "integer". */
yylval.TOK_INT = 42;
return TOK_INT;
This Bison extension cannot work if ‘%yacc’ (or ‘-y’/‘--yacc’) is
enabled, as POSIX mandates that Yacc generate tokens as macros (e.g.,
‘#define INT 258’, or ‘#define TOK_INT 258’).
A similar feature is provided for C++ that in addition overcomes C++
limitations (that forbid non-trivial objects to be part of a ‘union’):
‘%define api.value.type variant’, see *note C++ Variants::.
File: bison.info, Node: Union Decl, Next: Structured Value Type, Prev: Type Generation, Up: Semantics
3.4.4 The Union Declaration
---------------------------
The ‘%union’ declaration specifies the entire collection of possible
data types for semantic values. The keyword ‘%union’ is followed by
braced code containing the same thing that goes inside a ‘union’ in C.
For example:
%union {
double val;
symrec *tptr;
}
This says that the two alternative types are ‘double’ and ‘symrec *’.
They are given names ‘val’ and ‘tptr’; these names are used in the
‘%token’ and ‘%type’ declarations to pick one of the types for a
terminal or nonterminal symbol (*note Nonterminal Symbols: Type Decl.).
As an extension to POSIX, a tag is allowed after the ‘%union’. For
example:
%union value {
double val;
symrec *tptr;
}
specifies the union tag ‘value’, so the corresponding C type is ‘union
value’. If you do not specify a tag, it defaults to ‘YYSTYPE’ (*note
api.value.union.name: %define Summary.).
As another extension to POSIX, you may specify multiple ‘%union’
declarations; their contents are concatenated. However, only the first
‘%union’ declaration can specify a tag.
Note that, unlike making a ‘union’ declaration in C, you need not
write a semicolon after the closing brace.
File: bison.info, Node: Structured Value Type, Next: Actions, Prev: Union Decl, Up: Semantics
3.4.5 Providing a Structured Semantic Value Type
------------------------------------------------
Instead of ‘%union’, you can define and use your own union type
‘YYSTYPE’ if your grammar contains at least one ‘’ tag. For
example, you can put the following into a header file ‘parser.h’:
union YYSTYPE {
double val;
symrec *tptr;
};
and then your grammar can use the following instead of ‘%union’:
%{
#include "parser.h"
%}
%define api.value.type {union YYSTYPE}
%type expr
%token ID
Actually, you may also provide a ‘struct’ rather that a ‘union’,
which may be handy if you want to track information for every symbol
(such as preceding comments).
The type you provide may even be structured and include pointers, in
which case the type tags you provide may be composite, with ‘.’ and ‘->’
operators.
File: bison.info, Node: Actions, Next: Action Types, Prev: Structured Value Type, Up: Semantics
3.4.6 Actions
-------------
An action accompanies a syntactic rule and contains C code to be
executed each time an instance of that rule is recognized. The task of
most actions is to compute a semantic value for the grouping built by
the rule from the semantic values associated with tokens or smaller
groupings.
An action consists of braced code containing C statements, and can be
placed at any position in the rule; it is executed at that position.
Most rules have just one action at the end of the rule, following all
the components. Actions in the middle of a rule are tricky and used
only for special purposes (*note Actions in Midrule: Midrule Actions.).
The C code in an action can refer to the semantic values of the
components matched by the rule with the construct ‘$N’, which stands for
the value of the Nth component. The semantic value for the grouping
being constructed is ‘$$’. In addition, the semantic values of symbols
can be accessed with the named references construct ‘$NAME’ or
‘$[NAME]’. Bison translates both of these constructs into expressions
of the appropriate type when it copies the actions into the parser
implementation file. ‘$$’ (or ‘$NAME’, when it stands for the current
grouping) is translated to a modifiable lvalue, so it can be assigned
to.
Here is a typical example:
exp:
...
| exp '+' exp { $$ = $1 + $3; }
Or, in terms of named references:
exp[result]:
...
| exp[left] '+' exp[right] { $result = $left + $right; }
This rule constructs an ‘exp’ from two smaller ‘exp’ groupings connected
by a plus-sign token. In the action, ‘$1’ and ‘$3’ (‘$left’ and
‘$right’) refer to the semantic values of the two component ‘exp’
groupings, which are the first and third symbols on the right hand side
of the rule. The sum is stored into ‘$$’ (‘$result’) so that it becomes
the semantic value of the addition-expression just recognized by the
rule. If there were a useful semantic value associated with the ‘+’
token, it could be referred to as ‘$2’.
*Note Named References::, for more information about using the named
references construct.
Note that the vertical-bar character ‘|’ is really a rule separator,
and actions are attached to a single rule. This is a difference with
tools like Flex, for which ‘|’ stands for either “or”, or “the same
action as that of the next rule”. In the following example, the action
is triggered only when ‘b’ is found:
a-or-b: 'a'|'b' { a_or_b_found = 1; };
If you don’t specify an action for a rule, Bison supplies a default:
‘$$ = $1’. Thus, the value of the first symbol in the rule becomes the
value of the whole rule. Of course, the default action is valid only if
the two data types match. There is no meaningful default action for an
empty rule; every empty rule must have an explicit action unless the
rule’s value does not matter.
‘$N’ with N zero or negative is allowed for reference to tokens and
groupings on the stack _before_ those that match the current rule. This
is a very risky practice, and to use it reliably you must be certain of
the context in which the rule is applied. Here is a case in which you
can use this reliably:
foo:
expr bar '+' expr { ... }
| expr bar '-' expr { ... }
;
bar:
%empty { previous_expr = $0; }
;
As long as ‘bar’ is used only in the fashion shown here, ‘$0’ always
refers to the ‘expr’ which precedes ‘bar’ in the definition of ‘foo’.
It is also possible to access the semantic value of the lookahead
token, if any, from a semantic action. This semantic value is stored in
‘yylval’. *Note Special Features for Use in Actions: Action Features.
File: bison.info, Node: Action Types, Next: Midrule Actions, Prev: Actions, Up: Semantics
3.4.7 Data Types of Values in Actions
-------------------------------------
If you have chosen a single data type for semantic values, the ‘$$’ and
‘$N’ constructs always have that data type.
If you have used ‘%union’ to specify a variety of data types, then
you must declare a choice among these types for each terminal or
nonterminal symbol that can have a semantic value. Then each time you
use ‘$$’ or ‘$N’, its data type is determined by which symbol it refers
to in the rule. In this example,
exp:
...
| exp '+' exp { $$ = $1 + $3; }
‘$1’ and ‘$3’ refer to instances of ‘exp’, so they all have the data
type declared for the nonterminal symbol ‘exp’. If ‘$2’ were used, it
would have the data type declared for the terminal symbol ‘'+'’,
whatever that might be.
Alternatively, you can specify the data type when you refer to the
value, by inserting ‘’ after the ‘$’ at the beginning of the
reference. For example, if you have defined types as shown here:
%union {
int itype;
double dtype;
}
then you can write ‘$1’ to refer to the first subunit of the rule
as an integer, or ‘$1’ to refer to it as a double.
File: bison.info, Node: Midrule Actions, Prev: Action Types, Up: Semantics
3.4.8 Actions in Midrule
------------------------
Occasionally it is useful to put an action in the middle of a rule.
These actions are written just like usual end-of-rule actions, but they
are executed before the parser even recognizes the following components.
* Menu:
* Using Midrule Actions:: Putting an action in the middle of a rule.
* Typed Midrule Actions:: Specifying the semantic type of their values.
* Midrule Action Translation:: How midrule actions are actually processed.
* Midrule Conflicts:: Midrule actions can cause conflicts.
File: bison.info, Node: Using Midrule Actions, Next: Typed Midrule Actions, Up: Midrule Actions
3.4.8.1 Using Midrule Actions
.............................
A midrule action may refer to the components preceding it using ‘$N’,
but it may not refer to subsequent components because it is run before
they are parsed.
The midrule action itself counts as one of the components of the
rule. This makes a difference when there is another action later in the
same rule (and usually there is another at the end): you have to count
the actions along with the symbols when working out which number N to
use in ‘$N’.
The midrule action can also have a semantic value. The action can
set its value with an assignment to ‘$$’, and actions later in the rule
can refer to the value using ‘$N’. Since there is no symbol to name the
action, there is no way to declare a data type for the value in advance,
so you must use the ‘$<...>N’ construct to specify a data type each time
you refer to this value.
There is no way to set the value of the entire rule with a midrule
action, because assignments to ‘$$’ do not have that effect. The only
way to set the value for the entire rule is with an ordinary action at
the end of the rule.
Here is an example from a hypothetical compiler, handling a ‘let’
statement that looks like ‘let (VARIABLE) STATEMENT’ and serves to
create a variable named VARIABLE temporarily for the duration of
STATEMENT. To parse this construct, we must put VARIABLE into the
symbol table while STATEMENT is parsed, then remove it afterward. Here
is how it is done:
stmt:
"let" '(' var ')'
{
$$ = push_context ();
declare_variable ($3);
}
stmt
{
$$ = $6;
pop_context ($5);
}
As soon as ‘let (VARIABLE)’ has been recognized, the first action is
run. It saves a copy of the current semantic context (the list of
accessible variables) as its semantic value, using alternative ‘context’
in the data-type union. Then it calls ‘declare_variable’ to add the new
variable to that list. Once the first action is finished, the embedded
statement ‘stmt’ can be parsed.
Note that the midrule action is component number 5, so the ‘stmt’ is
component number 6. Named references can be used to improve the
readability and maintainability (*note Named References::):
stmt:
"let" '(' var ')'
{
$let = push_context ();
declare_variable ($3);
}[let]
stmt
{
$$ = $6;
pop_context ($let);
}
After the embedded statement is parsed, its semantic value becomes
the value of the entire ‘let’-statement. Then the semantic value from
the earlier action is used to restore the prior list of variables. This
removes the temporary ‘let’-variable from the list so that it won’t
appear to exist while the rest of the program is parsed.
Because the types of the semantic values of midrule actions are
unknown to Bison, type-based features (e.g., ‘%printer’, ‘%destructor’)
do not work, which could result in memory leaks. They also forbid the
use of the ‘variant’ implementation of the ‘api.value.type’ in C++
(*note C++ Variants::).
*Note Typed Midrule Actions::, for one way to address this issue, and
*note Midrule Action Translation::, for another: turning mid-action
actions into regular actions.
File: bison.info, Node: Typed Midrule Actions, Next: Midrule Action Translation, Prev: Using Midrule Actions, Up: Midrule Actions
3.4.8.2 Typed Midrule Actions
.............................
In the above example, if the parser initiates error recovery (*note
Error Recovery::) while parsing the tokens in the embedded statement
‘stmt’, it might discard the previous semantic context ‘$5’
without restoring it. Thus, ‘$5’ needs a destructor (*note
Freeing Discarded Symbols: Destructor Decl.), and Bison needs the type
of the semantic value (‘context’) to select the right destructor.
As an extension to Yacc’s midrule actions, Bison offers a means to
type their semantic value: specify its type tag (‘<...>’ before the
midrule action.
Consider the previous example, with an untyped midrule action:
stmt:
"let" '(' var ')'
{
$$ = push_context (); // ***
declare_variable ($3);
}
stmt
{
$$ = $6;
pop_context ($5); // ***
}
If instead you write:
stmt:
"let" '(' var ')'
{ // ***
$$ = push_context (); // ***
declare_variable ($3);
}
stmt
{
$$ = $6;
pop_context ($5); // ***
}
then ‘%printer’ and ‘%destructor’ work properly (no more leaks!), C++
‘variant’s can be used, and redundancy is reduced (‘’ is
specified once).
File: bison.info, Node: Midrule Action Translation, Next: Midrule Conflicts, Prev: Typed Midrule Actions, Up: Midrule Actions
3.4.8.3 Midrule Action Translation
..................................
Midrule actions are actually transformed into regular rules and actions.
The various reports generated by Bison (textual, graphical, etc., see
*note Understanding Your Parser: Understanding.) reveal this
translation, best explained by means of an example. The following rule:
exp: { a(); } "b" { c(); } { d(); } "e" { f(); };
is translated into:
$@1: %empty { a(); };
$@2: %empty { c(); };
$@3: %empty { d(); };
exp: $@1 "b" $@2 $@3 "e" { f(); };
with new nonterminal symbols ‘$@N’, where N is a number.
A midrule action is expected to generate a value if it uses ‘$$’, or
the (final) action uses ‘$N’ where N denote the midrule action. In that
case its nonterminal is rather named ‘@N’:
exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
is translated into
@1: %empty { a(); };
@2: %empty { $$ = c(); };
$@3: %empty { d(); };
exp: @1 "b" @2 $@3 "e" { f = $1; }
There are probably two errors in the above example: the first midrule
action does not generate a value (it does not use ‘$$’ although the
final action uses it), and the value of the second one is not used (the
final action does not use ‘$3’). Bison reports these errors when the
‘midrule-value’ warnings are enabled (*note Invoking Bison:
Invocation.):
$ bison -Wmidrule-value mid.y
mid.y:2.6-13: warning: unset value: $$
2 | exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
| ^~~~~~~~
mid.y:2.19-31: warning: unused value: $3
2 | exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
| ^~~~~~~~~~~~~
It is sometimes useful to turn midrule actions into regular actions,
e.g., to factor them, or to escape from their limitations. For
instance, as an alternative to _typed_ midrule action, you may bury the
midrule action inside a nonterminal symbol and to declare a printer and
a destructor for that symbol:
%type let
%destructor { pop_context ($$); } let
%printer { print_context (yyo, $$); } let
%%
stmt:
let stmt
{
$$ = $2;
pop_context ($let);
};
let:
"let" '(' var ')'
{
$let = push_context ();
declare_variable ($var);
};
File: bison.info, Node: Midrule Conflicts, Prev: Midrule Action Translation, Up: Midrule Actions
3.4.8.4 Conflicts due to Midrule Actions
........................................
Taking action before a rule is completely recognized often leads to
conflicts since the parser must commit to a parse in order to execute
the action. For example, the following two rules, without midrule
actions, can coexist in a working parser because the parser can shift
the open-brace token and look at what follows before deciding whether
there is a declaration or not:
compound:
'{' declarations statements '}'
| '{' statements '}'
;
But when we add a midrule action as follows, the rules become
nonfunctional:
compound:
{ prepare_for_local_variables (); }
'{' declarations statements '}'
| '{' statements '}'
;
Now the parser is forced to decide whether to run the midrule action
when it has read no farther than the open-brace. In other words, it
must commit to using one rule or the other, without sufficient
information to do it correctly. (The open-brace token is what is called
the “lookahead” token at this time, since the parser is still deciding
what to do about it. *Note Lookahead Tokens: Lookahead.)
You might think that you could correct the problem by putting
identical actions into the two rules, like this:
compound:
{ prepare_for_local_variables (); }
'{' declarations statements '}'
| { prepare_for_local_variables (); }
'{' statements '}'
;
But this does not help, because Bison does not realize that the two
actions are identical. (Bison never tries to understand the C code in
an action.)
If the grammar is such that a declaration can be distinguished from a
statement by the first token (which is true in C), then one solution
which does work is to put the action after the open-brace, like this:
compound:
'{' { prepare_for_local_variables (); }
declarations statements '}'
| '{' statements '}'
;
Now the first token of the following declaration or statement, which
would in any case tell Bison which rule to use, can still do so.
Another solution is to bury the action inside a nonterminal symbol
which serves as a subroutine:
subroutine:
%empty { prepare_for_local_variables (); }
;
compound:
subroutine '{' declarations statements '}'
| subroutine '{' statements '}'
;
Now Bison can execute the action in the rule for ‘subroutine’ without
deciding which rule for ‘compound’ it will eventually use.
File: bison.info, Node: Tracking Locations, Next: Named References, Prev: Semantics, Up: Grammar File
3.5 Tracking Locations
======================
Though grammar rules and semantic actions are enough to write a fully
functional parser, it can be useful to process some additional
information, especially symbol locations.
The way locations are handled is defined by providing a data type,
and actions to take when rules are matched.
* Menu:
* Location Type:: Specifying a data type for locations.
* Actions and Locations:: Using locations in actions.
* Location Default Action:: Defining a general way to compute locations.
File: bison.info, Node: Location Type, Next: Actions and Locations, Up: Tracking Locations
3.5.1 Data Type of Locations
----------------------------
Defining a data type for locations is much simpler than for semantic
values, since all tokens and groupings always use the same type.
You can specify the type of locations by defining a macro called
‘YYLTYPE’, just as you can specify the semantic value type by defining a
‘YYSTYPE’ macro (*note Value Type::). When ‘YYLTYPE’ is not defined,
Bison uses a default structure type with four members:
typedef struct YYLTYPE
{
int first_line;
int first_column;
int last_line;
int last_column;
} YYLTYPE;
When ‘YYLTYPE’ is not defined, at the beginning of the parsing, Bison
initializes all these fields to 1 for ‘yylloc’. To initialize ‘yylloc’
with a custom location type (or to chose a different initialization),
use the ‘%initial-action’ directive. *Note Performing Actions before
Parsing: Initial Action Decl.
File: bison.info, Node: Actions and Locations, Next: Location Default Action, Prev: Location Type, Up: Tracking Locations
3.5.2 Actions and Locations
---------------------------
Actions are not only useful for defining language semantics, but also
for describing the behavior of the output parser with locations.
The most obvious way for building locations of syntactic groupings is
very similar to the way semantic values are computed. In a given rule,
several constructs can be used to access the locations of the elements
being matched. The location of the Nth component of the right hand side
is ‘@N’, while the location of the left hand side grouping is ‘@$’.
In addition, the named references construct ‘@NAME’ and ‘@[NAME]’ may
also be used to address the symbol locations. *Note Named References::,
for more information about using the named references construct.
Here is a basic example using the default data type for locations:
exp:
...
| exp '/' exp
{
@$.first_column = @1.first_column;
@$.first_line = @1.first_line;
@$.last_column = @3.last_column;
@$.last_line = @3.last_line;
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
fprintf (stderr, "%d.%d-%d.%d: division by zero",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
As for semantic values, there is a default action for locations that
is run each time a rule is matched. It sets the beginning of ‘@$’ to
the beginning of the first symbol, and the end of ‘@$’ to the end of the
last symbol.
With this default action, the location tracking can be fully
automatic. The example above simply rewrites this way:
exp:
...
| exp '/' exp
{
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
fprintf (stderr, "%d.%d-%d.%d: division by zero",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
It is also possible to access the location of the lookahead token, if
any, from a semantic action. This location is stored in ‘yylloc’.
*Note Special Features for Use in Actions: Action Features.
File: bison.info, Node: Location Default Action, Prev: Actions and Locations, Up: Tracking Locations
3.5.3 Default Action for Locations
----------------------------------
Actually, actions are not the best place to compute locations. Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each rule.
The ‘YYLLOC_DEFAULT’ macro is invoked each time a rule is matched,
before the associated action is run. It is also invoked while
processing a syntax error, to compute the error’s location. Before
reporting an unresolvable syntactic ambiguity, a GLR parser invokes
‘YYLLOC_DEFAULT’ recursively to compute the location of that ambiguity.
Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.
The ‘YYLLOC_DEFAULT’ macro takes three parameters. The first one is
the location of the grouping (the result of the computation). When a
rule is matched, the second parameter identifies locations of all right
hand side elements of the rule being matched, and the third parameter is
the size of the rule’s right hand side. When a GLR parser reports an
ambiguity, which of multiple candidate right hand sides it passes to
‘YYLLOC_DEFAULT’ is undefined. When processing a syntax error, the
second parameter identifies locations of the symbols that were discarded
during error processing, and the third parameter is the number of
discarded symbols.
By default, ‘YYLLOC_DEFAULT’ is defined this way:
# define YYLLOC_DEFAULT(Cur, Rhs, N) \
do \
if (N) \
{ \
(Cur).first_line = YYRHSLOC(Rhs, 1).first_line; \
(Cur).first_column = YYRHSLOC(Rhs, 1).first_column; \
(Cur).last_line = YYRHSLOC(Rhs, N).last_line; \
(Cur).last_column = YYRHSLOC(Rhs, N).last_column; \
} \
else \
{ \
(Cur).first_line = (Cur).last_line = \
YYRHSLOC(Rhs, 0).last_line; \
(Cur).first_column = (Cur).last_column = \
YYRHSLOC(Rhs, 0).last_column; \
} \
while (0)
where ‘YYRHSLOC (rhs, k)’ is the location of the Kth symbol in RHS when
K is positive, and the location of the symbol just before the reduction
when K and N are both zero.
When defining ‘YYLLOC_DEFAULT’, you should consider that:
• All arguments are free of side-effects. However, only the first
one (the result) should be modified by ‘YYLLOC_DEFAULT’.
• For consistency with semantic actions, valid indexes within the
right hand side range from 1 to N. When N is zero, only 0 is a
valid index, and it refers to the symbol just before the reduction.
During error processing N is always positive.
• Your macro should parenthesize its arguments, if need be, since the
actual arguments may not be surrounded by parentheses. Also, your
macro should expand to something that can be used as a single
statement when it is followed by a semicolon.
File: bison.info, Node: Named References, Next: Declarations, Prev: Tracking Locations, Up: Grammar File
3.6 Named References
====================
As described in the preceding sections, the traditional way to refer to
any semantic value or location is a “positional reference”, which takes
the form ‘$N’, ‘$$’, ‘@N’, and ‘@$’. However, such a reference is not
very descriptive. Moreover, if you later decide to insert or remove
symbols in the right-hand side of a grammar rule, the need to renumber
such references can be tedious and error-prone.
To avoid these issues, you can also refer to a semantic value or
location using a “named reference”. First of all, original symbol names
may be used as named references. For example:
invocation: op '(' args ')'
{ $invocation = new_invocation ($op, $args, @invocation); }
Positional and named references can be mixed arbitrarily. For example:
invocation: op '(' args ')'
{ $$ = new_invocation ($op, $args, @$); }
However, sometimes regular symbol names are not sufficient due to
ambiguities:
exp: exp '/' exp
{ $exp = $exp / $exp; } // $exp is ambiguous.
exp: exp '/' exp
{ $$ = $1 / $exp; } // One usage is ambiguous.
exp: exp '/' exp
{ $$ = $1 / $3; } // No error.
When ambiguity occurs, explicitly declared names may be used for values
and locations. Explicit names are declared as a bracketed name after a
symbol appearance in rule definitions. For example:
exp[result]: exp[left] '/' exp[right]
{ $result = $left / $right; }
In order to access a semantic value generated by a midrule action, an
explicit name may also be declared by putting a bracketed name after the
closing brace of the midrule action code:
exp[res]: exp[x] '+' {$left = $x;}[left] exp[right]
{ $res = $left + $right; }
In references, in order to specify names containing dots and dashes,
an explicit bracketed syntax ‘$[name]’ and ‘@[name]’ must be used:
if-stmt: "if" '(' expr ')' "then" then.stmt ';'
{ $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); }
It often happens that named references are followed by a dot, dash or
other C punctuation marks and operators. By default, Bison will read
‘$name.suffix’ as a reference to symbol value ‘$name’ followed by
‘.suffix’, i.e., an access to the ‘suffix’ field of the semantic value.
In order to force Bison to recognize ‘name.suffix’ in its entirety as
the name of a semantic value, the bracketed syntax ‘$[name.suffix]’ must
be used.
The named references feature is experimental. More user feedback
will help to stabilize it.
File: bison.info, Node: Declarations, Next: Multiple Parsers, Prev: Named References, Up: Grammar File
3.7 Bison Declarations
======================
The “Bison declarations” section of a Bison grammar defines the symbols
used in formulating the grammar and the data types of semantic values.
*Note Symbols::.
All token type names (but not single-character literal tokens such as
‘'+'’ and ‘'*'’) must be declared. Nonterminal symbols must be declared
if you need to specify which data type to use for the semantic value
(*note More Than One Value Type: Multiple Types.).
The first rule in the grammar file also specifies the start symbol,
by default. If you want some other symbol to be the start symbol, you
must declare it explicitly (*note Languages and Context-Free Grammars:
Language and Grammar.).
* Menu:
* Require Decl:: Requiring a Bison version.
* Token Decl:: Declaring terminal symbols.
* Precedence Decl:: Declaring terminals with precedence and associativity.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
* Symbol Decls:: Summary of the Syntax of Symbol Declarations.
* Initial Action Decl:: Code run before parsing starts.
* Destructor Decl:: Declaring how symbols are freed.
* Printer Decl:: Declaring how symbol values are displayed.
* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
* Push Decl:: Requesting a push parser.
* Decl Summary:: Table of all Bison declarations.
* %define Summary:: Defining variables to adjust Bison’s behavior.
* %code Summary:: Inserting code into the parser source.
File: bison.info, Node: Require Decl, Next: Token Decl, Up: Declarations
3.7.1 Require a Version of Bison
--------------------------------
You may require the minimum version of Bison to process the grammar. If
the requirement is not met, ‘bison’ exits with an error (exit status
63).
%require "VERSION"
Some deprecated behaviors are disabled for some required VERSION:
‘"3.2"’ (or better)
The C++ deprecated files ‘position.hh’ and ‘stack.hh’ are no longer
generated.
‘"3.4"’ (or better)
To comply with the recommendations of the Graphviz team
(https://marc.info/?l=graphviz-devel&m=129418103126092), use the
‘.gv’ extension instead of ‘.dot’ for the name of the generated DOT
file. *Note Graphviz::.
File: bison.info, Node: Token Decl, Next: Precedence Decl, Prev: Require Decl, Up: Declarations
3.7.2 Token Type Names
----------------------
The basic way to declare a token type name (terminal symbol) is as
follows:
%token NAME
Bison will convert this into a definition in the parser, so that the
function ‘yylex’ (if it is in this file) can use the name NAME to stand
for this token type’s code.
Alternatively, you can use ‘%left’, ‘%right’, ‘%precedence’, or
‘%nonassoc’ instead of ‘%token’, if you wish to specify associativity
and precedence. *Note Operator Precedence: Precedence Decl.
You can explicitly specify the numeric code for a token type by
appending a nonnegative decimal or hexadecimal integer value in the
field immediately following the token name:
%token NUM 300
%token XNUM 0x12d // a GNU extension
It is generally best, however, to let Bison choose the numeric codes for
all token types. Bison will automatically select codes that don’t
conflict with each other or with normal characters.
In the event that the stack type is a union, you must augment the
‘%token’ or other token declaration to include the data type alternative
delimited by angle-brackets (*note More Than One Value Type: Multiple
Types.).
For example:
%union { /* define stack type */
double val;
symrec *tptr;
}
%token NUM /* define token NUM and its type */
You can associate a literal string token with a token type name by
writing the literal string at the end of a ‘%token’ declaration which
declares the name. For example:
%token arrow "=>"
For example, a grammar for the C language might specify these names with
equivalent literal string tokens:
%token OR "||"
%token LE 134 "<="
%left OR "<="
Once you equate the literal string and the token name, you can use them
interchangeably in further declarations or the grammar rules. The
‘yylex’ function can use the token name or the literal string to obtain
the token type code number (*note Calling Convention::). Syntax error
messages passed to ‘yyerror’ from the parser will reference the literal
string instead of the token name.
The token numbered as 0 corresponds to end of file; the following
line allows for nicer error messages referring to “end of file” instead
of “$end”:
%token END 0 "end of file"
File: bison.info, Node: Precedence Decl, Next: Type Decl, Prev: Token Decl, Up: Declarations
3.7.3 Operator Precedence
-------------------------
Use the ‘%left’, ‘%right’, ‘%nonassoc’, or ‘%precedence’ declaration to
declare a token and specify its precedence and associativity, all at
once. These are called “precedence declarations”. *Note Operator
Precedence: Precedence, for general information on operator precedence.
The syntax of a precedence declaration is nearly the same as that of
‘%token’: either
%left SYMBOLS...
or
%left SYMBOLS...
And indeed any of these declarations serves the purposes of ‘%token’.
But in addition, they specify the associativity and relative precedence
for all the SYMBOLS:
• The associativity of an operator OP determines how repeated uses of
the operator nest: whether ‘X OP Y OP Z’ is parsed by grouping X
with Y first or by grouping Y with Z first. ‘%left’ specifies
left-associativity (grouping X with Y first) and ‘%right’ specifies
right-associativity (grouping Y with Z first). ‘%nonassoc’
specifies no associativity, which means that ‘X OP Y OP Z’ is
considered a syntax error.
‘%precedence’ gives only precedence to the SYMBOLS, and defines no
associativity at all. Use this to define precedence only, and
leave any potential conflict due to associativity enabled.
• The precedence of an operator determines how it nests with other
operators. All the tokens declared in a single precedence
declaration have equal precedence and nest together according to
their associativity. When two tokens declared in different
precedence declarations associate, the one declared later has the
higher precedence and is grouped first.
For backward compatibility, there is a confusing difference between
the argument lists of ‘%token’ and precedence declarations. Only a
‘%token’ can associate a literal string with a token type name. A
precedence declaration always interprets a literal string as a reference
to a separate token. For example:
%left OR "<=" // Does not declare an alias.
%left OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=".
File: bison.info, Node: Type Decl, Next: Symbol Decls, Prev: Precedence Decl, Up: Declarations
3.7.4 Nonterminal Symbols
-------------------------
When you use ‘%union’ to specify multiple value types, you must declare
the value type of each nonterminal symbol for which values are used.
This is done with a ‘%type’ declaration, like this:
%type NONTERMINAL...
Here NONTERMINAL is the name of a nonterminal symbol, and TYPE is the
name given in the ‘%union’ to the alternative that you want (*note The
Union Declaration: Union Decl.). You can give any number of nonterminal
symbols in the same ‘%type’ declaration, if they have the same value
type. Use spaces to separate the symbol names.
While POSIX Yacc allows ‘%type’ only for nonterminals, Bison accepts
that this directive be also applied to terminal symbols. To declare
exclusively nonterminal symbols, use ‘%nterm’:
%nterm NONTERMINAL...
File: bison.info, Node: Symbol Decls, Next: Initial Action Decl, Prev: Type Decl, Up: Declarations
3.7.5 Syntax of Symbol Declarations
-----------------------------------
The syntax of the various directives to declare symbols is as follows.
%token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
%left TAG? ( ID NUMBER?)+ ( TAG ( ID NUMBER? )+ )*
%type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
%nterm TAG? ID+ ( TAG ID+ )*
where TAG denotes a type tag such as ‘’, ID denotes an identifier
such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
literal such as ‘"number"’. The postfix quantifiers are ‘?’ (zero or
one), ‘*’ (zero or more) and ‘+’ (one or more).
The directives ‘%precedence’, ‘%right’ and ‘%nonassoc’ behave like
‘%left’.
File: bison.info, Node: Initial Action Decl, Next: Destructor Decl, Prev: Symbol Decls, Up: Declarations
3.7.6 Performing Actions before Parsing
---------------------------------------
Sometimes your parser needs to perform some initializations before
parsing. The ‘%initial-action’ directive allows for such arbitrary
code.
-- Directive: %initial-action { CODE }
Declare that the braced CODE must be invoked before parsing each
time ‘yyparse’ is called. The CODE may use ‘$$’ (or ‘$$’) and
‘@$’ — initial value and location of the lookahead — and the
‘%parse-param’.
For instance, if your locations use a file name, you may use
%parse-param { char const *file_name };
%initial-action
{
@$.initialize (file_name);
};
File: bison.info, Node: Destructor Decl, Next: Printer Decl, Prev: Initial Action Decl, Up: Declarations
3.7.7 Freeing Discarded Symbols
-------------------------------
During error recovery (*note Error Recovery::), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded
until the parser falls on its feet. If the parser runs out of memory,
or if it returns via ‘YYABORT’ or ‘YYACCEPT’, all the symbols on the
stack must be discarded. Even if the parser succeeds, it must discard
the start symbol.
When discarded symbols convey heap based information, this memory is
lost. While this behavior can be tolerable for batch parsers, such as
in traditional compilers, it is unacceptable for programs like shells or
protocol implementations that may parse and execute indefinitely.
The ‘%destructor’ directive defines code that is called when a symbol
is automatically discarded.
-- Directive: %destructor { CODE } SYMBOLS
Invoke the braced CODE whenever the parser discards one of the
SYMBOLS. Within CODE, ‘$$’ (or ‘$$’) designates the semantic
value associated with the discarded symbol, and ‘@$’ designates its
location. The additional parser parameters are also available
(*note The Parser Function ‘yyparse’: Parser Function.).
When a symbol is listed among SYMBOLS, its ‘%destructor’ is called
a per-symbol ‘%destructor’. You may also define a per-type
‘%destructor’ by listing a semantic type tag among SYMBOLS. In
that case, the parser will invoke this CODE whenever it discards
any grammar symbol that has that semantic type tag unless that
symbol has its own per-symbol ‘%destructor’.
Finally, you can define two different kinds of default
‘%destructor’s. You can place each of ‘<*>’ and ‘<>’ in the
SYMBOLS list of exactly one ‘%destructor’ declaration in your
grammar file. The parser will invoke the CODE associated with one
of these whenever it discards any user-defined grammar symbol that
has no per-symbol and no per-type ‘%destructor’. The parser uses
the CODE for ‘<*>’ in the case of such a grammar symbol for which
you have formally declared a semantic type tag (‘%type’ counts as
such a declaration, but ‘$$’ does not). The parser uses the
CODE for ‘<>’ in the case of such a grammar symbol that has no
declared semantic type tag.
For example:
%union { char *string; }
%token STRING1 STRING2
%type string1 string2
%union { char character; }
%token CHR
%type chr
%token TAGLESS
%destructor { }
%destructor { free ($$); } <*>
%destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
%destructor { printf ("Discarding tagless symbol.\n"); } <>
guarantees that, when the parser discards any user-defined symbol that
has a semantic type tag other than ‘’, it passes its semantic
value to ‘free’ by default. However, when the parser discards a
‘STRING1’ or a ‘string1’, it uses the third ‘%destructor’, which frees
it and prints its line number to ‘stdout’ (‘free’ is invoked only once).
Finally, the parser merely prints a message whenever it discards any
symbol, such as ‘TAGLESS’, that has no semantic type tag.
A Bison-generated parser invokes the default ‘%destructor’s only for
user-defined as opposed to Bison-defined symbols. For example, the
parser will not invoke either kind of default ‘%destructor’ for the
special Bison-defined symbols ‘$accept’, ‘$undefined’, or ‘$end’ (*note
Bison Symbols: Table of Symbols.), none of which you can reference in
your grammar. It also will not invoke either for the ‘error’ token
(*note error: Table of Symbols.), which is always defined by Bison
regardless of whether you reference it in your grammar. However, it may
invoke one of them for the end token (token 0) if you redefine it from
‘$end’ to, for example, ‘END’:
%token END 0
Finally, Bison will never invoke a ‘%destructor’ for an unreferenced
midrule semantic value (*note Actions in Midrule: Midrule Actions.).
That is, Bison does not consider a midrule to have a semantic value if
you do not reference ‘$$’ in the midrule’s action or ‘$N’ (where N is
the right-hand side symbol position of the midrule) in any later action
in that rule. However, if you do reference either, the Bison-generated
parser will invoke the ‘<>’ ‘%destructor’ whenever it discards the
midrule symbol.
“Discarded symbols” are the following:
• stacked symbols popped during the first phase of error recovery,
• incoming terminals during the second phase of error recovery,
• the current lookahead and the entire stack (except the current
right-hand side symbols) when the parser returns immediately, and
• the current lookahead and the entire stack (including the current
right-hand side symbols) when the C++ parser (‘lalr1.cc’) catches
an exception in ‘parse’,
• the start symbol, when the parser succeeds.
The parser can “return immediately” because of an explicit call to
‘YYABORT’ or ‘YYACCEPT’, or failed error recovery, or memory exhaustion.
Right-hand side symbols of a rule that explicitly triggers a syntax
error via ‘YYERROR’ are not discarded automatically. As a rule of
thumb, destructors are invoked only when user actions cannot manage the
memory.
File: bison.info, Node: Printer Decl, Next: Expect Decl, Prev: Destructor Decl, Up: Declarations
3.7.8 Printing Semantic Values
------------------------------
When run-time traces are enabled (*note Tracing Your Parser: Tracing.),
the parser reports its actions, such as reductions. When a symbol
involved in an action is reported, only its kind is displayed, as the
parser cannot know how semantic values should be formatted.
The ‘%printer’ directive defines code that is called when a symbol is
reported. Its syntax is the same as ‘%destructor’ (*note Freeing
Discarded Symbols: Destructor Decl.).
-- Directive: %printer { CODE } SYMBOLS
Invoke the braced CODE whenever the parser displays one of the
SYMBOLS. Within CODE, ‘yyo’ denotes the output stream (a ‘FILE*’
in C, and an ‘std::ostream&’ in C++), ‘$$’ (or ‘$$’)
designates the semantic value associated with the symbol, and ‘@$’
its location. The additional parser parameters are also available
(*note The Parser Function ‘yyparse’: Parser Function.).
The SYMBOLS are defined as for ‘%destructor’ (*note Freeing
Discarded Symbols: Destructor Decl.): they can be per-type (e.g.,
‘’), per-symbol (e.g., ‘exp’, ‘NUM’, ‘"float"’), typed
per-default (i.e., ‘<*>’, or untyped per-default (i.e., ‘<>’).
For example:
%union { char *string; }
%token STRING1 STRING2
%type string1 string2
%union { char character; }
%token CHR
%type chr
%token TAGLESS
%printer { fprintf (yyo, "'%c'", $$); }
%printer { fprintf (yyo, "&%p", $$); } <*>
%printer { fprintf (yyo, "\"%s\"", $$); } STRING1 string1
%printer { fprintf (yyo, "<>"); } <>
guarantees that, when the parser print any symbol that has a semantic
type tag other than ‘’, it display the address of the
semantic value by default. However, when the parser displays a
‘STRING1’ or a ‘string1’, it formats it as a string in double quotes.
It performs only the second ‘%printer’ in this case, so it prints only
once. Finally, the parser print ‘<>’ for any symbol, such as ‘TAGLESS’,
that has no semantic type tag. *Note Enabling Debug Traces for
‘mfcalc’: Mfcalc Traces, for a complete example.
File: bison.info, Node: Expect Decl, Next: Start Decl, Prev: Printer Decl, Up: Declarations
3.7.9 Suppressing Conflict Warnings
-----------------------------------
Bison normally warns if there are any conflicts in the grammar (*note
Shift/Reduce Conflicts: Shift/Reduce.), but most real grammars have
harmless shift/reduce conflicts which are resolved in a predictable way
and would be difficult to eliminate. It is desirable to suppress the
warning about these conflicts unless the number of conflicts changes.
You can do this with the ‘%expect’ declaration.
The declaration looks like this:
%expect N
Here N is a decimal integer. The declaration says there should be N
shift/reduce conflicts and no reduce/reduce conflicts. Bison reports an
error if the number of shift/reduce conflicts differs from N, or if
there are any reduce/reduce conflicts.
For deterministic parsers, reduce/reduce conflicts are more serious,
and should be eliminated entirely. Bison will always report
reduce/reduce conflicts for these parsers. With GLR parsers, however,
both kinds of conflicts are routine; otherwise, there would be no need
to use GLR parsing. Therefore, it is also possible to specify an
expected number of reduce/reduce conflicts in GLR parsers, using the
declaration:
%expect-rr N
You may wish to be more specific in your specification of expected
conflicts. To this end, you can also attach ‘%expect’ and ‘%expect-rr’
modifiers to individual rules. The interpretation of these modifiers
differs from their use as declarations. When attached to rules, they
indicate the number of states in which the rule is involved in a
conflict. You will need to consult the output resulting from ‘-v’ to
determine appropriate numbers to use. For example, for the following
grammar fragment, the first rule for ‘empty_dims’ appears in two states
in which the ‘[’ token is a lookahead. Having determined that, you can
document this fact with an ‘%expect’ modifier as follows:
dims:
empty_dims
| '[' expr ']' dims
;
empty_dims:
%empty %expect 2
| empty_dims '[' ']'
;
Mid-rule actions generate implicit rules that are also subject to
conflicts (*note Conflicts due to Midrule Actions: Midrule Conflicts.).
To attach an ‘%expect’ or ‘%expect-rr’ annotation to an implicit
mid-rule action’s rule, put it before the action. For example,
%glr-parser
%expect-rr 1
%%
clause:
"condition" %expect-rr 1 { value_mode(); } '(' exprs ')'
| "condition" %expect-rr 1 { class_mode(); } '(' types ')'
;
Here, the appropriate mid-rule action will not be determined until after
the ‘(’ token is shifted. Thus, the two actions will clash with each
other, and we should expect one reduce/reduce conflict for each.
In general, using ‘%expect’ involves these steps:
• Compile your grammar without ‘%expect’. Use the ‘-v’ option to get
a verbose list of where the conflicts occur. Bison will also print
the number of conflicts.
• Check each of the conflicts to make sure that Bison’s default
resolution is what you really want. If not, rewrite the grammar
and go back to the beginning.
• Add an ‘%expect’ declaration, copying the number N from the number
that Bison printed. With GLR parsers, add an ‘%expect-rr’
declaration as well.
• Optionally, count up the number of states in which one or more
conflicted reductions for particular rules appear and add these
numbers to the affected rules as ‘%expect-rr’ or ‘%expect’
modifiers as appropriate. Rules that are in conflict appear in the
output listing surrounded by square brackets or, in the case of
reduce/reduce conflicts, as reductions having the same lookahead
symbol as a square-bracketed reduction in the same state.
Now Bison will report an error if you introduce an unexpected
conflict, but will keep silent otherwise.
File: bison.info, Node: Start Decl, Next: Pure Decl, Prev: Expect Decl, Up: Declarations
3.7.10 The Start-Symbol
-----------------------
Bison assumes by default that the start symbol for the grammar is the
first nonterminal specified in the grammar specification section. The
programmer may override this restriction with the ‘%start’ declaration
as follows:
%start SYMBOL
File: bison.info, Node: Pure Decl, Next: Push Decl, Prev: Start Decl, Up: Declarations
3.7.11 A Pure (Reentrant) Parser
--------------------------------
A “reentrant” program is one which does not alter in the course of
execution; in other words, it consists entirely of “pure” (read-only)
code. Reentrancy is important whenever asynchronous execution is
possible; for example, a nonreentrant program may not be safe to call
from a signal handler. In systems with multiple threads of control, a
nonreentrant program must be called only within interlocks.
Normally, Bison generates a parser which is not reentrant. This is
suitable for most uses, and it permits compatibility with Yacc. (The
standard Yacc interfaces are inherently nonreentrant, because they use
statically allocated variables for communication with ‘yylex’, including
‘yylval’ and ‘yylloc’.)
Alternatively, you can generate a pure, reentrant parser. The Bison
declaration ‘%define api.pure’ says that you want the parser to be
reentrant. It looks like this:
%define api.pure full
The result is that the communication variables ‘yylval’ and ‘yylloc’
become local variables in ‘yyparse’, and a different calling convention
is used for the lexical analyzer function ‘yylex’. *Note Calling
Conventions for Pure Parsers: Pure Calling, for the details of this.
The variable ‘yynerrs’ becomes local in ‘yyparse’ in pull mode but it
becomes a member of ‘yypstate’ in push mode. (*note The Error Reporting
Function ‘yyerror’: Error Reporting.). The convention for calling
‘yyparse’ itself is unchanged.
Whether the parser is pure has nothing to do with the grammar rules.
You can generate either a pure parser or a nonreentrant parser from any
valid grammar.
File: bison.info, Node: Push Decl, Next: Decl Summary, Prev: Pure Decl, Up: Declarations
3.7.12 A Push Parser
--------------------
A pull parser is called once and it takes control until all its input is
completely parsed. A push parser, on the other hand, is called each
time a new token is made available.
A push parser is typically useful when the parser is part of a main
event loop in the client’s application. This is typically a requirement
of a GUI, when the main event loop needs to be triggered within a
certain time period.
Normally, Bison generates a pull parser. The following Bison
declaration says that you want the parser to be a push parser (*note
api.push-pull: %define Summary.):
%define api.push-pull push
In almost all cases, you want to ensure that your push parser is also
a pure parser (*note A Pure (Reentrant) Parser: Pure Decl.). The only
time you should create an impure push parser is to have backwards
compatibility with the impure Yacc pull mode interface. Unless you know
what you are doing, your declarations should look like this:
%define api.pure full
%define api.push-pull push
There is a major notable functional difference between the pure push
parser and the impure push parser. It is acceptable for a pure push
parser to have many parser instances, of the same type of parser, in
memory at the same time. An impure push parser should only use one
parser at a time.
When a push parser is selected, Bison will generate some new symbols
in the generated parser. ‘yypstate’ is a structure that the generated
parser uses to store the parser’s state. ‘yypstate_new’ is the function
that will create a new parser instance. ‘yypstate_delete’ will free the
resources associated with the corresponding parser instance. Finally,
‘yypush_parse’ is the function that should be called whenever a token is
available to provide the parser. A trivial example of using a pure push
parser would look like this:
int status;
yypstate *ps = yypstate_new ();
do {
status = yypush_parse (ps, yylex (), NULL);
} while (status == YYPUSH_MORE);
yypstate_delete (ps);
If the user decided to use an impure push parser, a few things about
the generated parser will change. The ‘yychar’ variable becomes a
global variable instead of a variable in the ‘yypush_parse’ function.
For this reason, the signature of the ‘yypush_parse’ function is changed
to remove the token as a parameter. A nonreentrant push parser example
would thus look like this:
extern int yychar;
int status;
yypstate *ps = yypstate_new ();
do {
yychar = yylex ();
status = yypush_parse (ps);
} while (status == YYPUSH_MORE);
yypstate_delete (ps);
That’s it. Notice the next token is put into the global variable
‘yychar’ for use by the next invocation of the ‘yypush_parse’ function.
Bison also supports both the push parser interface along with the
pull parser interface in the same generated parser. In order to get
this functionality, you should replace the ‘%define api.push-pull push’
declaration with the ‘%define api.push-pull both’ declaration. Doing
this will create all of the symbols mentioned earlier along with the two
extra symbols, ‘yyparse’ and ‘yypull_parse’. ‘yyparse’ can be used
exactly as it normally would be used. However, the user should note
that it is implemented in the generated parser by calling
‘yypull_parse’. This makes the ‘yyparse’ function that is generated
with the ‘%define api.push-pull both’ declaration slower than the normal
‘yyparse’ function. If the user calls the ‘yypull_parse’ function it
will parse the rest of the input stream. It is possible to
‘yypush_parse’ tokens to select a subgrammar and then ‘yypull_parse’ the
rest of the input stream. If you would like to switch back and forth
between between parsing styles, you would have to write your own
‘yypull_parse’ function that knows when to quit looking for input. An
example of using the ‘yypull_parse’ function would look like this:
yypstate *ps = yypstate_new ();
yypull_parse (ps); /* Will call the lexer */
yypstate_delete (ps);
Adding the ‘%define api.pure’ declaration does exactly the same thing
to the generated parser with ‘%define api.push-pull both’ as it did for
‘%define api.push-pull push’.
File: bison.info, Node: Decl Summary, Next: %define Summary, Prev: Push Decl, Up: Declarations
3.7.13 Bison Declaration Summary
--------------------------------
Here is a summary of the declarations used to define a grammar:
-- Directive: %union
Declare the collection of data types that semantic values may have
(*note The Union Declaration: Union Decl.).
-- Directive: %token
Declare a terminal symbol (token type name) with no precedence or
associativity specified (*note Token Type Names: Token Decl.).
-- Directive: %right
Declare a terminal symbol (token type name) that is
right-associative (*note Operator Precedence: Precedence Decl.).
-- Directive: %left
Declare a terminal symbol (token type name) that is
left-associative (*note Operator Precedence: Precedence Decl.).
-- Directive: %nonassoc
Declare a terminal symbol (token type name) that is nonassociative
(*note Operator Precedence: Precedence Decl.). Using it in a way
that would be associative is a syntax error.
-- Directive: %nterm
Declare the type of semantic values for a nonterminal symbol (*note
Nonterminal Symbols: Type Decl.).
-- Directive: %type
Declare the type of semantic values for a symbol (*note Nonterminal
Symbols: Type Decl.).
-- Directive: %start
Specify the grammar’s start symbol (*note The Start-Symbol: Start
Decl.).
-- Directive: %expect
Declare the expected number of shift-reduce conflicts, either
overall or for a given rule (*note Suppressing Conflict Warnings:
Expect Decl.).
-- Directive: %expect-rr
Declare the expected number of reduce-reduce conflicts, either
overall or for a given rule (*note Suppressing Conflict Warnings:
Expect Decl.).
In order to change the behavior of ‘bison’, use the following
directives:
-- Directive: %code {CODE}
-- Directive: %code QUALIFIER {CODE}
Insert CODE verbatim into the output parser source at the default
location or at the location specified by QUALIFIER. *Note %code
Summary::.
-- Directive: %debug
Instrument the parser for traces. Obsoleted by ‘%define
parse.trace’. *Note Tracing Your Parser: Tracing.
-- Directive: %define VARIABLE
-- Directive: %define VARIABLE VALUE
-- Directive: %define VARIABLE {VALUE}
-- Directive: %define VARIABLE "VALUE"
Define a variable to adjust Bison’s behavior. *Note %define
Summary::.
-- Directive: %defines
Write a parser header file containing macro definitions for the
token type names defined in the grammar as well as a few other
declarations. If the parser implementation file is named ‘NAME.c’
then the parser header file is named ‘NAME.h’.
For C parsers, the parser header file declares ‘YYSTYPE’ unless
‘YYSTYPE’ is already defined as a macro or you have used a ‘’
tag without using ‘%union’. Therefore, if you are using a ‘%union’
(*note More Than One Value Type: Multiple Types.) with components
that require other definitions, or if you have defined a ‘YYSTYPE’
macro or type definition (*note Data Types of Semantic Values:
Value Type.), you need to arrange for these definitions to be
propagated to all modules, e.g., by putting them in a prerequisite
header that is included both by your parser and by any other module
that needs ‘YYSTYPE’.
Unless your parser is pure, the parser header file declares
‘yylval’ as an external variable. *Note A Pure (Reentrant) Parser:
Pure Decl.
If you have also used locations, the parser header file declares
‘YYLTYPE’ and ‘yylloc’ using a protocol similar to that of the
‘YYSTYPE’ macro and ‘yylval’. *Note Tracking Locations::.
This parser header file is normally essential if you wish to put
the definition of ‘yylex’ in a separate source file, because
‘yylex’ typically needs to be able to refer to the above-mentioned
declarations and to the token type codes. *Note Semantic Values of
Tokens: Token Values.
If you have declared ‘%code requires’ or ‘%code provides’, the
output header also contains their code. *Note %code Summary::.
The generated header is protected against multiple inclusions with
a C preprocessor guard: ‘YY_PREFIX_FILE_INCLUDED’, where PREFIX and
FILE are the prefix (*note Multiple Parsers in the Same Program:
Multiple Parsers.) and generated file name turned uppercase, with
each series of non alphanumerical characters converted to a single
underscore.
For instance with ‘%define api.prefix {calc}’ and ‘%defines
"lib/parse.h"’, the header will be guarded as follows.
#ifndef YY_CALC_LIB_PARSE_H_INCLUDED
# define YY_CALC_LIB_PARSE_H_INCLUDED
...
#endif /* ! YY_CALC_LIB_PARSE_H_INCLUDED */
-- Directive: %defines DEFINES-FILE
Same as above, but save in the file ‘DEFINES-FILE’.
-- Directive: %destructor
Specify how the parser should reclaim the memory associated to
discarded symbols. *Note Freeing Discarded Symbols: Destructor
Decl.
-- Directive: %file-prefix "PREFIX"
Specify a prefix to use for all Bison output file names. The names
are chosen as if the grammar file were named ‘PREFIX.y’.
-- Directive: %language "LANGUAGE"
Specify the programming language for the generated parser.
Currently supported languages include C, C++, and Java. LANGUAGE
is case-insensitive.
-- Directive: %locations
Generate the code processing the locations (*note Special Features
for Use in Actions: Action Features.). This mode is enabled as
soon as the grammar uses the special ‘@N’ tokens, but if your
grammar does not use it, using ‘%locations’ allows for more
accurate syntax error messages.
-- Directive: %name-prefix "PREFIX"
Obsoleted by ‘%define api.prefix {PREFIX}’. *Note Multiple Parsers
in the Same Program: Multiple Parsers. For C++ parsers, see the
‘%define api.namespace’ documentation in this section.
Rename the external symbols used in the parser so that they start
with PREFIX instead of ‘yy’. The precise list of symbols renamed
in C parsers is ‘yyparse’, ‘yylex’, ‘yyerror’, ‘yynerrs’, ‘yylval’,
‘yychar’, ‘yydebug’, and (if locations are used) ‘yylloc’. If you
use a push parser, ‘yypush_parse’, ‘yypull_parse’, ‘yypstate’,
‘yypstate_new’ and ‘yypstate_delete’ will also be renamed. For
example, if you use ‘%name-prefix "c_"’, the names become
‘c_parse’, ‘c_lex’, and so on.
Contrary to defining ‘api.prefix’, some symbols are _not_ renamed
by ‘%name-prefix’, for instance ‘YYDEBUG’, ‘YYTOKENTYPE’,
‘yytokentype’, ‘YYSTYPE’, ‘YYLTYPE’.
-- Directive: %no-lines
Don’t generate any ‘#line’ preprocessor commands in the parser
implementation file. Ordinarily Bison writes these commands in the
parser implementation file so that the C compiler and debuggers
will associate errors and object code with your source file (the
grammar file). This directive causes them to associate errors with
the parser implementation file, treating it as an independent
source file in its own right.
-- Directive: %output "FILE"
Generate the parser implementation in ‘FILE’.
-- Directive: %pure-parser
Deprecated version of ‘%define api.pure’ (*note api.pure: %define
Summary.), for which Bison is more careful to warn about
unreasonable usage.
-- Directive: %require "VERSION"
Require version VERSION or higher of Bison. *Note Require a
Version of Bison: Require Decl.
-- Directive: %skeleton "FILE"
Specify the skeleton to use.
If FILE does not contain a ‘/’, FILE is the name of a skeleton file
in the Bison installation directory. If it does, FILE is an
absolute file name or a file name relative to the directory of the
grammar file. This is similar to how most shells resolve commands.
-- Directive: %token-table
Generate an array of token names in the parser implementation file.
The name of the array is ‘yytname’; ‘yytname[I]’ is the name of the
token whose internal Bison token code number is I. The first three
elements of ‘yytname’ correspond to the predefined tokens ‘"$end"’,
‘"error"’, and ‘"$undefined"’; after these come the symbols defined
in the grammar file.
The name in the table includes all the characters needed to
represent the token in Bison. For single-character literals and
literal strings, this includes the surrounding quoting characters
and any escape sequences. For example, the Bison single-character
literal ‘'+'’ corresponds to a three-character name, represented in
C as ‘"'+'"’; and the Bison two-character literal string ‘"\\/"’
corresponds to a five-character name, represented in C as
‘"\"\\\\/\""’.
When you specify ‘%token-table’, Bison also generates macro
definitions for macros ‘YYNTOKENS’, ‘YYNNTS’, and ‘YYNRULES’, and
‘YYNSTATES’:
‘YYNTOKENS’
The highest token number, plus one.
‘YYNNTS’
The number of nonterminal symbols.
‘YYNRULES’
The number of grammar rules,
‘YYNSTATES’
The number of parser states (*note Parser States::).
-- Directive: %verbose
Write an extra output file containing verbose descriptions of the
parser states and what is done for each type of lookahead token in
that state. *Note Understanding Your Parser: Understanding, for
more information.
-- Directive: %yacc
Pretend the option ‘--yacc’ was given, i.e., imitate Yacc,
including its naming conventions. *Note Bison Options::, for more.
File: bison.info, Node: %define Summary, Next: %code Summary, Prev: Decl Summary, Up: Declarations
3.7.14 %define Summary
----------------------
There are many features of Bison’s behavior that can be controlled by
assigning the feature a single value. For historical reasons, some such
features are assigned values by dedicated directives, such as ‘%start’,
which assigns the start symbol. However, newer such features are
associated with variables, which are assigned by the ‘%define’
directive:
-- Directive: %define VARIABLE
-- Directive: %define VARIABLE VALUE
-- Directive: %define VARIABLE {VALUE}
-- Directive: %define VARIABLE "VALUE"
Define VARIABLE to VALUE.
The type of the values depend on the syntax. Braces denote value
in the target language (e.g., a namespace, a type, etc.). Keyword
values (no delimiters) denote finite choice (e.g., a variation of a
feature). String values denote remaining cases (e.g., a file
name).
It is an error if a VARIABLE is defined by ‘%define’ multiple
times, but see *note ‘-D NAME[=VALUE]’: Bison Options.
The rest of this section summarizes variables and values that
‘%define’ accepts.
Some VARIABLEs take Boolean values. In this case, Bison will
complain if the variable definition does not meet one of the following
four conditions:
1. ‘VALUE’ is ‘true’
2. ‘VALUE’ is omitted (or ‘""’ is specified). This is equivalent to
‘true’.
3. ‘VALUE’ is ‘false’.
4. VARIABLE is never defined. In this case, Bison selects a default
value.
What VARIABLEs are accepted, as well as their meanings and default
values, depend on the selected target language and/or the parser
skeleton (*note %language: Decl Summary, *note %skeleton: Decl
Summary.). Unaccepted VARIABLEs produce an error. Some of the accepted
VARIABLEs are described below.
-- Directive: %define api.namespace {NAMESPACE}
• Languages(s): C++
• Purpose: Specify the namespace for the parser class. For
example, if you specify:
%define api.namespace {foo::bar}
Bison uses ‘foo::bar’ verbatim in references such as:
foo::bar::parser::semantic_type
However, to open a namespace, Bison removes any leading ‘::’
and then splits on any remaining occurrences:
namespace foo { namespace bar {
class position;
class location;
} }
• Accepted Values: Any absolute or relative C++ namespace
reference without a trailing ‘"::"’. For example, ‘"foo"’ or
‘"::foo::bar"’.
• Default Value: ‘yy’, unless you used the obsolete
‘%name-prefix "PREFIX"’ directive.
-- Directive: %define api.location.file "FILE"
-- Directive: %define api.location.file ‘none’
• Language(s): C++
• Purpose: Define the name of the file in which Bison’s default
location and position types are generated. *Note Exposing the
Location Classes::.
• Accepted Values:
‘none’
If locations are enabled, generate the definition of the
‘position’ and ‘location’ classes in the header file if
‘%defines’, otherwise in the parser implementation.
"FILE"
Generate the definition of the ‘position’ and ‘location’
classes in FILE. This file name can be relative (to
where the parser file is output) or absolute.
• Default Value: Not applicable if locations are not enabled, or
if a user location type is specified (see
‘api.location.type’). Otherwise, Bison’s ‘location’ is
generated in ‘location.hh’ (*note C++ location::).
• History: Introduced in Bison 3.2.
-- Directive: %define api.location.include {"FILE"}
-- Directive: %define api.location.include {}
• Language(s): C++
• Purpose: Specify how the generated file that defines the
‘position’ and ‘location’ classes is included. This makes
sense when the ‘location’ class is exposed to the rest of your
application/library in another directory. *Note Exposing the
Location Classes::.
• Accepted Values: Argument for ‘#include’.
• Default Value: ‘"DIR/location.hh"’ where DIR is the directory
part of the output. For instance ‘src/parse’ if
‘--output=src/parse/parser.cc’ was given.
• History: Introduced in Bison 3.2.
-- Directive: %define api.location.type {TYPE}
• Language(s): C, C++, Java
• Purpose: Define the location type. *Note User Defined
Location Type::.
• Accepted Values: String
• Default Value: none
• History: Introduced in Bison 2.7 for C++ and Java, in Bison
3.4 for C.
-- Directive: %define api.parser.class {NAME}
• Language(s): C++, Java
• Purpose: The name of the parser class.
• Accepted Values: Any valid identifier.
• Default Value: In C++, ‘parser’. In Java, ‘YYParser’ or
‘API.PREFIXParser’ (*note Java Bison Interface::).
• History: Introduced in Bison 3.3 to replace
‘parser_class_name’.
-- Directive: %define api.prefix {PREFIX}
• Language(s): All
• Purpose: Rename exported symbols. *Note Multiple Parsers in
the Same Program: Multiple Parsers.
• Accepted Values: String
• Default Value: ‘yy’
• History: introduced in Bison 2.6
-- Directive: %define api.pure PURITY
• Language(s): C
• Purpose: Request a pure (reentrant) parser program. *Note A
Pure (Reentrant) Parser: Pure Decl.
• Accepted Values: ‘true’, ‘false’, ‘full’
The value may be omitted: this is equivalent to specifying
‘true’, as is the case for Boolean values.
When ‘%define api.pure full’ is used, the parser is made
reentrant. This changes the signature for ‘yylex’ (*note Pure
Calling::), and also that of ‘yyerror’ when the tracking of
locations has been activated, as shown below.
The ‘true’ value is very similar to the ‘full’ value, the only
difference is in the signature of ‘yyerror’ on Yacc parsers
without ‘%parse-param’, for historical reasons.
I.e., if ‘%locations %define api.pure’ is passed then the
prototypes for ‘yyerror’ are:
void yyerror (char const *msg); // Yacc parsers.
void yyerror (YYLTYPE *locp, char const *msg); // GLR parsers.
But if ‘%locations %define api.pure %parse-param {int
*nastiness}’ is used, then both parsers have the same
signature:
void yyerror (YYLTYPE *llocp, int *nastiness, char const *msg);
(*note The Error Reporting Function ‘yyerror’: Error
Reporting.)
• Default Value: ‘false’
• History: the ‘full’ value was introduced in Bison 2.7
-- Directive: %define api.push-pull KIND
• Language(s): C (deterministic parsers only)
• Purpose: Request a pull parser, a push parser, or both. *Note
A Push Parser: Push Decl.
• Accepted Values: ‘pull’, ‘push’, ‘both’
• Default Value: ‘pull’
-- Directive: %define api.token.constructor
• Language(s): C++
• Purpose: When variant-based semantic values are enabled (*note
C++ Variants::), request that symbols be handled as a whole
(type, value, and possibly location) in the scanner. *Note
Complete Symbols::, for details.
• Accepted Values: Boolean.
• Default Value: ‘false’
• History: introduced in Bison 3.0
-- Directive: %define api.token.prefix {PREFIX}
• Languages(s): all
• Purpose: Add a prefix to the token names when generating their
definition in the target language. For instance
%token FILE for ERROR
%define api.token.prefix {TOK_}
%%
start: FILE for ERROR;
generates the definition of the symbols ‘TOK_FILE’, ‘TOK_for’,
and ‘TOK_ERROR’ in the generated source files. In particular,
the scanner must use these prefixed token names, while the
grammar itself may still use the short names (as in the sample
rule given above). The generated informational files
(‘*.output’, ‘*.xml’, ‘*.gv’) are not modified by this prefix.
Bison also prefixes the generated member names of the semantic
value union. *Note Generating the Semantic Value Type: Type
Generation, for more details.
See *note Calc++ Parser:: and *note Calc++ Scanner::, for a
complete example.
• Accepted Values: Any string. Should be a valid identifier
prefix in the target language, in other words, it should
typically be an identifier itself (sequence of letters,
underscores, and —not at the beginning— digits).
• Default Value: empty
• History: introduced in Bison 3.0
-- Directive: %define api.value.automove
• Language(s): C++
• Purpose: Let occurrences of semantic values of the right-hand
sides of a rule be implicitly turned in rvalues. When
enabled, a grammar such as:
exp:
"number" { $$ = make_number ($1); }
| exp "+" exp { $$ = make_binary (add, $1, $3); }
| "(" exp ")" { $$ = $2; }
is actually compiled as if you had written:
exp:
"number" { $$ = make_number (std::move ($1)); }
| exp "+" exp { $$ = make_binary (add,
std::move ($1),
std::move ($3)); }
| "(" exp ")" { $$ = std::move ($2); }
Using a value several times with automove enabled is typically
an error. For instance, instead of:
exp: "twice" exp { $$ = make_binary (add, $2, $2); }
write:
exp: "twice" exp { auto v = $2; $$ = make_binary (add, v, v); }
It is tempting to use ‘std::move’ on one of the ‘v’, but the
argument evaluation order in C++ is unspecified.
• Accepted Values: Boolean.
• Default Value: ‘false’
• History: introduced in Bison 3.2
-- Directive: %define api.value.type SUPPORT
-- Directive: %define api.value.type {TYPE}
• Language(s): all
• Purpose: The type for semantic values.
• Accepted Values:
‘{}’
This grammar has no semantic value at all. This is not
properly supported yet.
‘union-directive’ (C, C++)
The type is defined thanks to the ‘%union’ directive.
You don’t have to define ‘api.value.type’ in that case,
using ‘%union’ suffices. *Note The Union Declaration:
Union Decl. For instance:
%define api.value.type union-directive
%union
{
int ival;
char *sval;
}
%token INT "integer"
%token STR "string"
‘union’ (C, C++)
The symbols are defined with type names, from which Bison
will generate a ‘union’. For instance:
%define api.value.type union
%token INT "integer"
%token STR "string"
Most C++ objects cannot be stored in a ‘union’, use
‘variant’ instead.
‘variant’ (C++)
This is similar to ‘union’, but special storage
techniques are used to allow any kind of C++ object to be
used. For instance:
%define api.value.type variant
%token INT "integer"
%token STR "string"
*Note C++ Variants::.
‘{TYPE}’
Use this TYPE as semantic value.
%code requires
{
struct my_value
{
enum
{
is_int, is_str
} kind;
union
{
int ival;
char *sval;
} u;
};
}
%define api.value.type {struct my_value}
%token INT "integer"
%token STR "string"
• Default Value:
− ‘union-directive’ if ‘%union’ is used, otherwise ...
− ‘int’ if type tags are used (i.e., ‘%token ...’ or
‘%type ...’ is used), otherwise ...
− undefined.
• History: introduced in Bison 3.0. Was introduced for Java
only in 2.3b as ‘stype’.
-- Directive: %define api.value.union.name NAME
• Language(s): C
• Purpose: The tag of the generated ‘union’ (_not_ the name of
the ‘typedef’). This variable is set to ‘ID’ when ‘%union ID’
is used. There is no clear reason to give this union a name.
• Accepted Values: Any valid identifier.
• Default Value: ‘YYSTYPE’.
• History: Introduced in Bison 3.0.3.
-- Directive: %define location_type
Obsoleted by ‘api.location.type’ since Bison 2.7.
-- Directive: %define lr.default-reduction WHEN
• Language(s): all
• Purpose: Specify the kind of states that are permitted to
contain default reductions. *Note Default Reductions::. (The
ability to specify where default reductions should be used is
experimental. More user feedback will help to stabilize it.)
• Accepted Values: ‘most’, ‘consistent’, ‘accepting’
• Default Value:
• ‘accepting’ if ‘lr.type’ is ‘canonical-lr’.
• ‘most’ otherwise.
• History: introduced as ‘lr.default-reductions’ in 2.5, renamed
as ‘lr.default-reduction’ in 3.0.
-- Directive: %define lr.keep-unreachable-state
• Language(s): all
• Purpose: Request that Bison allow unreachable parser states to
remain in the parser tables. *Note Unreachable States::.
• Accepted Values: Boolean
• Default Value: ‘false’
• History: introduced as ‘lr.keep_unreachable_states’ in 2.3b,
renamed as ‘lr.keep-unreachable-states’ in 2.5, and as
‘lr.keep-unreachable-state’ in 3.0.
-- Directive: %define lr.type TYPE
• Language(s): all
• Purpose: Specify the type of parser tables within the LR(1)
family. *Note LR Table Construction::.
• Accepted Values: ‘lalr’, ‘ielr’, ‘canonical-lr’
• Default Value: ‘lalr’
-- Directive: %define namespace {NAMESPACE}
Obsoleted by ‘api.namespace’
-- Directive: %define parse.assert
• Languages(s): C++
• Purpose: Issue runtime assertions to catch invalid uses. In
C++, when variants are used (*note C++ Variants::), symbols
must be constructed and destroyed properly. This option
checks these constraints.
• Accepted Values: Boolean
• Default Value: ‘false’
-- Directive: %define parse.error VERBOSITY
• Languages(s): all
• Purpose: Control the kind of error messages passed to the
error reporting function. *Note The Error Reporting Function
‘yyerror’: Error Reporting.
• Accepted Values:
• ‘simple’ Error messages passed to ‘yyerror’ are simply
‘"syntax error"’.
• ‘verbose’ Error messages report the unexpected token, and
possibly the expected ones. However, this report can
often be incorrect when LAC is not enabled (*note LAC::).
• Default Value: ‘simple’
-- Directive: %define parse.lac WHEN
• Languages(s): C (deterministic parsers only)
• Purpose: Enable LAC (lookahead correction) to improve syntax
error handling. *Note LAC::.
• Accepted Values: ‘none’, ‘full’
• Default Value: ‘none’
-- Directive: %define parse.trace
• Languages(s): C, C++, Java
• Purpose: Require parser instrumentation for tracing. *Note
Tracing Your Parser: Tracing.
In C/C++, define the macro ‘YYDEBUG’ (or ‘PREFIXDEBUG’ with
‘%define api.prefix {PREFIX}’), see *note Multiple Parsers in
the Same Program: Multiple Parsers.) to 1 in the parser
implementation file if it is not already defined, so that the
debugging facilities are compiled.
• Accepted Values: Boolean
• Default Value: ‘false’
-- Directive: %define parser_class_name {NAME}
Obsoleted by ‘api.parser.class’
File: bison.info, Node: %code Summary, Prev: %define Summary, Up: Declarations
3.7.15 %code Summary
--------------------
The ‘%code’ directive inserts code verbatim into the output parser
source at any of a predefined set of locations. It thus serves as a
flexible and user-friendly alternative to the traditional Yacc prologue,
‘%{CODE%}’. This section summarizes the functionality of ‘%code’ for
the various target languages supported by Bison. For a detailed
discussion of how to use ‘%code’ in place of ‘%{CODE%}’ for C/C++ and
why it is advantageous to do so, *note Prologue Alternatives::.
-- Directive: %code {CODE}
This is the unqualified form of the ‘%code’ directive. It inserts
CODE verbatim at a language-dependent default location in the
parser implementation.
For C/C++, the default location is the parser implementation file
after the usual contents of the parser header file. Thus, the
unqualified form replaces ‘%{CODE%}’ for most purposes.
For Java, the default location is inside the parser class.
-- Directive: %code QUALIFIER {CODE}
This is the qualified form of the ‘%code’ directive. QUALIFIER
identifies the purpose of CODE and thus the location(s) where Bison
should insert it. That is, if you need to specify
location-sensitive CODE that does not belong at the default
location selected by the unqualified ‘%code’ form, use this form
instead.
For any particular qualifier or for the unqualified form, if there
are multiple occurrences of the ‘%code’ directive, Bison concatenates
the specified code in the order in which it appears in the grammar file.
Not all qualifiers are accepted for all target languages. Unaccepted
qualifiers produce an error. Some of the accepted qualifiers are:
‘requires’
• Language(s): C, C++
• Purpose: This is the best place to write dependency code
required for ‘YYSTYPE’ and ‘YYLTYPE’. In other words, it’s
the best place to define types referenced in ‘%union’
directives. If you use ‘#define’ to override Bison’s default
‘YYSTYPE’ and ‘YYLTYPE’ definitions, then it is also the best
place. However you should rather ‘%define’ ‘api.value.type’
and ‘api.location.type’.
• Location(s): The parser header file and the parser
implementation file before the Bison-generated ‘YYSTYPE’ and
‘YYLTYPE’ definitions.
‘provides’
• Language(s): C, C++
• Purpose: This is the best place to write additional
definitions and declarations that should be provided to other
modules.
• Location(s): The parser header file and the parser
implementation file after the Bison-generated ‘YYSTYPE’,
‘YYLTYPE’, and token definitions.
‘top’
• Language(s): C, C++
• Purpose: The unqualified ‘%code’ or ‘%code requires’ should
usually be more appropriate than ‘%code top’. However,
occasionally it is necessary to insert code much nearer the
top of the parser implementation file. For example:
%code top {
#define _GNU_SOURCE
#include
}
• Location(s): Near the top of the parser implementation file.
‘imports’
• Language(s): Java
• Purpose: This is the best place to write Java import
directives.
• Location(s): The parser Java file after any Java package
directive and before any class definitions.
Though we say the insertion locations are language-dependent, they
are technically skeleton-dependent. Writers of non-standard skeletons
however should choose their locations consistently with the behavior of
the standard Bison skeletons.
File: bison.info, Node: Multiple Parsers, Prev: Declarations, Up: Grammar File
3.8 Multiple Parsers in the Same Program
========================================
Most programs that use Bison parse only one language and therefore
contain only one Bison parser. But what if you want to parse more than
one language with the same program? Then you need to avoid name
conflicts between different definitions of functions and variables such
as ‘yyparse’, ‘yylval’. To use different parsers from the same
compilation unit, you also need to avoid conflicts on types and macros
(e.g., ‘YYSTYPE’) exported in the generated header.
The easy way to do this is to define the ‘%define’ variable
‘api.prefix’. With different ‘api.prefix’s it is guaranteed that
headers do not conflict when included together, and that compiled
objects can be linked together too. Specifying ‘%define api.prefix
{PREFIX}’ (or passing the option ‘-Dapi.prefix={PREFIX}’, see *note
Invoking Bison: Invocation.) renames the interface functions and
variables of the Bison parser to start with PREFIX instead of ‘yy’, and
all the macros to start by PREFIX (i.e., PREFIX upper-cased) instead of
‘YY’.
The renamed symbols include ‘yyparse’, ‘yylex’, ‘yyerror’, ‘yynerrs’,
‘yylval’, ‘yylloc’, ‘yychar’ and ‘yydebug’. If you use a push parser,
‘yypush_parse’, ‘yypull_parse’, ‘yypstate’, ‘yypstate_new’ and
‘yypstate_delete’ will also be renamed. The renamed macros include
‘YYSTYPE’, ‘YYLTYPE’, and ‘YYDEBUG’, which is treated specifically —
more about this below.
For example, if you use ‘%define api.prefix {c}’, the names become
‘cparse’, ‘clex’, ..., ‘CSTYPE’, ‘CLTYPE’, and so on.
Users of Flex must update the signature of the generated ‘yylex’
function. Since the Flex scanner usually includes the generated header
of the parser (to get the definitions of the tokens, etc.), the most
convenient way is to insert the declaration of ‘yylex’ in the ‘provides’
section:
%define api.prefix {c}
// Emitted in the header file, after the definition of YYSTYPE.
%code provides
{
// Tell Flex the expected prototype of yylex.
#define YY_DECL \
int clex (CSTYPE *yylval, CLTYPE *yylloc)
// Declare the scanner.
YY_DECL;
}
The ‘%define’ variable ‘api.prefix’ works in two different ways. In
the implementation file, it works by adding macro definitions to the
beginning of the parser implementation file, defining ‘yyparse’ as
‘PREFIXparse’, and so on:
#define YYSTYPE CTYPE
#define yyparse cparse
#define yylval clval
...
YYSTYPE yylval;
int yyparse (void);
This effectively substitutes one name for the other in the entire
parser implementation file, thus the “original” names (‘yylex’,
‘YYSTYPE’, ...) are also usable in the parser implementation file.
However, in the parser header file, the symbols are defined renamed,
for instance:
extern CSTYPE clval;
int cparse (void);
The macro ‘YYDEBUG’ is commonly used to enable the tracing support in
parsers. To comply with this tradition, when ‘api.prefix’ is used,
‘YYDEBUG’ (not renamed) is used as a default value:
/* Debug traces. */
#ifndef CDEBUG
# if defined YYDEBUG
# if YYDEBUG
# define CDEBUG 1
# else
# define CDEBUG 0
# endif
# else
# define CDEBUG 0
# endif
#endif
#if CDEBUG
extern int cdebug;
#endif
Prior to Bison 2.6, a feature similar to ‘api.prefix’ was provided by
the obsolete directive ‘%name-prefix’ (*note Bison Symbols: Table of
Symbols.) and the option ‘--name-prefix’ (*note Bison Options::).
File: bison.info, Node: Interface, Next: Algorithm, Prev: Grammar File, Up: Top
4 Parser C-Language Interface
*****************************
The Bison parser is actually a C function named ‘yyparse’. Here we
describe the interface conventions of ‘yyparse’ and the other functions
that it needs to use.
Keep in mind that the parser uses many C identifiers starting with
‘yy’ and ‘YY’ for internal purposes. If you use such an identifier
(aside from those in this manual) in an action or in epilogue in the
grammar file, you are likely to run into trouble.
* Menu:
* Parser Function:: How to call ‘yyparse’ and what it returns.
* Push Parser Function:: How to call ‘yypush_parse’ and what it returns.
* Pull Parser Function:: How to call ‘yypull_parse’ and what it returns.
* Parser Create Function:: How to call ‘yypstate_new’ and what it returns.
* Parser Delete Function:: How to call ‘yypstate_delete’ and what it returns.
* Lexical:: You must supply a function ‘yylex’
which reads tokens.
* Error Reporting:: You must supply a function ‘yyerror’.
* Action Features:: Special features for use in actions.
* Internationalization:: How to let the parser speak in the user’s
native language.
File: bison.info, Node: Parser Function, Next: Push Parser Function, Up: Interface
4.1 The Parser Function ‘yyparse’
=================================
You call the function ‘yyparse’ to cause parsing to occur. This
function reads tokens, executes actions, and ultimately returns when it
encounters end-of-input or an unrecoverable syntax error. You can also
write an action which directs ‘yyparse’ to return immediately without
reading further.
-- Function: int yyparse (void)
The value returned by ‘yyparse’ is 0 if parsing was successful
(return is due to end-of-input).
The value is 1 if parsing failed because of invalid input, i.e.,
input that contains a syntax error or that causes ‘YYABORT’ to be
invoked.
The value is 2 if parsing failed due to memory exhaustion.
In an action, you can cause immediate return from ‘yyparse’ by using
these macros:
-- Macro: YYACCEPT
Return immediately with value 0 (to report success).
-- Macro: YYABORT
Return immediately with value 1 (to report failure).
If you use a reentrant parser, you can optionally pass additional
parameter information to it in a reentrant way. To do so, use the
declaration ‘%parse-param’:
-- Directive: %parse-param {ARGUMENT-DECLARATION} ...
Declare that one or more ARGUMENT-DECLARATION are additional
‘yyparse’ arguments. The ARGUMENT-DECLARATION is used when
declaring functions or prototypes. The last identifier in
ARGUMENT-DECLARATION must be the argument name.
Here’s an example. Write this in the parser:
%parse-param {int *nastiness} {int *randomness}
Then call the parser like this:
{
int nastiness, randomness;
... /* Store proper data in ‘nastiness’ and ‘randomness’. */
value = yyparse (&nastiness, &randomness);
...
}
In the grammar actions, use expressions like this to refer to the data:
exp: ... { ...; *randomness += 1; ... }
Using the following:
%parse-param {int *randomness}
Results in these signatures:
void yyerror (int *randomness, const char *msg);
int yyparse (int *randomness);
Or, if both ‘%define api.pure full’ (or just ‘%define api.pure’) and
‘%locations’ are used:
void yyerror (YYLTYPE *llocp, int *randomness, const char *msg);
int yyparse (int *randomness);
File: bison.info, Node: Push Parser Function, Next: Pull Parser Function, Prev: Parser Function, Up: Interface
4.2 The Push Parser Function ‘yypush_parse’
===========================================
You call the function ‘yypush_parse’ to parse a single token. This
function is available if either the ‘%define api.push-pull push’ or
‘%define api.push-pull both’ declaration is used. *Note A Push Parser:
Push Decl.
-- Function: int yypush_parse (yypstate *YYPS)
The value returned by ‘yypush_parse’ is the same as for ‘yyparse’
with the following exception: it returns ‘YYPUSH_MORE’ if more
input is required to finish parsing the grammar.
After ‘yypush_parse’ returns a status other than ‘YYPUSH_MORE’, the
parser instance ‘yyps’ may be reused for a new parse.
The fact that the parser state is reusable even after an error
simplifies reuse. For example, a calculator application which parses
each input line as an expression can just keep reusing the same ‘yyps’
even if an input was invalid.
File: bison.info, Node: Pull Parser Function, Next: Parser Create Function, Prev: Push Parser Function, Up: Interface
4.3 The Pull Parser Function ‘yypull_parse’
===========================================
You call the function ‘yypull_parse’ to parse the rest of the input
stream. This function is available if the ‘%define api.push-pull both’
declaration is used. *Note A Push Parser: Push Decl.
-- Function: int yypull_parse (yypstate *YYPS)
The value returned by ‘yypull_parse’ is the same as for ‘yyparse’.
The parser instance ‘yyps’ may be reused for new parses.
File: bison.info, Node: Parser Create Function, Next: Parser Delete Function, Prev: Pull Parser Function, Up: Interface
4.4 The Parser Create Function ‘yystate_new’
============================================
You call the function ‘yypstate_new’ to create a new parser instance.
This function is available if either the ‘%define api.push-pull push’ or
‘%define api.push-pull both’ declaration is used. *Note A Push Parser:
Push Decl.
-- Function: yypstate* yypstate_new (void)
The function will return a valid parser instance if there was
memory available or 0 if no memory was available. In impure mode,
it will also return 0 if a parser instance is currently allocated.
File: bison.info, Node: Parser Delete Function, Next: Lexical, Prev: Parser Create Function, Up: Interface
4.5 The Parser Delete Function ‘yystate_delete’
===============================================
You call the function ‘yypstate_delete’ to delete a parser instance.
function is available if either the ‘%define api.push-pull push’ or
‘%define api.push-pull both’ declaration is used. *Note A Push Parser:
Push Decl.
-- Function: void yypstate_delete (yypstate *YYPS)
This function will reclaim the memory associated with a parser
instance. After this call, you should no longer attempt to use the
parser instance.
File: bison.info, Node: Lexical, Next: Error Reporting, Prev: Parser Delete Function, Up: Interface
4.6 The Lexical Analyzer Function ‘yylex’
=========================================
The “lexical analyzer” function, ‘yylex’, recognizes tokens from the
input stream and returns them to the parser. Bison does not create this
function automatically; you must write it so that ‘yyparse’ can call it.
The function is sometimes referred to as a lexical scanner.
In simple programs, ‘yylex’ is often defined at the end of the Bison
grammar file. If ‘yylex’ is defined in a separate source file, you need
to arrange for the token-type macro definitions to be available there.
To do this, use the ‘-d’ option when you run Bison, so that it will
write these macro definitions into the separate parser header file,
‘NAME.tab.h’, which you can include in the other source files that need
it. *Note Invoking Bison: Invocation.
* Menu:
* Calling Convention:: How ‘yyparse’ calls ‘yylex’.
* Token Values:: How ‘yylex’ must return the semantic value
of the token it has read.
* Token Locations:: How ‘yylex’ must return the text location
(line number, etc.) of the token, if the
actions want that.
* Pure Calling:: How the calling convention differs in a pure parser
(*note A Pure (Reentrant) Parser: Pure Decl.).
File: bison.info, Node: Calling Convention, Next: Token Values, Up: Lexical
4.6.1 Calling Convention for ‘yylex’
------------------------------------
The value that ‘yylex’ returns must be the positive numeric code for the
type of token it has just found; a zero or negative value signifies
end-of-input.
When a token is referred to in the grammar rules by a name, that name
in the parser implementation file becomes a C macro whose definition is
the proper numeric code for that token type. So ‘yylex’ can use the
name to indicate that type. *Note Symbols::.
When a token is referred to in the grammar rules by a character
literal, the numeric code for that character is also the code for the
token type. So ‘yylex’ can simply return that character code, possibly
converted to ‘unsigned char’ to avoid sign-extension. The null
character must not be used this way, because its code is zero and that
signifies end-of-input.
Here is an example showing these things:
int
yylex (void)
{
...
if (c == EOF) /* Detect end-of-input. */
return 0;
...
if (c == '+' || c == '-')
return c; /* Assume token type for '+' is '+'. */
...
return INT; /* Return the type of the token. */
...
}
This interface has been designed so that the output from the ‘lex’
utility can be used without change as the definition of ‘yylex’.
If the grammar uses literal string tokens, there are two ways that
‘yylex’ can determine the token type codes for them:
• If the grammar defines symbolic token names as aliases for the
literal string tokens, ‘yylex’ can use these symbolic names like
all others. In this case, the use of the literal string tokens in
the grammar file has no effect on ‘yylex’.
• ‘yylex’ can find the multicharacter token in the ‘yytname’ table.
The index of the token in the table is the token type’s code. The
name of a multicharacter token is recorded in ‘yytname’ with a
double-quote, the token’s characters, and another double-quote.
The token’s characters are escaped as necessary to be suitable as
input to Bison.
Here’s code for looking up a multicharacter token in ‘yytname’,
assuming that the characters of the token are stored in
‘token_buffer’, and assuming that the token does not contain any
characters like ‘"’ that require escaping.
for (int i = 0; i < YYNTOKENS; i++)
{
if (yytname[i]
&& yytname[i][0] == '"'
&& ! strncmp (yytname[i] + 1, token_buffer,
strlen (token_buffer))
&& yytname[i][strlen (token_buffer) + 1] == '"'
&& yytname[i][strlen (token_buffer) + 2] == 0)
break;
}
The ‘yytname’ table is generated only if you use the ‘%token-table’
declaration. *Note Decl Summary::.
File: bison.info, Node: Token Values, Next: Token Locations, Prev: Calling Convention, Up: Lexical
4.6.2 Semantic Values of Tokens
-------------------------------
In an ordinary (nonreentrant) parser, the semantic value of the token
must be stored into the global variable ‘yylval’. When you are using
just one data type for semantic values, ‘yylval’ has that type. Thus,
if the type is ‘int’ (the default), you might write this in ‘yylex’:
...
yylval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
When you are using multiple data types, ‘yylval’’s type is a union
made from the ‘%union’ declaration (*note The Union Declaration: Union
Decl.). So when you store a token’s value, you must use the proper
member of the union. If the ‘%union’ declaration looks like this:
%union {
int intval;
double val;
symrec *tptr;
}
then the code in ‘yylex’ might look like this:
...
yylval.intval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
File: bison.info, Node: Token Locations, Next: Pure Calling, Prev: Token Values, Up: Lexical
4.6.3 Textual Locations of Tokens
---------------------------------
If you are using the ‘@N’-feature (*note Tracking Locations::) in
actions to keep track of the textual locations of tokens and groupings,
then you must provide this information in ‘yylex’. The function
‘yyparse’ expects to find the textual location of a token just parsed in
the global variable ‘yylloc’. So ‘yylex’ must store the proper data in
that variable.
By default, the value of ‘yylloc’ is a structure and you need only
initialize the members that are going to be used by the actions. The
four members are called ‘first_line’, ‘first_column’, ‘last_line’ and
‘last_column’. Note that the use of this feature makes the parser
noticeably slower.
The data type of ‘yylloc’ has the name ‘YYLTYPE’.
File: bison.info, Node: Pure Calling, Prev: Token Locations, Up: Lexical
4.6.4 Calling Conventions for Pure Parsers
------------------------------------------
When you use the Bison declaration ‘%define api.pure full’ to request a
pure, reentrant parser, the global communication variables ‘yylval’ and
‘yylloc’ cannot be used. (*Note A Pure (Reentrant) Parser: Pure Decl.)
In such parsers the two global variables are replaced by pointers passed
as arguments to ‘yylex’. You must declare them as shown here, and pass
the information back by storing it through those pointers.
int
yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
{
...
*lvalp = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
}
If the grammar file does not use the ‘@’ constructs to refer to
textual locations, then the type ‘YYLTYPE’ will not be defined. In this
case, omit the second argument; ‘yylex’ will be called with only one
argument.
If you wish to pass additional arguments to ‘yylex’, use ‘%lex-param’
just like ‘%parse-param’ (*note Parser Function::). To pass additional
arguments to both ‘yylex’ and ‘yyparse’, use ‘%param’.
-- Directive: %lex-param {ARGUMENT-DECLARATION} ...
Specify that ARGUMENT-DECLARATION are additional ‘yylex’ argument
declarations. You may pass one or more such declarations, which is
equivalent to repeating ‘%lex-param’.
-- Directive: %param {ARGUMENT-DECLARATION} ...
Specify that ARGUMENT-DECLARATION are additional ‘yylex’/‘yyparse’
argument declaration. This is equivalent to ‘%lex-param
{ARGUMENT-DECLARATION} ... %parse-param {ARGUMENT-DECLARATION}
...’. You may pass one or more declarations, which is equivalent
to repeating ‘%param’.
For instance:
%lex-param {scanner_mode *mode}
%parse-param {parser_mode *mode}
%param {environment_type *env}
results in the following signatures:
int yylex (scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);
If ‘%define api.pure full’ is added:
int yylex (YYSTYPE *lvalp, scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);
and finally, if both ‘%define api.pure full’ and ‘%locations’ are used:
int yylex (YYSTYPE *lvalp, YYLTYPE *llocp,
scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);
File: bison.info, Node: Error Reporting, Next: Action Features, Prev: Lexical, Up: Interface
4.7 The Error Reporting Function ‘yyerror’
==========================================
The Bison parser detects a “syntax error” (or “parse error”) whenever it
reads a token which cannot satisfy any syntax rule. An action in the
grammar can also explicitly proclaim an error, using the macro ‘YYERROR’
(*note Special Features for Use in Actions: Action Features.).
The Bison parser expects to report the error by calling an error
reporting function named ‘yyerror’, which you must supply. It is called
by ‘yyparse’ whenever a syntax error is found, and it receives one
argument. For a syntax error, the string is normally ‘"syntax error"’.
If you invoke ‘%define parse.error verbose’ in the Bison declarations
section (*note The Bison Declarations Section: Bison Declarations.),
then Bison provides a more verbose and specific error message string
instead of just plain ‘"syntax error"’. However, that message sometimes
contains incorrect information if LAC is not enabled (*note LAC::).
The parser can detect one other kind of error: memory exhaustion.
This can happen when the input contains constructions that are very
deeply nested. It isn’t likely you will encounter this, since the Bison
parser normally extends its stack automatically up to a very large
limit. But if memory is exhausted, ‘yyparse’ calls ‘yyerror’ in the
usual fashion, except that the argument string is ‘"memory exhausted"’.
In some cases diagnostics like ‘"syntax error"’ are translated
automatically from English to some other language before they are passed
to ‘yyerror’. *Note Internationalization::.
The following definition suffices in simple programs:
void
yyerror (char const *s)
{
fprintf (stderr, "%s\n", s);
}
After ‘yyerror’ returns to ‘yyparse’, the latter will attempt error
recovery if you have written suitable error recovery grammar rules
(*note Error Recovery::). If recovery is impossible, ‘yyparse’ will
immediately return 1.
Obviously, in location tracking pure parsers, ‘yyerror’ should have
an access to the current location. With ‘%define api.pure’, this is
indeed the case for the GLR parsers, but not for the Yacc parser, for
historical reasons, and this is the why ‘%define api.pure full’ should
be preferred over ‘%define api.pure’.
When ‘%locations %define api.pure full’ is used, ‘yyerror’ has the
following signature:
void yyerror (YYLTYPE *locp, char const *msg);
The prototypes are only indications of how the code produced by Bison
uses ‘yyerror’. Bison-generated code always ignores the returned value,
so ‘yyerror’ can return any type, including ‘void’. Also, ‘yyerror’ can
be a variadic function; that is why the message is always passed last.
Traditionally ‘yyerror’ returns an ‘int’ that is always ignored, but
this is purely for historical reasons, and ‘void’ is preferable since it
more accurately describes the return type for ‘yyerror’.
The variable ‘yynerrs’ contains the number of syntax errors reported
so far. Normally this variable is global; but if you request a pure
parser (*note A Pure (Reentrant) Parser: Pure Decl.) then it is a local
variable which only the actions can access.
File: bison.info, Node: Action Features, Next: Internationalization, Prev: Error Reporting, Up: Interface
4.8 Special Features for Use in Actions
=======================================
Here is a table of Bison constructs, variables and macros that are
useful in actions.
-- Variable: $$
Acts like a variable that contains the semantic value for the
grouping made by the current rule. *Note Actions::.
-- Variable: $N
Acts like a variable that contains the semantic value for the Nth
component of the current rule. *Note Actions::.
-- Variable: $$
Like ‘$$’ but specifies alternative TYPEALT in the union specified
by the ‘%union’ declaration. *Note Data Types of Values in
Actions: Action Types.
-- Variable: $N
Like ‘$N’ but specifies alternative TYPEALT in the union specified
by the ‘%union’ declaration. *Note Data Types of Values in
Actions: Action Types.
-- Macro: YYABORT ‘;’
Return immediately from ‘yyparse’, indicating failure. *Note The
Parser Function ‘yyparse’: Parser Function.
-- Macro: YYACCEPT ‘;’
Return immediately from ‘yyparse’, indicating success. *Note The
Parser Function ‘yyparse’: Parser Function.
-- Macro: YYBACKUP (TOKEN, VALUE)‘;’
Unshift a token. This macro is allowed only for rules that reduce
a single value, and only when there is no lookahead token. It is
also disallowed in GLR parsers. It installs a lookahead token with
token type TOKEN and semantic value VALUE; then it discards the
value that was going to be reduced by this rule.
If the macro is used when it is not valid, such as when there is a
lookahead token already, then it reports a syntax error with a
message ‘cannot back up’ and performs ordinary error recovery.
In either case, the rest of the action is not executed.
-- Macro: YYEMPTY
Value stored in ‘yychar’ when there is no lookahead token.
-- Macro: YYEOF
Value stored in ‘yychar’ when the lookahead is the end of the input
stream.
-- Macro: YYERROR ‘;’
Cause an immediate syntax error. This statement initiates error
recovery just as if the parser itself had detected an error;
however, it does not call ‘yyerror’, and does not print any
message. If you want to print an error message, call ‘yyerror’
explicitly before the ‘YYERROR;’ statement. *Note Error
Recovery::.
-- Macro: YYRECOVERING
The expression ‘YYRECOVERING ()’ yields 1 when the parser is
recovering from a syntax error, and 0 otherwise. *Note Error
Recovery::.
-- Variable: yychar
Variable containing either the lookahead token, or ‘YYEOF’ when the
lookahead is the end of the input stream, or ‘YYEMPTY’ when no
lookahead has been performed so the next token is not yet known.
Do not modify ‘yychar’ in a deferred semantic action (*note GLR
Semantic Actions::). *Note Lookahead Tokens: Lookahead.
-- Macro: yyclearin ‘;’
Discard the current lookahead token. This is useful primarily in
error rules. Do not invoke ‘yyclearin’ in a deferred semantic
action (*note GLR Semantic Actions::). *Note Error Recovery::.
-- Macro: yyerrok ‘;’
Resume generating error messages immediately for subsequent syntax
errors. This is useful primarily in error rules. *Note Error
Recovery::.
-- Variable: yylloc
Variable containing the lookahead token location when ‘yychar’ is
not set to ‘YYEMPTY’ or ‘YYEOF’. Do not modify ‘yylloc’ in a
deferred semantic action (*note GLR Semantic Actions::). *Note
Actions and Locations: Actions and Locations.
-- Variable: yylval
Variable containing the lookahead token semantic value when
‘yychar’ is not set to ‘YYEMPTY’ or ‘YYEOF’. Do not modify
‘yylval’ in a deferred semantic action (*note GLR Semantic
Actions::). *Note Actions: Actions.
-- Value: @$
Acts like a structure variable containing information on the
textual location of the grouping made by the current rule. *Note
Tracking Locations::.
-- Value: @N
Acts like a structure variable containing information on the
textual location of the Nth component of the current rule. *Note
Tracking Locations::.
File: bison.info, Node: Internationalization, Prev: Action Features, Up: Interface
4.9 Parser Internationalization
===============================
A Bison-generated parser can print diagnostics, including error and
tracing messages. By default, they appear in English. However, Bison
also supports outputting diagnostics in the user’s native language. To
make this work, the user should set the usual environment variables.
*Note The User’s View: (gettext)Users. For example, the shell command
‘export LC_ALL=fr_CA.UTF-8’ might set the user’s locale to French
Canadian using the UTF-8 encoding. The exact set of available locales
depends on the user’s installation.
The maintainer of a package that uses a Bison-generated parser
enables the internationalization of the parser’s output through the
following steps. Here we assume a package that uses GNU Autoconf and
GNU Automake.
1. Into the directory containing the GNU Autoconf macros used by the
package —often called ‘m4’— copy the ‘bison-i18n.m4’ file installed
by Bison under ‘share/aclocal/bison-i18n.m4’ in Bison’s
installation directory. For example:
cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4
2. In the top-level ‘configure.ac’, after the ‘AM_GNU_GETTEXT’
invocation, add an invocation of ‘BISON_I18N’. This macro is
defined in the file ‘bison-i18n.m4’ that you copied earlier. It
causes ‘configure’ to find the value of the ‘BISON_LOCALEDIR’
variable, and it defines the source-language symbol ‘YYENABLE_NLS’
to enable translations in the Bison-generated parser.
3. In the ‘main’ function of your program, designate the directory
containing Bison’s runtime message catalog, through a call to
‘bindtextdomain’ with domain name ‘bison-runtime’. For example:
bindtextdomain ("bison-runtime", BISON_LOCALEDIR);
Typically this appears after any other call ‘bindtextdomain
(PACKAGE, LOCALEDIR)’ that your package already has. Here we rely
on ‘BISON_LOCALEDIR’ to be defined as a string through the
‘Makefile’.
4. In the ‘Makefile.am’ that controls the compilation of the ‘main’
function, make ‘BISON_LOCALEDIR’ available as a C preprocessor
macro, either in ‘DEFS’ or in ‘AM_CPPFLAGS’. For example:
DEFS = @DEFS@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
or:
AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
5. Finally, invoke the command ‘autoreconf’ to generate the build
infrastructure.
File: bison.info, Node: Algorithm, Next: Error Recovery, Prev: Interface, Up: Top
5 The Bison Parser Algorithm
****************************
As Bison reads tokens, it pushes them onto a stack along with their
semantic values. The stack is called the “parser stack”. Pushing a
token is traditionally called “shifting”.
For example, suppose the infix calculator has read ‘1 + 5 *’, with a
‘3’ to come. The stack will have four elements, one for each token that
was shifted.
But the stack does not always have an element for each token read.
When the last N tokens and groupings shifted match the components of a
grammar rule, they can be combined according to that rule. This is
called “reduction”. Those tokens and groupings are replaced on the
stack by a single grouping whose symbol is the result (left hand side)
of that rule. Running the rule’s action is part of the process of
reduction, because this is what computes the semantic value of the
resulting grouping.
For example, if the infix calculator’s parser stack contains this:
1 + 5 * 3
and the next input token is a newline character, then the last three
elements can be reduced to 15 via the rule:
expr: expr '*' expr;
Then the stack contains just these three elements:
1 + 15
At this point, another reduction can be made, resulting in the single
value 16. Then the newline token can be shifted.
The parser tries, by shifts and reductions, to reduce the entire
input down to a single grouping whose symbol is the grammar’s
start-symbol (*note Languages and Context-Free Grammars: Language and
Grammar.).
This kind of parser is known in the literature as a bottom-up parser.
* Menu:
* Lookahead:: Parser looks one token ahead when deciding what to do.
* Shift/Reduce:: Conflicts: when either shifting or reduction is valid.
* Precedence:: Operator precedence works by resolving conflicts.
* Contextual Precedence:: When an operator’s precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
* Mysterious Conflicts:: Conflicts that look unjustified.
* Tuning LR:: How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted. How to avoid it.
File: bison.info, Node: Lookahead, Next: Shift/Reduce, Up: Algorithm
5.1 Lookahead Tokens
====================
The Bison parser does _not_ always reduce immediately as soon as the
last N tokens and groupings match a rule. This is because such a simple
strategy is inadequate to handle most languages. Instead, when a
reduction is possible, the parser sometimes “looks ahead” at the next
token in order to decide what to do.
When a token is read, it is not immediately shifted; first it becomes
the “lookahead token”, which is not on the stack. Now the parser can
perform one or more reductions of tokens and groupings on the stack,
while the lookahead token remains off to the side. When no more
reductions should take place, the lookahead token is shifted onto the
stack. This does not mean that all possible reductions have been done;
depending on the token type of the lookahead token, some rules may
choose to delay their application.
Here is a simple case where lookahead is needed. These three rules
define expressions which contain binary addition operators and postfix
unary factorial operators (‘!’), and allow parentheses for grouping.
expr:
term '+' expr
| term
;
term:
'(' expr ')'
| term '!'
| "number"
;
Suppose that the tokens ‘1 + 2’ have been read and shifted; what
should be done? If the following token is ‘)’, then the first three
tokens must be reduced to form an ‘expr’. This is the only valid
course, because shifting the ‘)’ would produce a sequence of symbols
‘term ')'’, and no rule allows this.
If the following token is ‘!’, then it must be shifted immediately so
that ‘2 !’ can be reduced to make a ‘term’. If instead the parser were
to reduce before shifting, ‘1 + 2’ would become an ‘expr’. It would
then be impossible to shift the ‘!’ because doing so would produce on
the stack the sequence of symbols ‘expr '!'’. No rule allows that
sequence.
The lookahead token is stored in the variable ‘yychar’. Its semantic
value and location, if any, are stored in the variables ‘yylval’ and
‘yylloc’. *Note Special Features for Use in Actions: Action Features.
File: bison.info, Node: Shift/Reduce, Next: Precedence, Prev: Lookahead, Up: Algorithm
5.2 Shift/Reduce Conflicts
==========================
Suppose we are parsing a language which has if-then and if-then-else
statements, with a pair of rules like this:
if_stmt:
"if" expr "then" stmt
| "if" expr "then" stmt "else" stmt
;
Here ‘"if"’, ‘"then"’ and ‘"else"’ are terminal symbols for specific
keyword tokens.
When the ‘"else"’ token is read and becomes the lookahead token, the
contents of the stack (assuming the input is valid) are just right for
reduction by the first rule. But it is also legitimate to shift the
‘"else"’, because that would lead to eventual reduction by the second
rule.
This situation, where either a shift or a reduction would be valid,
is called a “shift/reduce conflict”. Bison is designed to resolve these
conflicts by choosing to shift, unless otherwise directed by operator
precedence declarations. To see the reason for this, let’s contrast it
with the other alternative.
Since the parser prefers to shift the ‘"else"’, the result is to
attach the else-clause to the innermost if-statement, making these two
inputs equivalent:
if x then if y then win; else lose;
if x then do; if y then win; else lose; end;
But if the parser chose to reduce when possible rather than shift,
the result would be to attach the else-clause to the outermost
if-statement, making these two inputs equivalent:
if x then if y then win; else lose;
if x then do; if y then win; end; else lose;
The conflict exists because the grammar as written is ambiguous:
either parsing of the simple nested if-statement is legitimate. The
established convention is that these ambiguities are resolved by
attaching the else-clause to the innermost if-statement; this is what
Bison accomplishes by choosing to shift rather than reduce. (It would
ideally be cleaner to write an unambiguous grammar, but that is very
hard to do in this case.) This particular ambiguity was first
encountered in the specifications of Algol 60 and is called the
“dangling ‘else’” ambiguity.
To avoid warnings from Bison about predictable, legitimate
shift/reduce conflicts, you can use the ‘%expect N’ declaration. There
will be no warning as long as the number of shift/reduce conflicts is
exactly N, and Bison will report an error if there is a different
number. *Note Suppressing Conflict Warnings: Expect Decl. However, we
don’t recommend the use of ‘%expect’ (except ‘%expect 0’!), as an equal
number of conflicts does not mean that they are the _same_. When
possible, you should rather use precedence directives to _fix_ the
conflicts explicitly (*note Using Precedence For Non Operators: Non
Operators.).
The definition of ‘if_stmt’ above is solely to blame for the
conflict, but the conflict does not actually appear without additional
rules. Here is a complete Bison grammar file that actually manifests
the conflict:
%%
stmt:
expr
| if_stmt
;
if_stmt:
"if" expr "then" stmt
| "if" expr "then" stmt "else" stmt
;
expr:
"identifier"
;
File: bison.info, Node: Precedence, Next: Contextual Precedence, Prev: Shift/Reduce, Up: Algorithm
5.3 Operator Precedence
=======================
Another situation where shift/reduce conflicts appear is in arithmetic
expressions. Here shifting is not always the preferred resolution; the
Bison declarations for operator precedence allow you to specify when to
shift and when to reduce.
* Menu:
* Why Precedence:: An example showing why precedence is needed.
* Using Precedence:: How to specify precedence and associativity.
* Precedence Only:: How to specify precedence only.
* Precedence Examples:: How these features are used in the previous example.
* How Precedence:: How they work.
* Non Operators:: Using precedence for general conflicts.
File: bison.info, Node: Why Precedence, Next: Using Precedence, Up: Precedence
5.3.1 When Precedence is Needed
-------------------------------
Consider the following ambiguous grammar fragment (ambiguous because the
input ‘1 - 2 * 3’ can be parsed in two different ways):
expr:
expr '-' expr
| expr '*' expr
| expr '<' expr
| '(' expr ')'
...
;
Suppose the parser has seen the tokens ‘1’, ‘-’ and ‘2’; should it
reduce them via the rule for the subtraction operator? It depends on
the next token. Of course, if the next token is ‘)’, we must reduce;
shifting is invalid because no single rule can reduce the token sequence
‘- 2 )’ or anything starting with that. But if the next token is ‘*’ or
‘<’, we have a choice: either shifting or reduction would allow the
parse to complete, but with different results.
To decide which one Bison should do, we must consider the results.
If the next operator token OP is shifted, then it must be reduced first
in order to permit another opportunity to reduce the difference. The
result is (in effect) ‘1 - (2 OP 3)’. On the other hand, if the
subtraction is reduced before shifting OP, the result is ‘(1 - 2) OP 3’.
Clearly, then, the choice of shift or reduce should depend on the
relative precedence of the operators ‘-’ and OP: ‘*’ should be shifted
first, but not ‘<’.
What about input such as ‘1 - 2 - 5’; should this be ‘(1 - 2) - 5’ or
should it be ‘1 - (2 - 5)’? For most operators we prefer the former,
which is called “left association”. The latter alternative, “right
association”, is desirable for assignment operators. The choice of left
or right association is a matter of whether the parser chooses to shift
or reduce when the stack contains ‘1 - 2’ and the lookahead token is
‘-’: shifting makes right-associativity.
File: bison.info, Node: Using Precedence, Next: Precedence Only, Prev: Why Precedence, Up: Precedence
5.3.2 Specifying Operator Precedence
------------------------------------
Bison allows you to specify these choices with the operator precedence
declarations ‘%left’ and ‘%right’. Each such declaration contains a
list of tokens, which are operators whose precedence and associativity
is being declared. The ‘%left’ declaration makes all those operators
left-associative and the ‘%right’ declaration makes them
right-associative. A third alternative is ‘%nonassoc’, which declares
that it is a syntax error to find the same operator twice “in a row”.
The last alternative, ‘%precedence’, allows to define only precedence
and no associativity at all. As a result, any associativity-related
conflict that remains will be reported as an compile-time error. The
directive ‘%nonassoc’ creates run-time error: using the operator in a
associative way is a syntax error. The directive ‘%precedence’ creates
compile-time errors: an operator _can_ be involved in an
associativity-related conflict, contrary to what expected the grammar
author.
The relative precedence of different operators is controlled by the
order in which they are declared. The first precedence/associativity
declaration in the file declares the operators whose precedence is
lowest, the next such declaration declares the operators whose
precedence is a little higher, and so on.
File: bison.info, Node: Precedence Only, Next: Precedence Examples, Prev: Using Precedence, Up: Precedence
5.3.3 Specifying Precedence Only
--------------------------------
Since POSIX Yacc defines only ‘%left’, ‘%right’, and ‘%nonassoc’, which
all defines precedence and associativity, little attention is paid to
the fact that precedence cannot be defined without defining
associativity. Yet, sometimes, when trying to solve a conflict,
precedence suffices. In such a case, using ‘%left’, ‘%right’, or
‘%nonassoc’ might hide future (associativity related) conflicts that
would remain hidden.
The dangling ‘else’ ambiguity (*note Shift/Reduce Conflicts:
Shift/Reduce.) can be solved explicitly. This shift/reduce conflicts
occurs in the following situation, where the period denotes the current
parsing state:
if E1 then if E2 then S1 . else S2
The conflict involves the reduction of the rule ‘IF expr THEN stmt’,
which precedence is by default that of its last token (‘THEN’), and the
shifting of the token ‘ELSE’. The usual disambiguation (attach the
‘else’ to the closest ‘if’), shifting must be preferred, i.e., the
precedence of ‘ELSE’ must be higher than that of ‘THEN’. But neither is
expected to be involved in an associativity related conflict, which can
be specified as follows.
%precedence THEN
%precedence ELSE
The unary-minus is another typical example where associativity is
usually over-specified, see *note Infix Notation Calculator - ‘calc’:
Infix Calc. The ‘%left’ directive is traditionally used to declare the
precedence of ‘NEG’, which is more than needed since it also defines its
associativity. While this is harmless in the traditional example, who
knows how ‘NEG’ might be used in future evolutions of the grammar...
File: bison.info, Node: Precedence Examples, Next: How Precedence, Prev: Precedence Only, Up: Precedence
5.3.4 Precedence Examples
-------------------------
In our example, we would want the following declarations:
%left '<'
%left '-'
%left '*'
In a more complete example, which supports other operators as well,
we would declare them in groups of equal precedence. For example, ‘'+'’
is declared with ‘'-'’:
%left '<' '>' '=' "!=" "<=" ">="
%left '+' '-'
%left '*' '/'
File: bison.info, Node: How Precedence, Next: Non Operators, Prev: Precedence Examples, Up: Precedence
5.3.5 How Precedence Works
--------------------------
The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared. The second effect is to assign
precedence levels to certain rules: each rule gets its precedence from
the last terminal symbol mentioned in the components. (You can also
specify explicitly the precedence of a rule. *Note Context-Dependent
Precedence: Contextual Precedence.)
Finally, the resolution of conflicts works by comparing the
precedence of the rule being considered with that of the lookahead
token. If the token’s precedence is higher, the choice is to shift. If
the rule’s precedence is higher, the choice is to reduce. If they have
equal precedence, the choice is made based on the associativity of that
precedence level. The verbose output file made by ‘-v’ (*note Invoking
Bison: Invocation.) says how each conflict was resolved.
Not all rules and not all tokens have precedence. If either the rule
or the lookahead token has no precedence, then the default is to shift.
File: bison.info, Node: Non Operators, Prev: How Precedence, Up: Precedence
5.3.6 Using Precedence For Non Operators
----------------------------------------
Using properly precedence and associativity directives can help fixing
shift/reduce conflicts that do not involve arithmetics-like operators.
For instance, the “dangling ‘else’” problem (*note Shift/Reduce
Conflicts: Shift/Reduce.) can be solved elegantly in two different ways.
In the present case, the conflict is between the token ‘"else"’
willing to be shifted, and the rule ‘if_stmt: "if" expr "then" stmt’,
asking for reduction. By default, the precedence of a rule is that of
its last token, here ‘"then"’, so the conflict will be solved
appropriately by giving ‘"else"’ a precedence higher than that of
‘"then"’, for instance as follows:
%precedence "then"
%precedence "else"
Alternatively, you may give both tokens the same precedence, in which
case associativity is used to solve the conflict. To preserve the shift
action, use right associativity:
%right "then" "else"
Neither solution is perfect however. Since Bison does not provide,
so far, “scoped” precedence, both force you to declare the precedence of
these keywords with respect to the other operators your grammar.
Therefore, instead of being warned about new conflicts you would be
unaware of (e.g., a shift/reduce conflict due to ‘if test then 1 else 2
+ 3’ being ambiguous: ‘if test then 1 else (2 + 3)’ or ‘(if test then 1
else 2) + 3’?), the conflict will be already “fixed”.
File: bison.info, Node: Contextual Precedence, Next: Parser States, Prev: Precedence, Up: Algorithm
5.4 Context-Dependent Precedence
================================
Often the precedence of an operator depends on the context. This sounds
outlandish at first, but it is really very common. For example, a minus
sign typically has a very high precedence as a unary operator, and a
somewhat lower precedence (lower than multiplication) as a binary
operator.
The Bison precedence declarations can only be used once for a given
token; so a token has only one precedence declared in this way. For
context-dependent precedence, you need to use an additional mechanism:
the ‘%prec’ modifier for rules.
The ‘%prec’ modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that
rule. It’s not necessary for that symbol to appear otherwise in the
rule. The modifier’s syntax is:
%prec TERMINAL-SYMBOL
and it is written after the components of the rule. Its effect is to
assign the rule the precedence of TERMINAL-SYMBOL, overriding the
precedence that would be deduced for it in the ordinary way. The
altered rule precedence then affects how conflicts involving that rule
are resolved (*note Operator Precedence: Precedence.).
Here is how ‘%prec’ solves the problem of unary minus. First,
declare a precedence for a fictitious terminal symbol named ‘UMINUS’.
There are no tokens of this type, but the symbol serves to stand for its
precedence:
...
%left '+' '-'
%left '*'
%left UMINUS
Now the precedence of ‘UMINUS’ can be used in specific rules:
exp:
...
| exp '-' exp
...
| '-' exp %prec UMINUS
File: bison.info, Node: Parser States, Next: Reduce/Reduce, Prev: Contextual Precedence, Up: Algorithm
5.5 Parser States
=================
The function ‘yyparse’ is implemented using a finite-state machine. The
values pushed on the parser stack are not simply token type codes; they
represent the entire sequence of terminal and nonterminal symbols at or
near the top of the stack. The current state collects all the
information about previous input which is relevant to deciding what to
do next.
Each time a lookahead token is read, the current parser state
together with the type of lookahead token are looked up in a table.
This table entry can say, “Shift the lookahead token.” In this case, it
also specifies the new parser state, which is pushed onto the top of the
parser stack. Or it can say, “Reduce using rule number N.” This means
that a certain number of tokens or groupings are taken off the top of
the stack, and replaced by one grouping. In other words, that number of
states are popped from the stack, and one new state is pushed.
There is one other alternative: the table can say that the lookahead
token is erroneous in the current state. This causes error processing
to begin (*note Error Recovery::).
File: bison.info, Node: Reduce/Reduce, Next: Mysterious Conflicts, Prev: Parser States, Up: Algorithm
5.6 Reduce/Reduce Conflicts
===========================
A reduce/reduce conflict occurs if there are two or more rules that
apply to the same sequence of input. This usually indicates a serious
error in the grammar.
For example, here is an erroneous attempt to define a sequence of
zero or more ‘word’ groupings.
sequence:
%empty { printf ("empty sequence\n"); }
| maybeword
| sequence word { printf ("added word %s\n", $2); }
;
maybeword:
%empty { printf ("empty maybeword\n"); }
| word { printf ("single word %s\n", $1); }
;
The error is an ambiguity: there is more than one way to parse a single
‘word’ into a ‘sequence’. It could be reduced to a ‘maybeword’ and then
into a ‘sequence’ via the second rule. Alternatively, nothing-at-all
could be reduced into a ‘sequence’ via the first rule, and this could be
combined with the ‘word’ using the third rule for ‘sequence’.
There is also more than one way to reduce nothing-at-all into a
‘sequence’. This can be done directly via the first rule, or indirectly
via ‘maybeword’ and then the second rule.
You might think that this is a distinction without a difference,
because it does not change whether any particular input is valid or not.
But it does affect which actions are run. One parsing order runs the
second rule’s action; the other runs the first rule’s action and the
third rule’s action. In this example, the output of the program
changes.
Bison resolves a reduce/reduce conflict by choosing to use the rule
that appears first in the grammar, but it is very risky to rely on this.
Every reduce/reduce conflict must be studied and usually eliminated.
Here is the proper way to define ‘sequence’:
sequence:
%empty { printf ("empty sequence\n"); }
| sequence word { printf ("added word %s\n", $2); }
;
Here is another common error that yields a reduce/reduce conflict:
sequence:
%empty
| sequence words
| sequence redirects
;
words:
%empty
| words word
;
redirects:
%empty
| redirects redirect
;
The intention here is to define a sequence which can contain either
‘word’ or ‘redirect’ groupings. The individual definitions of
‘sequence’, ‘words’ and ‘redirects’ are error-free, but the three
together make a subtle ambiguity: even an empty input can be parsed in
infinitely many ways!
Consider: nothing-at-all could be a ‘words’. Or it could be two
‘words’ in a row, or three, or any number. It could equally well be a
‘redirects’, or two, or any number. Or it could be a ‘words’ followed
by three ‘redirects’ and another ‘words’. And so on.
Here are two ways to correct these rules. First, to make it a single
level of sequence:
sequence:
%empty
| sequence word
| sequence redirect
;
Second, to prevent either a ‘words’ or a ‘redirects’ from being
empty:
sequence:
%empty
| sequence words
| sequence redirects
;
words:
word
| words word
;
redirects:
redirect
| redirects redirect
;
Yet this proposal introduces another kind of ambiguity! The input
‘word word’ can be parsed as a single ‘words’ composed of two ‘word’s,
or as two one-‘word’ ‘words’ (and likewise for ‘redirect’/‘redirects’).
However this ambiguity is now a shift/reduce conflict, and therefore it
can now be addressed with precedence directives.
To simplify the matter, we will proceed with ‘word’ and ‘redirect’
being tokens: ‘"word"’ and ‘"redirect"’.
To prefer the longest ‘words’, the conflict between the token
‘"word"’ and the rule ‘sequence: sequence words’ must be resolved as a
shift. To this end, we use the same techniques as exposed above, see
*note Using Precedence For Non Operators: Non Operators. One solution
relies on precedences: use ‘%prec’ to give a lower precedence to the
rule:
%precedence "word"
%precedence "sequence"
%%
sequence:
%empty
| sequence word %prec "sequence"
| sequence redirect %prec "sequence"
;
words:
word
| words "word"
;
Another solution relies on associativity: provide both the token and
the rule with the same precedence, but make them right-associative:
%right "word" "redirect"
%%
sequence:
%empty
| sequence word %prec "word"
| sequence redirect %prec "redirect"
;
File: bison.info, Node: Mysterious Conflicts, Next: Tuning LR, Prev: Reduce/Reduce, Up: Algorithm
5.7 Mysterious Conflicts
========================
Sometimes reduce/reduce conflicts can occur that don’t look warranted.
Here is an example:
%%
def: param_spec return_spec ',';
param_spec:
type
| name_list ':' type
;
return_spec:
type
| name ':' type
;
type: "id";
name: "id";
name_list:
name
| name ',' name_list
;
It would seem that this grammar can be parsed with only a single
token of lookahead: when a ‘param_spec’ is being read, an ‘"id"’ is a
‘name’ if a comma or colon follows, or a ‘type’ if another ‘"id"’
follows. In other words, this grammar is LR(1).
However, for historical reasons, Bison cannot by default handle all
LR(1) grammars. In this grammar, two contexts, that after an ‘"id"’ at
the beginning of a ‘param_spec’ and likewise at the beginning of a
‘return_spec’, are similar enough that Bison assumes they are the same.
They appear similar because the same set of rules would be active—the
rule for reducing to a ‘name’ and that for reducing to a ‘type’. Bison
is unable to determine at that stage of processing that the rules would
require different lookahead tokens in the two contexts, so it makes a
single parser state for them both. Combining the two contexts causes a
conflict later. In parser terminology, this occurrence means that the
grammar is not LALR(1).
For many practical grammars (specifically those that fall into the
non-LR(1) class), the limitations of LALR(1) result in difficulties
beyond just mysterious reduce/reduce conflicts. The best way to fix all
these problems is to select a different parser table construction
algorithm. Either IELR(1) or canonical LR(1) would suffice, but the
former is more efficient and easier to debug during development. *Note
LR Table Construction::, for details. (Bison’s IELR(1) and canonical
LR(1) implementations are experimental. More user feedback will help to
stabilize them.)
If you instead wish to work around LALR(1)’s limitations, you can
often fix a mysterious conflict by identifying the two parser states
that are being confused, and adding something to make them look
distinct. In the above example, adding one rule to ‘return_spec’ as
follows makes the problem go away:
...
return_spec:
type
| name ':' type
| "id" "bogus" /* This rule is never used. */
;
This corrects the problem because it introduces the possibility of an
additional active rule in the context after the ‘"id"’ at the beginning
of ‘return_spec’. This rule is not active in the corresponding context
in a ‘param_spec’, so the two contexts receive distinct parser states.
As long as the token ‘"bogus"’ is never generated by ‘yylex’, the added
rule cannot alter the way actual input is parsed.
In this particular example, there is another way to solve the
problem: rewrite the rule for ‘return_spec’ to use ‘"id"’ directly
instead of via ‘name’. This also causes the two confusing contexts to
have different sets of active rules, because the one for ‘return_spec’
activates the altered rule for ‘return_spec’ rather than the one for
‘name’.
param_spec:
type
| name_list ':' type
;
return_spec:
type
| "id" ':' type
;
For a more detailed exposition of LALR(1) parsers and parser
generators, *note DeRemer 1982: Bibliography.
File: bison.info, Node: Tuning LR, Next: Generalized LR Parsing, Prev: Mysterious Conflicts, Up: Algorithm
5.8 Tuning LR
=============
The default behavior of Bison’s LR-based parsers is chosen mostly for
historical reasons, but that behavior is often not robust. For example,
in the previous section, we discussed the mysterious conflicts that can
be produced by LALR(1), Bison’s default parser table construction
algorithm. Another example is Bison’s ‘%define parse.error verbose’
directive, which instructs the generated parser to produce verbose
syntax error messages, which can sometimes contain incorrect
information.
In this section, we explore several modern features of Bison that
allow you to tune fundamental aspects of the generated LR-based parsers.
Some of these features easily eliminate shortcomings like those
mentioned above. Others can be helpful purely for understanding your
parser.
Most of the features discussed in this section are still
experimental. More user feedback will help to stabilize them.
* Menu:
* LR Table Construction:: Choose a different construction algorithm.
* Default Reductions:: Disable default reductions.
* LAC:: Correct lookahead sets in the parser states.
* Unreachable States:: Keep unreachable parser states for debugging.
File: bison.info, Node: LR Table Construction, Next: Default Reductions, Up: Tuning LR
5.8.1 LR Table Construction
---------------------------
For historical reasons, Bison constructs LALR(1) parser tables by
default. However, LALR does not possess the full language-recognition
power of LR. As a result, the behavior of parsers employing LALR parser
tables is often mysterious. We presented a simple example of this
effect in *note Mysterious Conflicts::.
As we also demonstrated in that example, the traditional approach to
eliminating such mysterious behavior is to restructure the grammar.
Unfortunately, doing so correctly is often difficult. Moreover, merely
discovering that LALR causes mysterious behavior in your parser can be
difficult as well.
Fortunately, Bison provides an easy way to eliminate the possibility
of such mysterious behavior altogether. You simply need to activate a
more powerful parser table construction algorithm by using the ‘%define
lr.type’ directive.
-- Directive: %define lr.type TYPE
Specify the type of parser tables within the LR(1) family. The
accepted values for TYPE are:
• ‘lalr’ (default)
• ‘ielr’
• ‘canonical-lr’
For example, to activate IELR, you might add the following directive
to you grammar file:
%define lr.type ielr
For the example in *note Mysterious Conflicts::, the mysterious conflict
is then eliminated, so there is no need to invest time in comprehending
the conflict or restructuring the grammar to fix it. If, during future
development, the grammar evolves such that all mysterious behavior would
have disappeared using just LALR, you need not fear that continuing to
use IELR will result in unnecessarily large parser tables. That is,
IELR generates LALR tables when LALR (using a deterministic parsing
algorithm) is sufficient to support the full language-recognition power
of LR. Thus, by enabling IELR at the start of grammar development, you
can safely and completely eliminate the need to consider LALR’s
shortcomings.
While IELR is almost always preferable, there are circumstances where
LALR or the canonical LR parser tables described by Knuth (*note Knuth
1965: Bibliography.) can be useful. Here we summarize the relative
advantages of each parser table construction algorithm within Bison:
• LALR
There are at least two scenarios where LALR can be worthwhile:
• GLR without static conflict resolution.
When employing GLR parsers (*note GLR Parsers::), if you do
not resolve any conflicts statically (for example, with
‘%left’ or ‘%precedence’), then the parser explores all
potential parses of any given input. In this case, the choice
of parser table construction algorithm is guaranteed not to
alter the language accepted by the parser. LALR parser tables
are the smallest parser tables Bison can currently construct,
so they may then be preferable. Nevertheless, once you begin
to resolve conflicts statically, GLR behaves more like a
deterministic parser in the syntactic contexts where those
conflicts appear, and so either IELR or canonical LR can then
be helpful to avoid LALR’s mysterious behavior.
• Malformed grammars.
Occasionally during development, an especially malformed
grammar with a major recurring flaw may severely impede the
IELR or canonical LR parser table construction algorithm.
LALR can be a quick way to construct parser tables in order to
investigate such problems while ignoring the more subtle
differences from IELR and canonical LR.
• IELR
IELR (Inadequacy Elimination LR) is a minimal LR algorithm. That
is, given any grammar (LR or non-LR), parsers using IELR or
canonical LR parser tables always accept exactly the same set of
sentences. However, like LALR, IELR merges parser states during
parser table construction so that the number of parser states is
often an order of magnitude less than for canonical LR. More
importantly, because canonical LR’s extra parser states may contain
duplicate conflicts in the case of non-LR grammars, the number of
conflicts for IELR is often an order of magnitude less as well.
This effect can significantly reduce the complexity of developing a
grammar.
• Canonical LR
While inefficient, canonical LR parser tables can be an interesting
means to explore a grammar because they possess a property that
IELR and LALR tables do not. That is, if ‘%nonassoc’ is not used
and default reductions are left disabled (*note Default
Reductions::), then, for every left context of every canonical LR
state, the set of tokens accepted by that state is guaranteed to be
the exact set of tokens that is syntactically acceptable in that
left context. It might then seem that an advantage of canonical LR
parsers in production is that, under the above constraints, they
are guaranteed to detect a syntax error as soon as possible without
performing any unnecessary reductions. However, IELR parsers that
use LAC are also able to achieve this behavior without sacrificing
‘%nonassoc’ or default reductions. For details and a few caveats
of LAC, *note LAC::.
For a more detailed exposition of the mysterious behavior in LALR
parsers and the benefits of IELR, *note Denny 2008 March: Bibliography,
and *note Denny 2010 November: Bibliography.
File: bison.info, Node: Default Reductions, Next: LAC, Prev: LR Table Construction, Up: Tuning LR
5.8.2 Default Reductions
------------------------
After parser table construction, Bison identifies the reduction with the
largest lookahead set in each parser state. To reduce the size of the
parser state, traditional Bison behavior is to remove that lookahead set
and to assign that reduction to be the default parser action. Such a
reduction is known as a “default reduction”.
Default reductions affect more than the size of the parser tables.
They also affect the behavior of the parser:
• Delayed ‘yylex’ invocations.
A “consistent state” is a state that has only one possible parser
action. If that action is a reduction and is encoded as a default
reduction, then that consistent state is called a “defaulted
state”. Upon reaching a defaulted state, a Bison-generated parser
does not bother to invoke ‘yylex’ to fetch the next token before
performing the reduction. In other words, whether default
reductions are enabled in consistent states determines how soon a
Bison-generated parser invokes ‘yylex’ for a token: immediately
when it _reaches_ that token in the input or when it eventually
_needs_ that token as a lookahead to determine the next parser
action. Traditionally, default reductions are enabled, and so the
parser exhibits the latter behavior.
The presence of defaulted states is an important consideration when
designing ‘yylex’ and the grammar file. That is, if the behavior
of ‘yylex’ can influence or be influenced by the semantic actions
associated with the reductions in defaulted states, then the delay
of the next ‘yylex’ invocation until after those reductions is
significant. For example, the semantic actions might pop a scope
stack that ‘yylex’ uses to determine what token to return. Thus,
the delay might be necessary to ensure that ‘yylex’ does not look
up the next token in a scope that should already be considered
closed.
• Delayed syntax error detection.
When the parser fetches a new token by invoking ‘yylex’, it checks
whether there is an action for that token in the current parser
state. The parser detects a syntax error if and only if either (1)
there is no action for that token or (2) the action for that token
is the error action (due to the use of ‘%nonassoc’). However, if
there is a default reduction in that state (which might or might
not be a defaulted state), then it is impossible for condition 1 to
exist. That is, all tokens have an action. Thus, the parser
sometimes fails to detect the syntax error until it reaches a later
state.
While default reductions never cause the parser to accept
syntactically incorrect sentences, the delay of syntax error
detection can have unexpected effects on the behavior of the
parser. However, the delay can be caused anyway by parser state
merging and the use of ‘%nonassoc’, and it can be fixed by another
Bison feature, LAC. We discuss the effects of delayed syntax error
detection and LAC more in the next section (*note LAC::).
For canonical LR, the only default reduction that Bison enables by
default is the accept action, which appears only in the accepting state,
which has no other action and is thus a defaulted state. However, the
default accept action does not delay any ‘yylex’ invocation or syntax
error detection because the accept action ends the parse.
For LALR and IELR, Bison enables default reductions in nearly all
states by default. There are only two exceptions. First, states that
have a shift action on the ‘error’ token do not have default reductions
because delayed syntax error detection could then prevent the ‘error’
token from ever being shifted in that state. However, parser state
merging can cause the same effect anyway, and LAC fixes it in both
cases, so future versions of Bison might drop this exception when LAC is
activated. Second, GLR parsers do not record the default reduction as
the action on a lookahead token for which there is a conflict. The
correct action in this case is to split the parse instead.
To adjust which states have default reductions enabled, use the
‘%define lr.default-reduction’ directive.
-- Directive: %define lr.default-reduction WHERE
Specify the kind of states that are permitted to contain default
reductions. The accepted values of WHERE are:
• ‘most’ (default for LALR and IELR)
• ‘consistent’
• ‘accepting’ (default for canonical LR)
(The ability to specify where default reductions are permitted is
experimental. More user feedback will help to stabilize it.)
File: bison.info, Node: LAC, Next: Unreachable States, Prev: Default Reductions, Up: Tuning LR
5.8.3 LAC
---------
Canonical LR, IELR, and LALR can suffer from a couple of problems upon
encountering a syntax error. First, the parser might perform additional
parser stack reductions before discovering the syntax error. Such
reductions can perform user semantic actions that are unexpected because
they are based on an invalid token, and they cause error recovery to
begin in a different syntactic context than the one in which the invalid
token was encountered. Second, when verbose error messages are enabled
(*note Error Reporting::), the expected token list in the syntax error
message can both contain invalid tokens and omit valid tokens.
The culprits for the above problems are ‘%nonassoc’, default
reductions in inconsistent states (*note Default Reductions::), and
parser state merging. Because IELR and LALR merge parser states, they
suffer the most. Canonical LR can suffer only if ‘%nonassoc’ is used or
if default reductions are enabled for inconsistent states.
LAC (Lookahead Correction) is a new mechanism within the parsing
algorithm that solves these problems for canonical LR, IELR, and LALR
without sacrificing ‘%nonassoc’, default reductions, or state merging.
You can enable LAC with the ‘%define parse.lac’ directive.
-- Directive: %define parse.lac VALUE
Enable LAC to improve syntax error handling.
• ‘none’ (default)
• ‘full’
(This feature is experimental. More user feedback will help to
stabilize it. Moreover, it is currently only available for
deterministic parsers in C.)
Conceptually, the LAC mechanism is straight-forward. Whenever the
parser fetches a new token from the scanner so that it can determine the
next parser action, it immediately suspends normal parsing and performs
an exploratory parse using a temporary copy of the normal parser state
stack. During this exploratory parse, the parser does not perform user
semantic actions. If the exploratory parse reaches a shift action,
normal parsing then resumes on the normal parser stacks. If the
exploratory parse reaches an error instead, the parser reports a syntax
error. If verbose syntax error messages are enabled, the parser must
then discover the list of expected tokens, so it performs a separate
exploratory parse for each token in the grammar.
There is one subtlety about the use of LAC. That is, when in a
consistent parser state with a default reduction, the parser will not
attempt to fetch a token from the scanner because no lookahead is needed
to determine the next parser action. Thus, whether default reductions
are enabled in consistent states (*note Default Reductions::) affects
how soon the parser detects a syntax error: immediately when it
_reaches_ an erroneous token or when it eventually _needs_ that token as
a lookahead to determine the next parser action. The latter behavior is
probably more intuitive, so Bison currently provides no way to achieve
the former behavior while default reductions are enabled in consistent
states.
Thus, when LAC is in use, for some fixed decision of whether to
enable default reductions in consistent states, canonical LR and IELR
behave almost exactly the same for both syntactically acceptable and
syntactically unacceptable input. While LALR still does not support the
full language-recognition power of canonical LR and IELR, LAC at least
enables LALR’s syntax error handling to correctly reflect LALR’s
language-recognition power.
There are a few caveats to consider when using LAC:
• Infinite parsing loops.
IELR plus LAC does have one shortcoming relative to canonical LR.
Some parsers generated by Bison can loop infinitely. LAC does not
fix infinite parsing loops that occur between encountering a syntax
error and detecting it, but enabling canonical LR or disabling
default reductions sometimes does.
• Verbose error message limitations.
Because of internationalization considerations, Bison-generated
parsers limit the size of the expected token list they are willing
to report in a verbose syntax error message. If the number of
expected tokens exceeds that limit, the list is simply dropped from
the message. Enabling LAC can increase the size of the list and
thus cause the parser to drop it. Of course, dropping the list is
better than reporting an incorrect list.
• Performance.
Because LAC requires many parse actions to be performed twice, it
can have a performance penalty. However, not all parse actions
must be performed twice. Specifically, during a series of default
reductions in consistent states and shift actions, the parser never
has to initiate an exploratory parse. Moreover, the most
time-consuming tasks in a parse are often the file I/O, the lexical
analysis performed by the scanner, and the user’s semantic actions,
but none of these are performed during the exploratory parse.
Finally, the base of the temporary stack used during an exploratory
parse is a pointer into the normal parser state stack so that the
stack is never physically copied. In our experience, the
performance penalty of LAC has proved insignificant for practical
grammars.
While the LAC algorithm shares techniques that have been recognized
in the parser community for years, for the publication that introduces
LAC, *note Denny 2010 May: Bibliography.
File: bison.info, Node: Unreachable States, Prev: LAC, Up: Tuning LR
5.8.4 Unreachable States
------------------------
If there exists no sequence of transitions from the parser’s start state
to some state S, then Bison considers S to be an “unreachable state”. A
state can become unreachable during conflict resolution if Bison
disables a shift action leading to it from a predecessor state.
By default, Bison removes unreachable states from the parser after
conflict resolution because they are useless in the generated parser.
However, keeping unreachable states is sometimes useful when trying to
understand the relationship between the parser and the grammar.
-- Directive: %define lr.keep-unreachable-state VALUE
Request that Bison allow unreachable states to remain in the parser
tables. VALUE must be a Boolean. The default is ‘false’.
There are a few caveats to consider:
• Missing or extraneous warnings.
Unreachable states may contain conflicts and may use rules not used
in any other state. Thus, keeping unreachable states may induce
warnings that are irrelevant to your parser’s behavior, and it may
eliminate warnings that are relevant. Of course, the change in
warnings may actually be relevant to a parser table analysis that
wants to keep unreachable states, so this behavior will likely
remain in future Bison releases.
• Other useless states.
While Bison is able to remove unreachable states, it is not
guaranteed to remove other kinds of useless states. Specifically,
when Bison disables reduce actions during conflict resolution, some
goto actions may become useless, and thus some additional states
may become useless. If Bison were to compute which goto actions
were useless and then disable those actions, it could identify such
states as unreachable and then remove those states. However, Bison
does not compute which goto actions are useless.
File: bison.info, Node: Generalized LR Parsing, Next: Memory Management, Prev: Tuning LR, Up: Algorithm
5.9 Generalized LR (GLR) Parsing
================================
Bison produces _deterministic_ parsers that choose uniquely when to
reduce and which reduction to apply based on a summary of the preceding
input and on one extra token of lookahead. As a result, normal Bison
handles a proper subset of the family of context-free languages.
Ambiguous grammars, since they have strings with more than one possible
sequence of reductions cannot have deterministic parsers in this sense.
The same is true of languages that require more than one symbol of
lookahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
Finally, as previously mentioned (*note Mysterious Conflicts::), there
are languages where Bison’s default choice of how to summarize the input
seen so far loses necessary information.
When you use the ‘%glr-parser’ declaration in your grammar file,
Bison generates a parser that uses a different algorithm, called
Generalized LR (or GLR). A Bison GLR parser uses the same basic
algorithm for parsing as an ordinary Bison parser, but behaves
differently in cases where there is a shift-reduce conflict that has not
been resolved by precedence rules (*note Precedence::) or a
reduce-reduce conflict. When a GLR parser encounters such a situation,
it effectively _splits_ into a several parsers, one for each possible
shift or reduction. These parsers then proceed as usual, consuming
tokens in lock-step. Some of the stacks may encounter other conflicts
and split further, with the result that instead of a sequence of states,
a Bison GLR parsing stack is what is in effect a tree of states.
In effect, each stack represents a guess as to what the proper parse
is. Additional input may indicate that a guess was wrong, in which case
the appropriate stack silently disappears. Otherwise, the semantics
actions generated in each stack are saved, rather than being executed
immediately. When a stack disappears, its saved semantic actions never
get executed. When a reduction causes two stacks to become equivalent,
their sets of semantic actions are both saved with the state that
results from the reduction. We say that two stacks are equivalent when
they both represent the same sequence of states, and each pair of
corresponding states represents a grammar symbol that produces the same
segment of the input token stream.
Whenever the parser makes a transition from having multiple states to
having one, it reverts to the normal deterministic parsing algorithm,
after resolving and executing the saved-up actions. At this transition,
some of the states on the stack will have semantic values that are sets
(actually multisets) of possible actions. The parser tries to pick one
of the actions by first finding one whose rule has the highest dynamic
precedence, as set by the ‘%dprec’ declaration. Otherwise, if the
alternative actions are not ordered by precedence, but there the same
merging function is declared for both rules by the ‘%merge’ declaration,
Bison resolves and evaluates both and then calls the merge function on
the result. Otherwise, it reports an ambiguity.
It is possible to use a data structure for the GLR parsing tree that
permits the processing of any LR(1) grammar in linear time (in the size
of the input), any unambiguous (not necessarily LR(1)) grammar in
quadratic worst-case time, and any general (possibly ambiguous)
context-free grammar in cubic worst-case time. However, Bison currently
uses a simpler data structure that requires time proportional to the
length of the input times the maximum number of stacks required for any
prefix of the input. Thus, really ambiguous or nondeterministic
grammars can require exponential time and space to process. Such badly
behaving examples, however, are not generally of practical interest.
Usually, nondeterminism in a grammar is local—the parser is “in doubt”
only for a few tokens at a time. Therefore, the current data structure
should generally be adequate. On LR(1) portions of a grammar, in
particular, it is only slightly slower than with the deterministic LR(1)
Bison parser.
For a more detailed exposition of GLR parsers, *note Scott 2000:
Bibliography.
File: bison.info, Node: Memory Management, Prev: Generalized LR Parsing, Up: Algorithm
5.10 Memory Management, and How to Avoid Memory Exhaustion
==========================================================
The Bison parser stack can run out of memory if too many tokens are
shifted and not reduced. When this happens, the parser function
‘yyparse’ calls ‘yyerror’ and then returns 2.
Because Bison parsers have growing stacks, hitting the upper limit
usually results from using a right recursion instead of a left
recursion, see *note Recursive Rules: Recursion.
By defining the macro ‘YYMAXDEPTH’, you can control how deep the
parser stack can become before memory is exhausted. Define the macro
with a value that is an integer. This value is the maximum number of
tokens that can be shifted (and not reduced) before overflow.
The stack space allowed is not necessarily allocated. If you specify
a large value for ‘YYMAXDEPTH’, the parser normally allocates a small
stack at first, and then makes it bigger by stages as needed. This
increasing allocation happens automatically and silently. Therefore,
you do not need to make ‘YYMAXDEPTH’ painfully small merely to save
space for ordinary inputs that do not need much stack.
However, do not allow ‘YYMAXDEPTH’ to be a value so large that
arithmetic overflow could occur when calculating the size of the stack
space. Also, do not allow ‘YYMAXDEPTH’ to be less than ‘YYINITDEPTH’.
The default value of ‘YYMAXDEPTH’, if you do not define it, is 10000.
You can control how much stack is allocated initially by defining the
macro ‘YYINITDEPTH’ to a positive integer. For the deterministic parser
in C, this value must be a compile-time constant unless you are assuming
C99 or some other target language or compiler that allows
variable-length arrays. The default is 200.
Do not allow ‘YYINITDEPTH’ to be greater than ‘YYMAXDEPTH’.
You can generate a deterministic parser containing C++ user code from
the default (C) skeleton, as well as from the C++ skeleton (*note C++
Parsers::). However, if you do use the default skeleton and want to
allow the parsing stack to grow, be careful not to use semantic types or
location types that require non-trivial copy constructors. The C
skeleton bypasses these constructors when copying data to new, larger
stacks.
File: bison.info, Node: Error Recovery, Next: Context Dependency, Prev: Algorithm, Up: Top
6 Error Recovery
****************
It is not usually acceptable to have a program terminate on a syntax
error. For example, a compiler should recover sufficiently to parse the
rest of the input file and check it for errors; a calculator should
accept another expression.
In a simple interactive command parser where each input is one line,
it may be sufficient to allow ‘yyparse’ to return 1 on error and have
the caller ignore the rest of the input line when that happens (and then
call ‘yyparse’ again). But this is inadequate for a compiler, because
it forgets all the syntactic context leading up to the error. A syntax
error deep within a function in the compiler input should not cause the
compiler to treat the following line like the beginning of a source
file.
You can define how to recover from a syntax error by writing rules to
recognize the special token ‘error’. This is a terminal symbol that is
always defined (you need not declare it) and reserved for error
handling. The Bison parser generates an ‘error’ token whenever a syntax
error happens; if you have provided a rule to recognize this token in
the current context, the parse can continue.
For example:
stmts:
%empty
| stmts '\n'
| stmts exp '\n'
| stmts error '\n'
The fourth rule in this example says that an error followed by a
newline makes a valid addition to any ‘stmts’.
What happens if a syntax error occurs in the middle of an ‘exp’? The
error recovery rule, interpreted strictly, applies to the precise
sequence of a ‘stmts’, an ‘error’ and a newline. If an error occurs in
the middle of an ‘exp’, there will probably be some additional tokens
and subexpressions on the stack after the last ‘stmts’, and there will
be tokens to read before the next newline. So the rule is not
applicable in the ordinary way.
But Bison can force the situation to fit the rule, by discarding part
of the semantic context and part of the input. First it discards states
and objects from the stack until it gets back to a state in which the
‘error’ token is acceptable. (This means that the subexpressions
already parsed are discarded, back to the last complete ‘stmts’.) At
this point the ‘error’ token can be shifted. Then, if the old lookahead
token is not acceptable to be shifted next, the parser reads tokens and
discards them until it finds a token which is acceptable. In this
example, Bison reads and discards input until the next newline so that
the fourth rule can apply. Note that discarded symbols are possible
sources of memory leaks, see *note Freeing Discarded Symbols: Destructor
Decl, for a means to reclaim this memory.
The choice of error rules in the grammar is a choice of strategies
for error recovery. A simple and useful strategy is simply to skip the
rest of the current input line or current statement if an error is
detected:
stmt: error ';' /* On error, skip until ';' is read. */
It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed. Otherwise the
close-delimiter will probably appear to be unmatched, and generate
another, spurious error message:
primary:
'(' expr ')'
| '(' error ')'
...
;
Error recovery strategies are necessarily guesses. When they guess
wrong, one syntax error often leads to another. In the above example,
the error recovery rule guesses that an error is due to bad input within
one ‘stmt’. Suppose that instead a spurious semicolon is inserted in
the middle of a valid ‘stmt’. After the error recovery rule recovers
from the first error, another syntax error will be found straightaway,
since the text following the spurious semicolon is also an invalid
‘stmt’.
To prevent an outpouring of error messages, the parser will output no
error message for another syntax error that happens shortly after the
first; only after three consecutive input tokens have been successfully
shifted will error messages resume.
Note that rules which accept the ‘error’ token may have actions, just
as any other rules can.
You can make error messages resume immediately by using the macro
‘yyerrok’ in an action. If you do this in the error rule’s action, no
error messages will be suppressed. This macro requires no arguments;
‘yyerrok;’ is a valid C statement.
The previous lookahead token is reanalyzed immediately after an
error. If this is unacceptable, then the macro ‘yyclearin’ may be used
to clear this token. Write the statement ‘yyclearin;’ in the error
rule’s action. *Note Special Features for Use in Actions: Action
Features.
For example, suppose that on a syntax error, an error handling
routine is called that advances the input stream to some point where
parsing should once again commence. The next symbol returned by the
lexical scanner is probably correct. The previous lookahead token ought
to be discarded with ‘yyclearin;’.
The expression ‘YYRECOVERING ()’ yields 1 when the parser is
recovering from a syntax error, and 0 otherwise. Syntax error
diagnostics are suppressed while recovering from a syntax error.
File: bison.info, Node: Context Dependency, Next: Debugging, Prev: Error Recovery, Up: Top
7 Handling Context Dependencies
*******************************
The Bison paradigm is to parse tokens first, then group them into larger
syntactic units. In many languages, the meaning of a token is affected
by its context. Although this violates the Bison paradigm, certain
techniques (known as “kludges”) may enable you to write Bison parsers
for such languages.
* Menu:
* Semantic Tokens:: Token parsing can depend on the semantic context.
* Lexical Tie-ins:: Token parsing can depend on the syntactic context.
* Tie-in Recovery:: Lexical tie-ins have implications for how
error recovery rules must be written.
(Actually, “kludge” means any technique that gets its job done but is
neither clean nor robust.)
File: bison.info, Node: Semantic Tokens, Next: Lexical Tie-ins, Up: Context Dependency
7.1 Semantic Info in Token Types
================================
The C language has a context dependency: the way an identifier is used
depends on what its current meaning is. For example, consider this:
foo (x);
This looks like a function call statement, but if ‘foo’ is a typedef
name, then this is actually a declaration of ‘x’. How can a Bison
parser for C decide how to parse this input?
The method used in GNU C is to have two different token types,
‘IDENTIFIER’ and ‘TYPENAME’. When ‘yylex’ finds an identifier, it looks
up the current declaration of the identifier in order to decide which
token type to return: ‘TYPENAME’ if the identifier is declared as a
typedef, ‘IDENTIFIER’ otherwise.
The grammar rules can then express the context dependency by the
choice of token type to recognize. ‘IDENTIFIER’ is accepted as an
expression, but ‘TYPENAME’ is not. ‘TYPENAME’ can start a declaration,
but ‘IDENTIFIER’ cannot. In contexts where the meaning of the
identifier is _not_ significant, such as in declarations that can shadow
a typedef name, either ‘TYPENAME’ or ‘IDENTIFIER’ is accepted—there is
one rule for each of the two token types.
This technique is simple to use if the decision of which kinds of
identifiers to allow is made at a place close to where the identifier is
parsed. But in C this is not always so: C allows a declaration to
redeclare a typedef name provided an explicit type has been specified
earlier:
typedef int foo, bar;
int baz (void)
{
static bar (bar); /* redeclare ‘bar’ as static variable */
extern foo foo (foo); /* redeclare ‘foo’ as function */
return foo (bar);
}
Unfortunately, the name being declared is separated from the
declaration construct itself by a complicated syntactic structure—the
“declarator”.
As a result, part of the Bison parser for C needs to be duplicated,
with all the nonterminal names changed: once for parsing a declaration
in which a typedef name can be redefined, and once for parsing a
declaration in which that can’t be done. Here is a part of the
duplication, with actions omitted for brevity:
initdcl:
declarator maybeasm '=' init
| declarator maybeasm
;
notype_initdcl:
notype_declarator maybeasm '=' init
| notype_declarator maybeasm
;
Here ‘initdcl’ can redeclare a typedef name, but ‘notype_initdcl’
cannot. The distinction between ‘declarator’ and ‘notype_declarator’ is
the same sort of thing.
There is some similarity between this technique and a lexical tie-in
(described next), in that information which alters the lexical analysis
is changed during parsing by other parts of the program. The difference
is here the information is global, and is used for other purposes in the
program. A true lexical tie-in has a special-purpose flag controlled by
the syntactic context.
File: bison.info, Node: Lexical Tie-ins, Next: Tie-in Recovery, Prev: Semantic Tokens, Up: Context Dependency
7.2 Lexical Tie-ins
===================
One way to handle context-dependency is the “lexical tie-in”: a flag
which is set by Bison actions, whose purpose is to alter the way tokens
are parsed.
For example, suppose we have a language vaguely like C, but with a
special construct ‘hex (HEX-EXPR)’. After the keyword ‘hex’ comes an
expression in parentheses in which all integers are hexadecimal. In
particular, the token ‘a1b’ must be treated as an integer rather than as
an identifier if it appears in that context. Here is how you can do it:
%{
int hexflag;
int yylex (void);
void yyerror (char const *);
%}
%%
...
expr:
IDENTIFIER
| constant
| HEX '(' { hexflag = 1; }
expr ')' { hexflag = 0; $$ = $4; }
| expr '+' expr { $$ = make_sum ($1, $3); }
...
;
constant:
INTEGER
| STRING
;
Here we assume that ‘yylex’ looks at the value of ‘hexflag’; when it is
nonzero, all integers are parsed in hexadecimal, and tokens starting
with letters are parsed as integers if possible.
The declaration of ‘hexflag’ shown in the prologue of the grammar
file is needed to make it accessible to the actions (*note The Prologue:
Prologue.). You must also write the code in ‘yylex’ to obey the flag.
File: bison.info, Node: Tie-in Recovery, Prev: Lexical Tie-ins, Up: Context Dependency
7.3 Lexical Tie-ins and Error Recovery
======================================
Lexical tie-ins make strict demands on any error recovery rules you
have. *Note Error Recovery::.
The reason for this is that the purpose of an error recovery rule is
to abort the parsing of one construct and resume in some larger
construct. For example, in C-like languages, a typical error recovery
rule is to skip tokens until the next semicolon, and then start a new
statement, like this:
stmt:
expr ';'
| IF '(' expr ')' stmt { ... }
...
| error ';' { hexflag = 0; }
;
If there is a syntax error in the middle of a ‘hex (EXPR)’ construct,
this error rule will apply, and then the action for the completed ‘hex
(EXPR)’ will never run. So ‘hexflag’ would remain set for the entire
rest of the input, or until the next ‘hex’ keyword, causing identifiers
to be misinterpreted as integers.
To avoid this problem the error recovery rule itself clears
‘hexflag’.
There may also be an error recovery rule that works within
expressions. For example, there could be a rule which applies within
parentheses and skips to the close-parenthesis:
expr:
...
| '(' expr ')' { $$ = $2; }
| '(' error ')'
...
If this rule acts within the ‘hex’ construct, it is not going to
abort that construct (since it applies to an inner level of parentheses
within the construct). Therefore, it should not clear the flag: the
rest of the ‘hex’ construct should be parsed with the flag still in
effect.
What if there is an error recovery rule which might abort out of the
‘hex’ construct or might not, depending on circumstances? There is no
way you can write the action to determine whether a ‘hex’ construct is
being aborted or not. So if you are using a lexical tie-in, you had
better make sure your error recovery rules are not of this kind. Each
rule must be such that you can be sure that it always will, or always
won’t, have to clear the flag.
File: bison.info, Node: Debugging, Next: Invocation, Prev: Context Dependency, Up: Top
8 Debugging Your Parser
***********************
Developing a parser can be a challenge, especially if you don’t
understand the algorithm (*note The Bison Parser Algorithm: Algorithm.).
This chapter explains how to understand and debug a parser.
The first sections focus on the static part of the parser: its
structure. They explain how to generate and read the detailed
description of the automaton. There are several formats available:
− as text, see *note Understanding Your Parser: Understanding.;
− as a graph, see *note Visualizing Your Parser: Graphviz.;
− or as a markup report that can be turned, for instance, into HTML,
see *note Visualizing your parser in multiple formats: Xml.
The last section focuses on the dynamic part of the parser: how to
enable and understand the parser run-time traces (*note Tracing Your
Parser: Tracing.).
* Menu:
* Understanding:: Understanding the structure of your parser.
* Graphviz:: Getting a visual representation of the parser.
* Xml:: Getting a markup representation of the parser.
* Tracing:: Tracing the execution of your parser.
File: bison.info, Node: Understanding, Next: Graphviz, Up: Debugging
8.1 Understanding Your Parser
=============================
As documented elsewhere (*note The Bison Parser Algorithm: Algorithm.)
Bison parsers are “shift/reduce automata”. In some cases (much more
frequent than one would hope), looking at this automaton is required to
tune or simply fix a parser.
The textual file is generated when the options ‘--report’ or
‘--verbose’ are specified, see *note Invoking Bison: Invocation. Its
name is made by removing ‘.tab.c’ or ‘.c’ from the parser implementation
file name, and adding ‘.output’ instead. Therefore, if the grammar file
is ‘foo.y’, then the parser implementation file is called ‘foo.tab.c’ by
default. As a consequence, the verbose output file is called
‘foo.output’.
The following grammar file, ‘calc.y’, will be used in the sequel:
%union
{
int ival;
const char *sval;
}
%token NUM
%type exp
%token STR
%type useless
%left '+' '-'
%left '*'
%%
exp:
exp '+' exp
| exp '-' exp
| exp '*' exp
| exp '/' exp
| NUM
;
useless: STR;
%%
‘bison’ reports:
calc.y: warning: 1 nonterminal useless in grammar [-Wother]
calc.y: warning: 1 rule useless in grammar [-Wother]
calc.y:19.1-7: warning: nonterminal useless in grammar: useless [-Wother]
19 | useless: STR;
| ^~~~~~~
calc.y: warning: 7 shift/reduce conflicts [-Wconflicts-sr]
When given ‘--report=state’, in addition to ‘calc.tab.c’, it creates
a file ‘calc.output’ with contents detailed below. The order of the
output and the exact presentation might vary, but the interpretation is
the same.
The first section reports useless tokens, nonterminals and rules.
Useless nonterminals and rules are removed in order to produce a smaller
parser, but useless tokens are preserved, since they might be used by
the scanner (note the difference between “useless” and “unused” below):
Nonterminals useless in grammar
useless
Terminals unused in grammar
STR
Rules useless in grammar
6 useless: STR
The next section lists states that still have conflicts.
State 8 conflicts: 1 shift/reduce
State 9 conflicts: 1 shift/reduce
State 10 conflicts: 1 shift/reduce
State 11 conflicts: 4 shift/reduce
Then Bison reproduces the exact grammar it used:
Grammar
0 $accept: exp $end
1 exp: exp '+' exp
2 | exp '-' exp
3 | exp '*' exp
4 | exp '/' exp
5 | NUM
and reports the uses of the symbols:
Terminals, with rules where they appear
$end (0) 0
'*' (42) 3
'+' (43) 1
'-' (45) 2
'/' (47) 4
error (256)
NUM (258) 5
STR (259)
Nonterminals, with rules where they appear
$accept (9)
on left: 0
exp (10)
on left: 1 2 3 4 5, on right: 0 1 2 3 4
Bison then proceeds onto the automaton itself, describing each state
with its set of “items”, also known as “pointed rules”. Each item is a
production rule together with a point (‘.’) marking the location of the
input cursor.
State 0
0 $accept: . exp $end
NUM shift, and go to state 1
exp go to state 2
This reads as follows: “state 0 corresponds to being at the very
beginning of the parsing, in the initial rule, right before the start
symbol (here, ‘exp’). When the parser returns to this state right after
having reduced a rule that produced an ‘exp’, the control flow jumps to
state 2. If there is no such transition on a nonterminal symbol, and
the lookahead is a ‘NUM’, then this token is shifted onto the parse
stack, and the control flow jumps to state 1. Any other lookahead
triggers a syntax error.”
Even though the only active rule in state 0 seems to be rule 0, the
report lists ‘NUM’ as a lookahead token because ‘NUM’ can be at the
beginning of any rule deriving an ‘exp’. By default Bison reports the
so-called “core” or “kernel” of the item set, but if you want to see
more detail you can invoke ‘bison’ with ‘--report=itemset’ to list the
derived items as well:
State 0
0 $accept: . exp $end
1 exp: . exp '+' exp
2 | . exp '-' exp
3 | . exp '*' exp
4 | . exp '/' exp
5 | . NUM
NUM shift, and go to state 1
exp go to state 2
In the state 1...
State 1
5 exp: NUM .
$default reduce using rule 5 (exp)
the rule 5, ‘exp: NUM;’, is completed. Whatever the lookahead token
(‘$default’), the parser will reduce it. If it was coming from State 0,
then, after this reduction it will return to state 0, and will jump to
state 2 (‘exp: go to state 2’).
State 2
0 $accept: exp . $end
1 exp: exp . '+' exp
2 | exp . '-' exp
3 | exp . '*' exp
4 | exp . '/' exp
$end shift, and go to state 3
'+' shift, and go to state 4
'-' shift, and go to state 5
'*' shift, and go to state 6
'/' shift, and go to state 7
In state 2, the automaton can only shift a symbol. For instance,
because of the item ‘exp: exp . '+' exp’, if the lookahead is ‘+’ it is
shifted onto the parse stack, and the automaton jumps to state 4,
corresponding to the item ‘exp: exp '+' . exp’. Since there is no
default action, any lookahead not listed triggers a syntax error.
The state 3 is named the “final state”, or the “accepting state”:
State 3
0 $accept: exp $end .
$default accept
the initial rule is completed (the start symbol and the end-of-input
were read), the parsing exits successfully.
The interpretation of states 4 to 7 is straightforward, and is left
to the reader.
State 4
1 exp: exp '+' . exp
NUM shift, and go to state 1
exp go to state 8
State 5
2 exp: exp '-' . exp
NUM shift, and go to state 1
exp go to state 9
State 6
3 exp: exp '*' . exp
NUM shift, and go to state 1
exp go to state 10
State 7
4 exp: exp '/' . exp
NUM shift, and go to state 1
exp go to state 11
As was announced in beginning of the report, ‘State 8 conflicts: 1
shift/reduce’:
State 8
1 exp: exp . '+' exp
1 | exp '+' exp .
2 | exp . '-' exp
3 | exp . '*' exp
4 | exp . '/' exp
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
Indeed, there are two actions associated to the lookahead ‘/’: either
shifting (and going to state 7), or reducing rule 1. The conflict means
that either the grammar is ambiguous, or the parser lacks information to
make the right decision. Indeed the grammar is ambiguous, as, since we
did not specify the precedence of ‘/’, the sentence ‘NUM + NUM / NUM’
can be parsed as ‘NUM + (NUM / NUM)’, which corresponds to shifting ‘/’,
or as ‘(NUM + NUM) / NUM’, which corresponds to reducing rule 1.
Because in deterministic parsing a single decision can be made, Bison
arbitrarily chose to disable the reduction, see *note Shift/Reduce
Conflicts: Shift/Reduce. Discarded actions are reported between square
brackets.
Note that all the previous states had a single possible action:
either shifting the next token and going to the corresponding state, or
reducing a single rule. In the other cases, i.e., when shifting _and_
reducing is possible or when _several_ reductions are possible, the
lookahead is required to select the action. State 8 is one such state:
if the lookahead is ‘*’ or ‘/’ then the action is shifting, otherwise
the action is reducing rule 1. In other words, the first two items,
corresponding to rule 1, are not eligible when the lookahead token is
‘*’, since we specified that ‘*’ has higher precedence than ‘+’. More
generally, some items are eligible only with some set of possible
lookahead tokens. When run with ‘--report=lookahead’, Bison specifies
these lookahead tokens:
State 8
1 exp: exp . '+' exp
1 | exp '+' exp . [$end, '+', '-', '/']
2 | exp . '-' exp
3 | exp . '*' exp
4 | exp . '/' exp
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
Note however that while ‘NUM + NUM / NUM’ is ambiguous (which results
in the conflicts on ‘/’), ‘NUM + NUM * NUM’ is not: the conflict was
solved thanks to associativity and precedence directives. If invoked
with ‘--report=solved’, Bison includes information about the solved
conflicts in the report:
Conflict between rule 1 and token '+' resolved as reduce (%left '+').
Conflict between rule 1 and token '-' resolved as reduce (%left '-').
Conflict between rule 1 and token '*' resolved as shift ('+' < '*').
The remaining states are similar:
State 9
1 exp: exp . '+' exp
2 | exp . '-' exp
2 | exp '-' exp .
3 | exp . '*' exp
4 | exp . '/' exp
'*' shift, and go to state 6
'/' shift, and go to state 7
'/' [reduce using rule 2 (exp)]
$default reduce using rule 2 (exp)
State 10
1 exp: exp . '+' exp
2 | exp . '-' exp
3 | exp . '*' exp
3 | exp '*' exp .
4 | exp . '/' exp
'/' shift, and go to state 7
'/' [reduce using rule 3 (exp)]
$default reduce using rule 3 (exp)
State 11
1 exp: exp . '+' exp
2 | exp . '-' exp
3 | exp . '*' exp
4 | exp . '/' exp
4 | exp '/' exp .
'+' shift, and go to state 4
'-' shift, and go to state 5
'*' shift, and go to state 6
'/' shift, and go to state 7
'+' [reduce using rule 4 (exp)]
'-' [reduce using rule 4 (exp)]
'*' [reduce using rule 4 (exp)]
'/' [reduce using rule 4 (exp)]
$default reduce using rule 4 (exp)
Observe that state 11 contains conflicts not only due to the lack of
precedence of ‘/’ with respect to ‘+’, ‘-’, and ‘*’, but also because
the associativity of ‘/’ is not specified.
Bison may also produce an HTML version of this output, via an XML
file and XSLT processing (*note Visualizing your parser in multiple
formats: Xml.).
File: bison.info, Node: Graphviz, Next: Xml, Prev: Understanding, Up: Debugging
8.2 Visualizing Your Parser
===========================
As another means to gain better understanding of the shift/reduce
automaton corresponding to the Bison parser, a DOT file can be
generated. Note that debugging a real grammar with this is tedious at
best, and impractical most of the times, because the generated files are
huge (the generation of a PDF or PNG file from it will take very long,
and more often than not it will fail due to memory exhaustion). This
option was rather designed for beginners, to help them understand LR
parsers.
This file is generated when the ‘--graph’ option is specified (*note
Invoking Bison: Invocation.). Its name is made by removing ‘.tab.c’ or
‘.c’ from the parser implementation file name, and adding ‘.gv’ instead.
If the grammar file is ‘foo.y’, the Graphviz output file is called
‘foo.gv’. A DOT file may also be produced via an XML file and XSLT
processing (*note Visualizing your parser in multiple formats: Xml.).
The following grammar file, ‘rr.y’, will be used in the sequel:
%%
exp: a ";" | b ".";
a: "0";
b: "0";
The graphical output is very similar to the textual one, and as such
it is easier understood by making direct comparisons between them.
*Note Debugging Your Parser: Debugging, for a detailed analysis of the
textual report.
Graphical Representation of States
----------------------------------
The items (pointed rules) for each state are grouped together in graph
nodes. Their numbering is the same as in the verbose file. See the
following points, about transitions, for examples
When invoked with ‘--report=lookaheads’, the lookahead tokens, when
needed, are shown next to the relevant rule between square brackets as a
comma separated list. This is the case in the figure for the
representation of reductions, below.
The transitions are represented as directed edges between the current
and the target states.
Graphical Representation of Shifts
----------------------------------
Shifts are shown as solid arrows, labeled with the lookahead token for
that shift. The following describes a reduction in the ‘rr.output’
file:
State 3
1 exp: a . ";"
";" shift, and go to state 6
A Graphviz rendering of this portion of the graph could be:
[image src="figs/example-shift.svg" text=".----------------.
| State 3 |
| 1 exp: a . \";\" |
`----------------'
|
| \";\"
|
v
.----------------.
| State 6 |
| 1 exp: a \";\" . |
`----------------'" ]
Graphical Representation of Reductions
--------------------------------------
Reductions are shown as solid arrows, leading to a diamond-shaped node
bearing the number of the reduction rule. The arrow is labeled with the
appropriate comma separated lookahead tokens. If the reduction is the
default action for the given state, there is no such label.
This is how reductions are represented in the verbose file
‘rr.output’:
State 1
3 a: "0" . [";"]
4 b: "0" . ["."]
"." reduce using rule 4 (b)
$default reduce using rule 3 (a)
A Graphviz rendering of this portion of the graph could be:
[image src="figs/example-reduce.svg" text=" .------------------.
| State 1 |
| 3 a: \"0\" . [\";\"] |
| 4 b: \"0\" . [\".\"] |
`------------------'
/ \\
/ \\ [\".\"]
/ \\
v v
/ \\ / \\
/ R \\ / R \\
(green) \\ 3 / \\ 4 / (green)
\\ / \\ /" ]
When unresolved conflicts are present, because in deterministic
parsing a single decision can be made, Bison can arbitrarily choose to
disable a reduction, see *note Shift/Reduce Conflicts: Shift/Reduce.
Discarded actions are distinguished by a red filling color on these
nodes, just like how they are reported between square brackets in the
verbose file.
The reduction corresponding to the rule number 0 is the acceptation
state. It is shown as a blue diamond, labeled “Acc”.
Graphical Representation of Gotos
---------------------------------
The ‘go to’ jump transitions are represented as dotted lines bearing the
name of the rule being jumped to.
File: bison.info, Node: Xml, Next: Tracing, Prev: Graphviz, Up: Debugging
8.3 Visualizing your parser in multiple formats
===============================================
Bison supports two major report formats: textual output (*note
Understanding Your Parser: Understanding.) when invoked with option
‘--verbose’, and DOT (*note Visualizing Your Parser: Graphviz.) when
invoked with option ‘--graph’. However, another alternative is to
output an XML file that may then be, with ‘xsltproc’, rendered as either
a raw text format equivalent to the verbose file, or as an HTML version
of the same file, with clickable transitions, or even as a DOT. The
‘.output’ and DOT files obtained via XSLT have no difference whatsoever
with those obtained by invoking ‘bison’ with options ‘--verbose’ or
‘--graph’.
The XML file is generated when the options ‘-x’ or ‘--xml[=FILE]’ are
specified, see *note Invoking Bison: Invocation. If not specified, its
name is made by removing ‘.tab.c’ or ‘.c’ from the parser implementation
file name, and adding ‘.xml’ instead. For instance, if the grammar file
is ‘foo.y’, the default XML output file is ‘foo.xml’.
Bison ships with a ‘data/xslt’ directory, containing XSL
Transformation files to apply to the XML file. Their names are
non-ambiguous:
‘xml2dot.xsl’
Used to output a copy of the DOT visualization of the automaton.
‘xml2text.xsl’
Used to output a copy of the ‘.output’ file.
‘xml2xhtml.xsl’
Used to output an xhtml enhancement of the ‘.output’ file.
Sample usage (requires ‘xsltproc’):
$ bison -x gr.y
$ bison --print-datadir
/usr/local/share/bison
$ xsltproc /usr/local/share/bison/xslt/xml2xhtml.xsl gr.xml >gr.html
File: bison.info, Node: Tracing, Prev: Xml, Up: Debugging
8.4 Tracing Your Parser
=======================
When a Bison grammar compiles properly but parses “incorrectly”, the
‘yydebug’ parser-trace feature helps figuring out why.
* Menu:
* Enabling Traces:: Activating run-time trace support
* Mfcalc Traces:: Extending ‘mfcalc’ to support traces
* The YYPRINT Macro:: Obsolete interface for semantic value reports
File: bison.info, Node: Enabling Traces, Next: Mfcalc Traces, Up: Tracing
8.4.1 Enabling Traces
---------------------
There are several means to enable compilation of trace facilities:
the macro ‘YYDEBUG’
Define the macro ‘YYDEBUG’ to a nonzero value when you compile the
parser. This is compliant with POSIX Yacc. You could use
‘-DYYDEBUG=1’ as a compiler option or you could put ‘#define
YYDEBUG 1’ in the prologue of the grammar file (*note The Prologue:
Prologue.).
If the ‘%define’ variable ‘api.prefix’ is used (*note Multiple
Parsers in the Same Program: Multiple Parsers.), for instance
‘%define api.prefix x’, then if ‘CDEBUG’ is defined, its value
controls the tracing feature (enabled if and only if nonzero);
otherwise tracing is enabled if and only if ‘YYDEBUG’ is nonzero.
the option ‘-t’ (POSIX Yacc compliant)
the option ‘--debug’ (Bison extension)
Use the ‘-t’ option when you run Bison (*note Invoking Bison:
Invocation.). With ‘%define api.prefix {c}’, it defines ‘CDEBUG’
to 1, otherwise it defines ‘YYDEBUG’ to 1.
the directive ‘%debug’
Add the ‘%debug’ directive (*note Bison Declaration Summary: Decl
Summary.). This Bison extension is maintained for backward
compatibility with previous versions of Bison.
the variable ‘parse.trace’
Add the ‘%define parse.trace’ directive (*note parse.trace: %define
Summary.), or pass the ‘-Dparse.trace’ option (*note Bison
Options::). This is a Bison extension, which is especially useful
for languages that don’t use a preprocessor. Unless POSIX and Yacc
portability matter to you, this is the preferred solution.
We suggest that you always enable the trace option so that debugging
is always possible.
The trace facility outputs messages with macro calls of the form
‘YYFPRINTF (stderr, FORMAT, ARGS)’ where FORMAT and ARGS are the usual
‘printf’ format and variadic arguments. If you define ‘YYDEBUG’ to a
nonzero value but do not define ‘YYFPRINTF’, ‘’ is
automatically included and ‘YYFPRINTF’ is defined to ‘fprintf’.
Once you have compiled the program with trace facilities, the way to
request a trace is to store a nonzero value in the variable ‘yydebug’.
You can do this by making the C code do it (in ‘main’, perhaps), or you
can alter the value with a C debugger.
Each step taken by the parser when ‘yydebug’ is nonzero produces a
line or two of trace information, written on ‘stderr’. The trace
messages tell you these things:
• Each time the parser calls ‘yylex’, what kind of token was read.
• Each time a token is shifted, the depth and complete contents of
the state stack (*note Parser States::).
• Each time a rule is reduced, which rule it is, and the complete
contents of the state stack afterward.
To make sense of this information, it helps to refer to the automaton
description file (*note Understanding Your Parser: Understanding.).
This file shows the meaning of each state in terms of positions in
various rules, and also what each state will do with each possible input
token. As you read the successive trace messages, you can see that the
parser is functioning according to its specification in the listing
file. Eventually you will arrive at the place where something
undesirable happens, and you will see which parts of the grammar are to
blame.
The parser implementation file is a C/C++/Java program and you can
use debuggers on it, but it’s not easy to interpret what it is doing.
The parser function is a finite-state machine interpreter, and aside
from the actions it executes the same code over and over. Only the
values of variables show where in the grammar it is working.
File: bison.info, Node: Mfcalc Traces, Next: The YYPRINT Macro, Prev: Enabling Traces, Up: Tracing
8.4.2 Enabling Debug Traces for ‘mfcalc’
----------------------------------------
The debugging information normally gives the token type of each token
read, but not its semantic value. The ‘%printer’ directive allows
specify how semantic values are reported, see *note Printing Semantic
Values: Printer Decl. For backward compatibility, Yacc like C parsers
may also use the ‘YYPRINT’ (*note The ‘YYPRINT’ Macro: The YYPRINT
Macro.), but its use is discouraged.
As a demonstration of ‘%printer’, consider the multi-function
calculator, ‘mfcalc’ (*note Multi-function Calc::). To enable run-time
traces, and semantic value reports, insert the following directives in
its prologue:
/* Generate the parser description file. */
%verbose
/* Enable run-time traces (yydebug). */
%define parse.trace
/* Formatting semantic values. */
%printer { fprintf (yyo, "%s", $$->name); } VAR;
%printer { fprintf (yyo, "%s()", $$->name); } FUN;
%printer { fprintf (yyo, "%g", $$); } ;
The ‘%define’ directive instructs Bison to generate run-time trace
support. Then, activation of these traces is controlled at run-time by
the ‘yydebug’ variable, which is disabled by default. Because these
traces will refer to the “states” of the parser, it is helpful to ask
for the creation of a description of that parser; this is the purpose of
(admittedly ill-named) ‘%verbose’ directive.
The set of ‘%printer’ directives demonstrates how to format the
semantic value in the traces. Note that the specification can be done
either on the symbol type (e.g., ‘VAR’ or ‘FUN’), or on the type tag:
since ‘’ is the type for both ‘NUM’ and ‘exp’, this printer will
be used for them.
Here is a sample of the information provided by run-time traces. The
traces are sent onto standard error.
$ echo 'sin(1-1)' | ./mfcalc -p
Starting parse
Entering state 0
Reducing stack by rule 1 (line 34):
-> $$ = nterm input ()
Stack now 0
Entering state 1
This first batch shows a specific feature of this grammar: the first
rule (which is in line 34 of ‘mfcalc.y’ can be reduced without even
having to look for the first token. The resulting left-hand symbol
(‘$$’) is a valueless (‘()’) ‘input’ nonterminal (‘nterm’).
Then the parser calls the scanner.
Reading a token: Next token is token FUN (sin())
Shifting token FUN (sin())
Entering state 6
That token (‘token’) is a function (‘FUN’) whose value is ‘sin’ as
formatted per our ‘%printer’ specification: ‘sin()’. The parser stores
(‘Shifting’) that token, and others, until it can do something about it.
Reading a token: Next token is token '(' ()
Shifting token '(' ()
Entering state 14
Reading a token: Next token is token NUM (1.000000)
Shifting token NUM (1.000000)
Entering state 4
Reducing stack by rule 6 (line 44):
$1 = token NUM (1.000000)
-> $$ = nterm exp (1.000000)
Stack now 0 1 6 14
Entering state 24
The previous reduction demonstrates the ‘%printer’ directive for
‘’: both the token ‘NUM’ and the resulting nonterminal ‘exp’
have ‘1’ as value.
Reading a token: Next token is token '-' ()
Shifting token '-' ()
Entering state 17
Reading a token: Next token is token NUM (1.000000)
Shifting token NUM (1.000000)
Entering state 4
Reducing stack by rule 6 (line 44):
$1 = token NUM (1.000000)
-> $$ = nterm exp (1.000000)
Stack now 0 1 6 14 24 17
Entering state 26
Reading a token: Next token is token ')' ()
Reducing stack by rule 11 (line 49):
$1 = nterm exp (1.000000)
$2 = token '-' ()
$3 = nterm exp (1.000000)
-> $$ = nterm exp (0.000000)
Stack now 0 1 6 14
Entering state 24
The rule for the subtraction was just reduced. The parser is about to
discover the end of the call to ‘sin’.
Next token is token ')' ()
Shifting token ')' ()
Entering state 31
Reducing stack by rule 9 (line 47):
$1 = token FUN (sin())
$2 = token '(' ()
$3 = nterm exp (0.000000)
$4 = token ')' ()
-> $$ = nterm exp (0.000000)
Stack now 0 1
Entering state 11
Finally, the end-of-line allow the parser to complete the computation,
and display its result.
Reading a token: Next token is token '\n' ()
Shifting token '\n' ()
Entering state 22
Reducing stack by rule 4 (line 40):
$1 = nterm exp (0.000000)
$2 = token '\n' ()
⇒ 0
-> $$ = nterm line ()
Stack now 0 1
Entering state 10
Reducing stack by rule 2 (line 35):
$1 = nterm input ()
$2 = nterm line ()
-> $$ = nterm input ()
Stack now 0
Entering state 1
The parser has returned into state 1, in which it is waiting for the
next expression to evaluate, or for the end-of-file token, which causes
the completion of the parsing.
Reading a token: Now at end of input.
Shifting token $end ()
Entering state 2
Stack now 0 1 2
Cleanup: popping token $end ()
Cleanup: popping nterm input ()
File: bison.info, Node: The YYPRINT Macro, Prev: Mfcalc Traces, Up: Tracing
8.4.3 The ‘YYPRINT’ Macro
-------------------------
Before ‘%printer’ support, semantic values could be displayed using the
‘YYPRINT’ macro, which works only for terminal symbols and only with the
‘yacc.c’ skeleton.
-- Macro: YYPRINT (STREAM, TOKEN, VALUE);
If you define ‘YYPRINT’, it should take three arguments. The
parser will pass a standard I/O stream, the numeric code for the
token type, and the token value (from ‘yylval’).
For ‘yacc.c’ only. Obsoleted by ‘%printer’.
Here is an example of ‘YYPRINT’ suitable for the multi-function
calculator (*note Declarations for ‘mfcalc’: Mfcalc Declarations.):
%{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(File, Type, Value) \
print_token_value (File, Type, Value)
%}
... %% ... %% ...
static void
print_token_value (FILE *file, int type, YYSTYPE value)
{
if (type == VAR)
fprintf (file, "%s", value.tptr->name);
else if (type == NUM)
fprintf (file, "%d", value.val);
}
File: bison.info, Node: Invocation, Next: Other Languages, Prev: Debugging, Up: Top
9 Invoking Bison
****************
The usual way to invoke Bison is as follows:
$ bison FILE
Here FILE is the grammar file name, which usually ends in ‘.y’. The
parser implementation file’s name is made by replacing the ‘.y’ with
‘.tab.c’ and removing any leading directory. Thus, the ‘bison foo.y’
file name yields ‘foo.tab.c’, and the ‘bison hack/foo.y’ file name
yields ‘foo.tab.c’. It’s also possible, in case you are writing C++
code instead of C in your grammar file, to name it ‘foo.ypp’ or
‘foo.y++’. Then, the output files will take an extension like the given
one as input (respectively ‘foo.tab.cpp’ and ‘foo.tab.c++’). This
feature takes effect with all options that manipulate file names like
‘-o’ or ‘-d’.
For example:
$ bison -d FILE.YXX
will produce ‘file.tab.cxx’ and ‘file.tab.hxx’, and
$ bison -d -o OUTPUT.C++ FILE.Y
will produce ‘output.c++’ and ‘output.h++’.
For compatibility with POSIX, the standard Bison distribution also
contains a shell script called ‘yacc’ that invokes Bison with the ‘-y’
option.
* Menu:
* Bison Options:: All the options described in detail,
in alphabetical order by short options.
* Option Cross Key:: Alphabetical list of long options.
* Yacc Library:: Yacc-compatible ‘yylex’ and ‘main’.
File: bison.info, Node: Bison Options, Next: Option Cross Key, Up: Invocation
9.1 Bison Options
=================
Bison supports both traditional single-letter options and mnemonic long
option names. Long option names are indicated with ‘--’ instead of ‘-’.
Abbreviations for option names are allowed as long as they are unique.
When a long option takes an argument, like ‘--file-prefix’, connect the
option name and the argument with ‘=’.
Here is a list of options that can be used with Bison, alphabetized
by short option. It is followed by a cross key alphabetized by long
option.
Operations modes:
‘-h’
‘--help’
Print a summary of the command-line options to Bison and exit.
‘-V’
‘--version’
Print the version number of Bison and exit.
‘--print-localedir’
Print the name of the directory containing locale-dependent data.
‘--print-datadir’
Print the name of the directory containing skeletons and XSLT.
‘-u’
‘--update’
Update the grammar file (remove duplicates, update deprecated
directives, etc.) and exit (i.e., do not generate any of the
output files). Leaves a backup of the original file with a ‘~’
appended. For instance:
$ cat foo.y
%error-verbose
%define parse.error verbose
%%
exp:;
$ bison -u foo.y
foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1 | %error-verbose
| ^~~~~~~~~~~~~~
foo.y:2.1-27: warning: %define variable 'parse.error' redefined [-Wother]
2 | %define parse.error verbose
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.y:1.1-14: previous definition
1 | %error-verbose
| ^~~~~~~~~~~~~~
bison: file 'foo.y' was updated (backup: 'foo.y~')
$ cat foo.y
%define parse.error verbose
%%
exp:;
See the documentation of ‘--feature=fixit’ below for more details.
‘-y’
‘--yacc’
Act more like the traditional ‘yacc’ command. This can cause
different diagnostics to be generated (it implies ‘-Wyacc’), and
may change behavior in other minor ways. Most importantly, imitate
Yacc’s output file name conventions, so that the parser
implementation file is called ‘y.tab.c’, and the other outputs are
called ‘y.output’ and ‘y.tab.h’. Also, if generating a
deterministic parser in C, generate ‘#define’ statements in
addition to an ‘enum’ to associate token numbers with token names.
Thus, the following shell script can substitute for Yacc, and the
Bison distribution contains such a script for compatibility with
POSIX:
#! /bin/sh
bison -y "$@"
The ‘-y’/‘--yacc’ option is intended for use with traditional Yacc
grammars. If your grammar uses Bison extensions like
‘%glr-parser’, Bison might not be Yacc-compatible even if this
option is specified.
‘-W [CATEGORY]’
‘--warnings[=CATEGORY]’
Output warnings falling in CATEGORY. CATEGORY can be one of:
‘conflicts-sr’
‘conflicts-rr’
S/R and R/R conflicts. These warnings are enabled by default.
However, if the ‘%expect’ or ‘%expect-rr’ directive is
specified, an unexpected number of conflicts is an error, and
an expected number of conflicts is not reported, so ‘-W’ and
‘--warning’ then have no effect on the conflict report.
‘deprecated’
Deprecated constructs whose support will be removed in future
versions of Bison.
‘empty-rule’
Empty rules without ‘%empty’. *Note Empty Rules::. Disabled
by default, but enabled by uses of ‘%empty’, unless
‘-Wno-empty-rule’ was specified.
‘midrule-values’
Warn about midrule values that are set but not used within any
of the actions of the parent rule. For example, warn about
unused ‘$2’ in:
exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
Also warn about midrule values that are used but not set. For
example, warn about unset ‘$$’ in the midrule action in:
exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
These warnings are not enabled by default since they sometimes
prove to be false alarms in existing grammars employing the
Yacc constructs ‘$0’ or ‘$-N’ (where N is some positive
integer).
‘precedence’
Useless precedence and associativity directives. Disabled by
default.
Consider for instance the following grammar:
%nonassoc "="
%left "+"
%left "*"
%precedence "("
%%
stmt:
exp
| "var" "=" exp
;
exp:
exp "+" exp
| exp "*" "num"
| "(" exp ")"
| "num"
;
Bison reports:
warning: useless precedence and associativity for "="
| %nonassoc "="
| ^~~
warning: useless associativity for "*", use %precedence
| %left "*"
| ^~~
warning: useless precedence for "("
| %precedence "("
| ^~~
One would get the exact same parser with the following
directives instead:
%left "+"
%precedence "*"
‘yacc’
Incompatibilities with POSIX Yacc.
‘other’
All warnings not categorized above. These warnings are
enabled by default.
This category is provided merely for the sake of completeness.
Future releases of Bison may move warnings from this category
to new, more specific categories.
‘all’
All the warnings except ‘yacc’.
‘none’
Turn off all the warnings.
‘error’
See ‘-Werror’, below.
A category can be turned off by prefixing its name with ‘no-’. For
instance, ‘-Wno-yacc’ will hide the warnings about POSIX Yacc
incompatibilities.
‘-Werror’
Turn enabled warnings for every CATEGORY into errors, unless they
are explicitly disabled by ‘-Wno-error=CATEGORY’.
‘-Werror=CATEGORY’
Enable warnings falling in CATEGORY, and treat them as errors.
CATEGORY is the same as for ‘--warnings’, with the exception that
it may not be prefixed with ‘no-’ (see above).
Note that the precedence of the ‘=’ and ‘,’ operators is such that
the following commands are _not_ equivalent, as the first will not
treat S/R conflicts as errors.
$ bison -Werror=yacc,conflicts-sr input.y
$ bison -Werror=yacc,error=conflicts-sr input.y
‘-Wno-error’
Do not turn enabled warnings for every CATEGORY into errors, unless
they are explicitly enabled by ‘-Werror=CATEGORY’.
‘-Wno-error=CATEGORY’
Deactivate the error treatment for this CATEGORY. However, the
warning itself won’t be disabled, or enabled, by this option.
‘-f [FEATURE]’
‘--feature[=FEATURE]’
Activate miscellaneous FEATURE. FEATURE can be one of:
‘caret’
‘diagnostics-show-caret’
Show caret errors, in a manner similar to GCC’s
‘-fdiagnostics-show-caret’, or Clang’s ‘-fcaret-diagnostics’.
The location provided with the message is used to quote the
corresponding line of the source file, underlining the
important part of it with carets (‘^’). Here is an example,
using the following file ‘in.y’:
%type exp
%%
exp: exp '+' exp { $exp = $1 + $2; };
When invoked with ‘-fcaret’ (or nothing), Bison will report:
in.y:3.20-23: error: ambiguous reference: '$exp'
3 | exp: exp '+' exp { $exp = $1 + $2; };
| ^~~~
in.y:3.1-3: refers to: $exp at $$
3 | exp: exp '+' exp { $exp = $1 + $2; };
| ^~~
in.y:3.6-8: refers to: $exp at $1
3 | exp: exp '+' exp { $exp = $1 + $2; };
| ^~~
in.y:3.14-16: refers to: $exp at $3
3 | exp: exp '+' exp { $exp = $1 + $2; };
| ^~~
in.y:3.32-33: error: $2 of 'exp' has no declared type
3 | exp: exp '+' exp { $exp = $1 + $2; };
| ^~
Whereas, when invoked with ‘-fno-caret’, Bison will only
report:
in.y:3.20-23: error: ambiguous reference: ‘$exp’
in.y:3.1-3: refers to: $exp at $$
in.y:3.6-8: refers to: $exp at $1
in.y:3.14-16: refers to: $exp at $3
in.y:3.32-33: error: $2 of ‘exp’ has no declared type
This option is activated by default.
‘fixit’
‘diagnostics-parseable-fixits’
Show machine-readable fixes, in a manner similar to GCC’s and
Clang’s ‘-fdiagnostics-parseable-fixits’.
Fix-its are generated for duplicate directives:
$ cat foo.y
%define api.prefix {foo}
%define api.prefix {bar}
%%
exp:;
$ bison -ffixit foo.y
foo.y:2.1-24: error: %define variable 'api.prefix' redefined
2 | %define api.prefix {bar}
| ^~~~~~~~~~~~~~~~~~~~~~~~
foo.y:1.1-24: previous definition
1 | %define api.prefix {foo}
| ^~~~~~~~~~~~~~~~~~~~~~~~
fix-it:"foo.y":{2:1-2:25}:""
foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
They are also generated to update deprecated directives,
unless ‘-Wno-deprecated’ was given:
$ cat /tmp/foo.yy
%error-verbose
%name-prefix "foo"
%%
exp:;
$ bison foo.y
foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1 | %error-verbose
| ^~~~~~~~~~~~~~
foo.y:2.1-18: warning: deprecated directive, use '%define api.prefix {foo}' [-Wdeprecated]
2 | %name-prefix "foo"
| ^~~~~~~~~~~~~~~~~~
foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
The fix-its are applied by ‘bison’ itself when given the
option ‘-u’/‘--update’. See its documentation above.
‘syntax-only’
Do not generate the output files. The name of this feature is
somewhat misleading as more than just checking the syntax is
done: every stage is run (including checking for conflicts for
instance), except the generation of the output files.
Tuning the parser:
‘-t’
‘--debug’
In the parser implementation file, define the macro ‘YYDEBUG’ to 1
if it is not already defined, so that the debugging facilities are
compiled. *Note Tracing Your Parser: Tracing.
‘-D NAME[=VALUE]’
‘--define=NAME[=VALUE]’
‘-F NAME[=VALUE]’
‘--force-define=NAME[=VALUE]’
Each of these is equivalent to ‘%define NAME "VALUE"’ (*note
%define Summary::) except that Bison processes multiple definitions
for the same NAME as follows:
• Bison quietly ignores all command-line definitions for NAME
except the last.
• If that command-line definition is specified by a ‘-D’ or
‘--define’, Bison reports an error for any ‘%define’
definition for NAME.
• If that command-line definition is specified by a ‘-F’ or
‘--force-define’ instead, Bison quietly ignores all ‘%define’
definitions for NAME.
• Otherwise, Bison reports an error if there are multiple
‘%define’ definitions for NAME.
You should avoid using ‘-F’ and ‘--force-define’ in your make files
unless you are confident that it is safe to quietly ignore any
conflicting ‘%define’ that may be added to the grammar file.
‘-L LANGUAGE’
‘--language=LANGUAGE’
Specify the programming language for the generated parser, as if
‘%language’ was specified (*note Bison Declaration Summary: Decl
Summary.). Currently supported languages include C, C++, and Java.
LANGUAGE is case-insensitive.
‘--locations’
Pretend that ‘%locations’ was specified. *Note Decl Summary::.
‘-p PREFIX’
‘--name-prefix=PREFIX’
Pretend that ‘%name-prefix "PREFIX"’ was specified (*note Decl
Summary::). Obsoleted by ‘-Dapi.prefix=PREFIX’. *Note Multiple
Parsers in the Same Program: Multiple Parsers.
‘-l’
‘--no-lines’
Don’t put any ‘#line’ preprocessor commands in the parser
implementation file. Ordinarily Bison puts them in the parser
implementation file so that the C compiler and debuggers will
associate errors with your source file, the grammar file. This
option causes them to associate errors with the parser
implementation file, treating it as an independent source file in
its own right.
‘-S FILE’
‘--skeleton=FILE’
Specify the skeleton to use, similar to ‘%skeleton’ (*note Bison
Declaration Summary: Decl Summary.).
If FILE does not contain a ‘/’, FILE is the name of a skeleton file
in the Bison installation directory. If it does, FILE is an
absolute file name or a file name relative to the current working
directory. This is similar to how most shells resolve commands.
‘-k’
‘--token-table’
Pretend that ‘%token-table’ was specified. *Note Decl Summary::.
Adjust the output:
‘--defines[=FILE]’
Pretend that ‘%defines’ was specified, i.e., write an extra output
file containing macro definitions for the token type names defined
in the grammar, as well as a few other declarations. *Note Decl
Summary::.
‘-d’
This is the same as ‘--defines’ except ‘-d’ does not accept a FILE
argument since POSIX Yacc requires that ‘-d’ can be bundled with
other short options.
‘-b FILE-PREFIX’
‘--file-prefix=PREFIX’
Pretend that ‘%file-prefix’ was specified, i.e., specify prefix to
use for all Bison output file names. *Note Decl Summary::.
‘-r THINGS’
‘--report=THINGS’
Write an extra output file containing verbose description of the
comma separated list of THINGS among:
‘state’
Description of the grammar, conflicts (resolved and
unresolved), and parser’s automaton.
‘itemset’
Implies ‘state’ and augments the description of the automaton
with the full set of items for each state, instead of its core
only.
‘lookahead’
Implies ‘state’ and augments the description of the automaton
with each rule’s lookahead set.
‘solved’
Implies ‘state’. Explain how conflicts were solved thanks to
precedence and associativity directives.
‘all’
Enable all the items.
‘none’
Do not generate the report.
‘--report-file=FILE’
Specify the FILE for the verbose description.
‘-v’
‘--verbose’
Pretend that ‘%verbose’ was specified, i.e., write an extra output
file containing verbose descriptions of the grammar and parser.
*Note Decl Summary::.
‘-o FILE’
‘--output=FILE’
Specify the FILE for the parser implementation file.
The other output files’ names are constructed from FILE as
described under the ‘-v’ and ‘-d’ options.
‘-g [FILE]’
‘--graph[=FILE]’
Output a graphical representation of the parser’s automaton
computed by Bison, in Graphviz (http://www.graphviz.org/) DOT
(http://www.graphviz.org/doc/info/lang.html) format. ‘FILE’ is
optional. If omitted and the grammar file is ‘foo.y’, the output
file will be ‘foo.gv’ if the ‘%required’ version is 3.4 or better,
‘foo.dot’ otherwise.
‘-x [FILE]’
‘--xml[=FILE]’
Output an XML report of the parser’s automaton computed by Bison.
‘FILE’ is optional. If omitted and the grammar file is ‘foo.y’,
the output file will be ‘foo.xml’.
File: bison.info, Node: Option Cross Key, Next: Yacc Library, Prev: Bison Options, Up: Invocation
9.2 Option Cross Key
====================
Here is a list of options, alphabetized by long option, to help you find
the corresponding short option and directive.
Long Option Short Option Bison Directive
---------------------------------------------------------------------------------
‘--debug’ ‘-t’ ‘%debug’
‘--define=NAME[=VALUE]’ ‘-D NAME[=VALUE]’ ‘%define NAME [VALUE]’
‘--defines[=FILE]’ ‘-d’ ‘%defines ["FILE"]’
‘--feature[=FEATURES]’ ‘-f [FEATURES]’
‘--file-prefix=PREFIX’ ‘-b PREFIX’ ‘%file-prefix "PREFIX"’
‘--force-define=NAME[=VALUE]’ ‘-F NAME[=VALUE]’ ‘%define NAME [VALUE]’
‘--graph[=FILE]’ ‘-g [FILE]’
‘--help’ ‘-h’
‘--language=LANGUAGE’ ‘-L LANGUAGE’ ‘%language "LANGUAGE"’
‘--locations’ ‘%locations’
‘--name-prefix=PREFIX’ ‘-p PREFIX’ ‘%name-prefix "PREFIX"’
‘--no-lines’ ‘-l’ ‘%no-lines’
‘--output=FILE’ ‘-o FILE’ ‘%output "FILE"’
‘--print-datadir’
‘--print-localedir’
‘--report-file=FILE’
‘--report=THINGS’ ‘-r THINGS’
‘--skeleton=FILE’ ‘-S FILE’ ‘%skeleton "FILE"’
‘--token-table’ ‘-k’ ‘%token-table’
‘--update’ ‘-u’
‘--verbose’ ‘-v’ ‘%verbose’
‘--version’ ‘-V’
‘--warnings[=CATEGORY]’ ‘-W [CATEGORY]’
‘--xml[=FILE]’ ‘-x [FILE]’
‘--yacc’ ‘-y’ ‘%yacc’
File: bison.info, Node: Yacc Library, Prev: Option Cross Key, Up: Invocation
9.3 Yacc Library
================
The Yacc library contains default implementations of the ‘yyerror’ and
‘main’ functions. These default implementations are normally not
useful, but POSIX requires them. To use the Yacc library, link your
program with the ‘-ly’ option. Note that Bison’s implementation of the
Yacc library is distributed under the terms of the GNU General Public
License (*note Copying::).
If you use the Yacc library’s ‘yyerror’ function, you should declare
‘yyerror’ as follows:
int yyerror (char const *);
The ‘int’ value returned by this ‘yyerror’ is ignored.
The implementation of Yacc library’s ‘main’ function is:
int main (void)
{
setlocale (LC_ALL, "");
return yyparse ();
}
so if you use it, the internationalization support is enabled (e.g.,
error messages are translated), and your ‘yyparse’ function should have
the following type signature:
int yyparse (void);
File: bison.info, Node: Other Languages, Next: History, Prev: Invocation, Up: Top
10 Parsers Written In Other Languages
*************************************
In addition to C, Bison can generate parsers in C++ and Java. This
chapter is devoted to these languages. The reader is expected to
understand how Bison works; read the introductory chapters first if you
don’t.
* Menu:
* C++ Parsers:: The interface to generate C++ parser classes
* Java Parsers:: The interface to generate Java parser classes
File: bison.info, Node: C++ Parsers, Next: Java Parsers, Up: Other Languages
10.1 C++ Parsers
================
The Bison parser in C++ is an object, an instance of the class
‘yy::parser’.
* Menu:
* A Simple C++ Example:: A short introduction to C++ parsers
* C++ Bison Interface:: Asking for C++ parser generation
* C++ Parser Interface:: Instantiating and running the parser
* C++ Semantic Values:: %union vs. C++
* C++ Location Values:: The position and location classes
* C++ Scanner Interface:: Exchanges between yylex and parse
* A Complete C++ Example:: Demonstrating their use
File: bison.info, Node: A Simple C++ Example, Next: C++ Bison Interface, Up: C++ Parsers
10.1.1 A Simple C++ Example
---------------------------
This tutorial about C++ parsers is based on a simple, self contained
example. The following sections are the reference manual for Bison with
C++, the last one showing a fully blown example (*note A Complete C++
Example::).
To look nicer, our example will be in C++14. It is not required:
Bison supports the original C++98 standard.
A Bison file has three parts. In the first part, the prologue, we
start by making sure we run a version of Bison which is recent enough,
and that we generate C++.
%require "3.2"
%language "c++"
Let’s dive directly into the middle part: the grammar. Our input is
a simple list of strings, that we display once the parsing is done.
%%
result:
list { std::cout << $1 << '\n'; }
;
%type > list;
list:
%empty { /* Generates an empty string list */ }
| list item { $$ = $1; $$.push_back ($2); }
;
We used a vector of strings as a semantic value! To use genuine C++
objects as semantic values—not just PODs—we cannot rely on the union
that Bison uses by default to store them, we need _variants_ (*note C++
Variants::):
%define api.value.type variant
Obviously, the rule for ‘result’ needs to print a vector of strings.
In the prologue, we add:
%code
{
// Print a list of strings.
auto
operator<< (std::ostream& o, const std::vector& ss)
-> std::ostream&
{
o << '{';
const char *sep = "";
for (const auto& s: ss)
{
o << sep << s;
sep = ", ";
}
return o << '}';
}
}
You may want to move it into the ‘yy’ namespace to avoid leaking it in
your default namespace. We recommend that you keep the actions simple,
and move details into auxiliary functions, as we did with ‘operator<<’.
Our list of strings will be built from two types of items: numbers
and strings:
%type item;
%token TEXT;
%token NUMBER;
item:
TEXT
| NUMBER { $$ = std::to_string ($1); }
;
In the case of ‘TEXT’, the implicit default action applies:
‘$$ = $1’.
Our scanner deserves some attention. The traditional interface of
‘yylex’ is not type safe: since the token type and the token value are
not correlated, you may return a ‘NUMBER’ with a string as semantic
value. To avoid this, we use _token constructors_ (*note Complete
Symbols::). This directive:
%define api.token.constructor
requests that Bison generates the functions ‘make_TEXT’ and
‘make_NUMBER’. As a matter of fact, it is convenient to have also a
symbol to mark the end of input, say ‘END_OF_FILE’:
%token END_OF_FILE 0
The ‘0’ tells Bison this token is special: when it is reached, parsing
finishes.
Everything is in place for our scanner:
%code
{
namespace yy
{
// Return the next token.
auto yylex () -> parser::symbol_type
{
static int count = 0;
switch (int stage = count++)
{
case 0:
return parser::make_TEXT ("I have three numbers for you.");
case 1: case 2: case 3:
return parser::make_NUMBER (stage);
case 4:
return parser::make_TEXT ("And that's all!");
default:
return parser::make_END_OF_FILE ();
}
}
}
}
In the epilogue, the third part of a Bison grammar file, we leave
simple details: the error reporting function, and the main function.
%%
namespace yy
{
// Report an error to the user.
auto parser::error (const std::string& msg) -> void
{
std::cerr << msg << '\n';
}
}
int main ()
{
yy::parser parse;
return parse ();
}
Compile, and run!
$ bison simple.yy -o simple.cc
$ g++ -std=c++14 simple.cc -o simple
$ ./simple
{I have three numbers for you., 1, 2, 3, And that's all!}
File: bison.info, Node: C++ Bison Interface, Next: C++ Parser Interface, Prev: A Simple C++ Example, Up: C++ Parsers
10.1.2 C++ Bison Interface
--------------------------
The C++ deterministic parser is selected using the skeleton directive,
‘%skeleton "lalr1.cc"’. *Note Decl Summary::.
When run, ‘bison’ will create several entities in the ‘yy’ namespace.
Use the ‘%define api.namespace’ directive to change the namespace name,
see *note api.namespace: %define Summary. The various classes are
generated in the following files:
‘FILE.hh’
(Assuming the extension of the grammar file was ‘.yy’.) The
declaration of the C++ parser class and auxiliary types. By
default, this file is not generated (*note %defines: Decl
Summary.).
‘FILE.cc’
The implementation of the C++ parser class. The basename and
extension of these two files (‘FILE.hh’ and ‘FILE.cc’) follow the
same rules as with regular C parsers (*note Invocation::).
‘location.hh’
Generated when both ‘%defines’ and ‘%locations’ are enabled, this
file contains the definition of the classes ‘position’ and
‘location’, used for location tracking. It is not generated if
‘%define api.location.file none’ is specified, or if user defined
locations are used. *Note C++ Location Values::.
‘position.hh’
‘stack.hh’
Useless legacy files. To get rid of then, use ‘%require "3.2"’ or
newer.
All these files are documented using Doxygen; run ‘doxygen’ for a
complete and accurate documentation.
File: bison.info, Node: C++ Parser Interface, Next: C++ Semantic Values, Prev: C++ Bison Interface, Up: C++ Parsers
10.1.3 C++ Parser Interface
---------------------------
The output files ‘FILE.hh’ and ‘FILE.cc’ declare and define the parser
class in the namespace ‘yy’. The class name defaults to ‘parser’, but
may be changed using ‘%define api.parser.class {NAME}’. The interface
of this class is detailed below. It can be extended using the
‘%parse-param’ feature: its semantics is slightly changed since it
describes an additional member of the parser class, and an additional
argument for its constructor.
-- Type of parser: semantic_type
The types for semantic values. *Note C++ Semantic Values::.
-- Type of parser: location_type
The type of locations, if location tracking is enabled. *Note C++
Location Values::.
-- Type of parser: token
A structure that contains (only) the ‘yytokentype’ enumeration,
which defines the tokens. To refer to the token ‘FOO’, use
‘yy::parser::token::FOO’. The scanner can use ‘typedef
yy::parser::token token;’ to “import” the token enumeration (*note
Calc++ Scanner::).
-- Type of parser: syntax_error
This class derives from ‘std::runtime_error’. Throw instances of
it from the scanner or from the actions to raise parse errors.
This is equivalent with first invoking ‘error’ to report the
location and message of the syntax error, and then to invoke
‘YYERROR’ to enter the error-recovery mode. But contrary to
‘YYERROR’ which can only be invoked from user actions (i.e.,
written in the action itself), the exception can be thrown from
functions invoked from the user action.
-- Constructor on parser: parser ()
-- Constructor on parser: parser (TYPE1 ARG1, ...)
Build a new parser object. There are no arguments, unless
‘%parse-param {TYPE1 ARG1}’ was used.
-- Constructor on syntax_error: syntax_error (const location_type& L,
const std::string& M)
-- Constructor on syntax_error: syntax_error (const std::string& M)
Instantiate a syntax-error exception.
-- Method on parser: int operator() ()
-- Method on parser: int parse ()
Run the syntactic analysis, and return 0 on success, 1 otherwise.
Both routines are equivalent, ‘operator()’ being more C++ish.
The whole function is wrapped in a ‘try’/‘catch’ block, so that
when an exception is thrown, the ‘%destructor’s are called to
release the lookahead symbol, and the symbols pushed on the stack.
Exception related code in the generated parser is protected by CPP
guards (‘#if’) and disabled when exceptions are not supported
(i.e., passing ‘-fno-exceptions’ to the C++ compiler).
-- Method on parser: std::ostream& debug_stream ()
-- Method on parser: void set_debug_stream (std::ostream& O)
Get or set the stream used for tracing the parsing. It defaults to
‘std::cerr’.
-- Method on parser: debug_level_type debug_level ()
-- Method on parser: void set_debug_level (debug_level_type L)
Get or set the tracing level (an integral). Currently its value is
either 0, no trace, or nonzero, full tracing.
-- Method on parser: void error (const location_type& L, const
std::string& M)
-- Method on parser: void error (const std::string& M)
The definition for this member function must be supplied by the
user: the parser uses it to report a parser error occurring at L,
described by M. If location tracking is not enabled, the second
signature is used.
File: bison.info, Node: C++ Semantic Values, Next: C++ Location Values, Prev: C++ Parser Interface, Up: C++ Parsers
10.1.4 C++ Semantic Values
--------------------------
Bison supports two different means to handle semantic values in C++.
One is alike the C interface, and relies on unions. As C++
practitioners know, unions are inconvenient in C++, therefore another
approach is provided, based on variants.
* Menu:
* C++ Unions:: Semantic values cannot be objects
* C++ Variants:: Using objects as semantic values
File: bison.info, Node: C++ Unions, Next: C++ Variants, Up: C++ Semantic Values
10.1.4.1 C++ Unions
...................
The ‘%union’ directive works as for C, see *note The Union Declaration:
Union Decl. In particular it produces a genuine ‘union’, which have a
few specific features in C++.
− The type ‘YYSTYPE’ is defined but its use is discouraged: rather
you should refer to the parser’s encapsulated type
‘yy::parser::semantic_type’.
− Non POD (Plain Old Data) types cannot be used. C++98 forbids any
instance of classes with constructors in unions: only _pointers_ to
such objects are allowed. C++11 relaxed this constraints, but at
the cost of safety.
Because objects have to be stored via pointers, memory is not
reclaimed automatically: using the ‘%destructor’ directive is the only
means to avoid leaks. *Note Freeing Discarded Symbols: Destructor Decl.
File: bison.info, Node: C++ Variants, Prev: C++ Unions, Up: C++ Semantic Values
10.1.4.2 C++ Variants
.....................
Bison provides a _variant_ based implementation of semantic values for
C++. This alleviates all the limitations reported in the previous
section, and in particular, object types can be used without pointers.
To enable variant-based semantic values, set the ‘%define’ variable
‘api.value.type’ to ‘variant’ (*note ‘api.value.type’: %define
Summary.). Then ‘%union’ is ignored; instead of using the name of the
fields of the ‘%union’ to “type” the symbols, use genuine types.
For instance, instead of:
%union
{
int ival;
std::string* sval;
}
%token NUMBER;
%token STRING;
write:
%token NUMBER;
%token STRING;
‘STRING’ is no longer a pointer, which should fairly simplify the
user actions in the grammar and in the scanner (in particular the memory
management).
Since C++ features destructors, and since it is customary to
specialize ‘operator<<’ to support uniform printing of values, variants
also typically simplify Bison printers and destructors.
Variants are stricter than unions. When based on unions, you may
play any dirty game with ‘yylval’, say storing an ‘int’, reading a
‘char*’, and then storing a ‘double’ in it. This is no longer possible
with variants: they must be initialized, then assigned to, and
eventually, destroyed. As a matter of fact, Bison variants forbid the
use of alternative types such as ‘$2’ or ‘$$’, even in
midrule actions. It is mandatory to use typed midrule actions (*note
Typed Midrule Actions::).
-- Method on semantic_type: T& emplace ()
-- Method on semantic_type: T& emplace (const T& T)
Available in C++98/C++03 only. Default construct/copy-construct
from T. Return a reference to where the actual value may be
stored. Requires that the variant was not initialized yet.
-- Method on semantic_type: T& emplace (U&&... U)
Available in C++11 and later only. Build a variant of type ‘T’
from the variadic forwarding references U....
*Warning*: We do not use Boost.Variant, for two reasons. First, it
appeared unacceptable to require Boost on the user’s machine (i.e., the
machine on which the generated parser will be compiled, not the machine
on which ‘bison’ was run). Second, for each possible semantic value,
Boost.Variant not only stores the value, but also a tag specifying its
type. But the parser already “knows” the type of the semantic value, so
that would be duplicating the information.
We do not use C++17’s ‘std::variant’ either: we want to support all
the C++ standards, and of course ‘std::variant’ also stores a tag to
record the current type.
Therefore we developed light-weight variants whose type tag is
external (so they are really like ‘unions’ for C++ actually). There is
a number of limitations in (the current implementation of) variants:
• Alignment must be enforced: values should be aligned in memory
according to the most demanding type. Computing the smallest
alignment possible requires meta-programming techniques that are
not currently implemented in Bison, and therefore, since, as far as
we know, ‘double’ is the most demanding type on all platforms,
alignments are enforced for ‘double’ whatever types are actually
used. This may waste space in some cases.
• There might be portability issues we are not aware of.
As far as we know, these limitations _can_ be alleviated. All it
takes is some time and/or some talented C++ hacker willing to contribute
to Bison.
File: bison.info, Node: C++ Location Values, Next: C++ Scanner Interface, Prev: C++ Semantic Values, Up: C++ Parsers
10.1.5 C++ Location Values
--------------------------
When the directive ‘%locations’ is used, the C++ parser supports
location tracking, see *note Tracking Locations::.
By default, two auxiliary classes define a ‘position’, a single point
in a file, and a ‘location’, a range composed of a pair of ‘position’s
(possibly spanning several files). But if the ‘%define’ variable
‘api.location.type’ is defined, then these classes will not be
generated, and the user defined type will be used.
* Menu:
* C++ position:: One point in the source file
* C++ location:: Two points in the source file
* Exposing the Location Classes:: Using the Bison location class in your
project
* User Defined Location Type:: Required interface for locations
File: bison.info, Node: C++ position, Next: C++ location, Up: C++ Location Values
10.1.5.1 C++ ‘position’
.......................
-- Constructor on position: position (std::string* FILE = nullptr,
unsigned LINE = 1, unsigned COL = 1)
Create a ‘position’ denoting a given point. Note that ‘file’ is
not reclaimed when the ‘position’ is destroyed: memory managed must
be handled elsewhere.
-- Method on position: void initialize (std::string* FILE = nullptr,
unsigned LINE = 1, unsigned COL = 1)
Reset the position to the given values.
-- Instance Variable of position: std::string* file
The name of the file. It will always be handled as a pointer, the
parser will never duplicate nor deallocate it. As an experimental
feature you may change it to ‘TYPE*’ using ‘%define filename_type
"TYPE"’.
-- Instance Variable of position: unsigned line
The line, starting at 1.
-- Method on position: void lines (int HEIGHT = 1)
If HEIGHT is not null, advance by HEIGHT lines, resetting the
column number. The resulting line number cannot be less than 1.
-- Instance Variable of position: unsigned column
The column, starting at 1.
-- Method on position: void columns (int WIDTH = 1)
Advance by WIDTH columns, without changing the line number. The
resulting column number cannot be less than 1.
-- Method on position: position& operator+= (int WIDTH)
-- Method on position: position operator+ (int WIDTH)
-- Method on position: position& operator-= (int WIDTH)
-- Method on position: position operator- (int WIDTH)
Various forms of syntactic sugar for ‘columns’.
-- Method on position: bool operator== (const position& THAT)
-- Method on position: bool operator!= (const position& THAT)
Whether ‘*this’ and ‘that’ denote equal/different positions.
-- Function: std::ostream& operator<< (std::ostream& O, const position&
P)
Report P on O like this: ‘FILE:LINE.COLUMN’, or ‘LINE.COLUMN’ if
FILE is null.
File: bison.info, Node: C++ location, Next: Exposing the Location Classes, Prev: C++ position, Up: C++ Location Values
10.1.5.2 C++ ‘location’
.......................
-- Constructor on location: location (const position& BEGIN, const
position& END)
Create a ‘Location’ from the endpoints of the range.
-- Constructor on location: location (const position& POS = position())
-- Constructor on location: location (std::string* FILE, unsigned LINE,
unsigned COL)
Create a ‘Location’ denoting an empty range located at a given
point.
-- Method on location: void initialize (std::string* FILE = nullptr,
unsigned LINE = 1, unsigned COL = 1)
Reset the location to an empty range at the given values.
-- Instance Variable of location: position begin
-- Instance Variable of location: position end
The first, inclusive, position of the range, and the first beyond.
-- Method on location: void columns (int WIDTH = 1)
-- Method on location: void lines (int HEIGHT = 1)
Forwarded to the ‘end’ position.
-- Method on location: location operator+ (int WIDTH)
-- Method on location: location operator+= (int WIDTH)
-- Method on location: location operator- (int WIDTH)
-- Method on location: location operator-= (int WIDTH)
Various forms of syntactic sugar for ‘columns’.
-- Method on location: location operator+ (const location& END)
-- Method on location: location operator+= (const location& END)
Join two locations: starts at the position of the first one, and
ends at the position of the second.
-- Method on location: void step ()
Move ‘begin’ onto ‘end’.
-- Method on location: bool operator== (const location& THAT)
-- Method on location: bool operator!= (const location& THAT)
Whether ‘*this’ and ‘that’ denote equal/different ranges of
positions.
-- Function: std::ostream& operator<< (std::ostream& O, const location&
P)
Report P on O, taking care of special cases such as: no ‘filename’
defined, or equal filename/line or column.
File: bison.info, Node: Exposing the Location Classes, Next: User Defined Location Type, Prev: C++ location, Up: C++ Location Values
10.1.5.3 Exposing the Location Classes
......................................
When both ‘%defines’ and ‘%locations’ are enabled, Bison generates an
additional file: ‘location.hh’. If you don’t use locations outside of
the parser, you may avoid its creation with ‘%define api.location.file
none’.
However this file is useful if, for instance, your parser builds an
abstract syntax tree decorated with locations: you may use Bison’s
‘location’ type independently of Bison’s parser. You may name the file
differently, e.g., ‘%define api.location.file
"include/ast/location.hh"’: this name can have directory components, or
even be absolute. The way the location file is included is controlled
by ‘api.location.include’.
This way it is possible to have several parsers share the same
location file.
For instance, in ‘src/foo/parser.yy’, generate the
‘include/ast/loc.hh’ file:
// src/foo/parser.yy
%locations
%define api.namespace {foo}
%define api.location.file "include/ast/loc.hh"
%define api.location.include {}
and use it in ‘src/bar/parser.yy’:
// src/bar/parser.yy
%locations
%define api.namespace {bar}
%code requires {#include }
%define api.location.type {bar::location}
Absolute file names are supported; it is safe in your ‘Makefile’ to
pass the flag ‘-Dapi.location.file='"$(top_srcdir)/include/ast/loc.hh"'’
to ‘bison’ for ‘src/foo/parser.yy’. The generated file will not have
references to this absolute path, thanks to ‘%define
api.location.include {}’. Adding ‘-I $(top_srcdir)/include’
to your ‘CPPFLAGS’ will suffice for the compiler to find ‘ast/loc.hh’.
File: bison.info, Node: User Defined Location Type, Prev: Exposing the Location Classes, Up: C++ Location Values
10.1.5.4 User Defined Location Type
...................................
Instead of using the built-in types you may use the ‘%define’ variable
‘api.location.type’ to specify your own type:
%define api.location.type {LOCATIONTYPE}
The requirements over your LOCATIONTYPE are:
• it must be copyable;
• in order to compute the (default) value of ‘@$’ in a reduction, the
parser basically runs
@$.begin = @1.begin;
@$.end = @N.end; // The location of last right-hand side symbol.
so there must be copyable ‘begin’ and ‘end’ members;
• alternatively you may redefine the computation of the default
location, in which case these members are not required (*note
Location Default Action::);
• if traces are enabled, then there must exist an ‘std::ostream&
operator<< (std::ostream& o, const LOCATIONTYPE& s)’ function.
In programs with several C++ parsers, you may also use the ‘%define’
variable ‘api.location.type’ to share a common set of built-in
definitions for ‘position’ and ‘location’. For instance, one parser
‘master/parser.yy’ might use:
%defines
%locations
%define api.namespace {master::}
to generate the ‘master/position.hh’ and ‘master/location.hh’ files,
reused by other parsers as follows:
%define api.location.type {master::location}
%code requires { #include }
File: bison.info, Node: C++ Scanner Interface, Next: A Complete C++ Example, Prev: C++ Location Values, Up: C++ Parsers
10.1.6 C++ Scanner Interface
----------------------------
The parser invokes the scanner by calling ‘yylex’. Contrary to C
parsers, C++ parsers are always pure: there is no point in using the
‘%define api.pure’ directive. The actual interface with ‘yylex’ depends
whether you use unions, or variants.
* Menu:
* Split Symbols:: Passing symbols as two/three components
* Complete Symbols:: Making symbols a whole
File: bison.info, Node: Split Symbols, Next: Complete Symbols, Up: C++ Scanner Interface
10.1.6.1 Split Symbols
......................
The generated parser expects ‘yylex’ to have the following prototype.
-- Function: int yylex (semantic_type* YYLVAL, location_type* YYLLOC,
TYPE1 ARG1, ...)
-- Function: int yylex (semantic_type* YYLVAL, TYPE1 ARG1, ...)
Return the next token. Its type is the return value, its semantic
value and location (if enabled) being YYLVAL and YYLLOC.
Invocations of ‘%lex-param {TYPE1 ARG1}’ yield additional
arguments.
Note that when using variants, the interface for ‘yylex’ is the same,
but ‘yylval’ is handled differently.
Regular union-based code in Lex scanner typically look like:
[0-9]+ {
yylval->ival = text_to_int (yytext);
return yy::parser::token::INTEGER;
}
[a-z]+ {
yylval->sval = new std::string (yytext);
return yy::parser::token::IDENTIFIER;
}
Using variants, ‘yylval’ is already constructed, but it is not
initialized. So the code would look like:
[0-9]+ {
yylval->emplace () = text_to_int (yytext);
return yy::parser::token::INTEGER;
}
[a-z]+ {
yylval->emplace () = yytext;
return yy::parser::token::IDENTIFIER;
}
or
[0-9]+ {
yylval->emplace (text_to_int (yytext));
return yy::parser::token::INTEGER;
}
[a-z]+ {
yylval->emplace (yytext);
return yy::parser::token::IDENTIFIER;
}
File: bison.info, Node: Complete Symbols, Prev: Split Symbols, Up: C++ Scanner Interface
10.1.6.2 Complete Symbols
.........................
With both ‘%define api.value.type variant’ and ‘%define
api.token.constructor’, the parser defines the type ‘symbol_type’, and
expects ‘yylex’ to have the following prototype.
-- Function: parser::symbol_type yylex ()
-- Function: parser::symbol_type yylex (TYPE1 ARG1, ...)
Return a _complete_ symbol, aggregating its type (i.e., the
traditional value returned by ‘yylex’), its semantic value, and
possibly its location. Invocations of ‘%lex-param {TYPE1 ARG1}’
yield additional arguments.
For each token type, Bison generates named constructors as follows.
-- Constructor on parser::symbol_type: symbol_type (int TOKEN, const
VALUE_TYPE& VALUE, const location_type& LOCATION)
-- Constructor on parser::symbol_type: symbol_type (int TOKEN, const
location_type& LOCATION)
-- Constructor on parser::symbol_type: symbol_type (int TOKEN, const
VALUE_TYPE& VALUE)
-- Constructor on parser::symbol_type: symbol_type (int TOKEN)
Build a complete terminal symbol for the token type TOKEN
(including the ‘api.token.prefix’), whose semantic value, if it has
one, is VALUE of adequate VALUE_TYPE. Pass the LOCATION iff
location tracking is enabled.
Consistency between TOKEN and VALUE_TYPE is checked via an
‘assert’.
For instance, given the following declarations:
%define api.token.prefix {TOK_}
%token IDENTIFIER;
%token INTEGER;
%token ':';
you may use these constructors:
symbol_type (int token, const std::string&, const location_type&);
symbol_type (int token, const int&, const location_type&);
symbol_type (int token, const location_type&);
which should be used in a Flex-scanner as follows.
%%
[a-z]+ return yy::parser::symbol_type (TOK_IDENTIFIER, yytext, loc);
[0-9]+ return yy::parser::symbol_type (TOK_INTEGER, text_to_int (yytext), loc);
":" return yy::parser::symbol_type (':', loc);
<> return yy::parser::symbol_type (0, loc);
Note that it is possible to generate and compile type incorrect code
(e.g. ‘symbol_type (':', yytext, loc)’). It will fail at run time,
provided the assertions are enabled (i.e., ‘-DNDEBUG’ was not passed to
the compiler). Bison supports an alternative that guarantees that type
incorrect code will not even compile. Indeed, it generates _named
constructors_ as follows.
-- Method on parser: symbol_type make_TOKEN (const VALUE_TYPE& VALUE,
const location_type& LOCATION)
-- Method on parser: symbol_type make_TOKEN (const location_type&
LOCATION)
-- Method on parser: symbol_type make_TOKEN (const VALUE_TYPE& VALUE)
-- Method on parser: symbol_type make_TOKEN ()
Build a complete terminal symbol for the token type TOKEN (not
including the ‘api.token.prefix’), whose semantic value, if it has
one, is VALUE of adequate VALUE_TYPE. Pass the LOCATION iff
location tracking is enabled.
For instance, given the following declarations:
%define api.token.prefix {TOK_}
%token IDENTIFIER;
%token INTEGER;
%token COLON;
%token EOF 0;
Bison generates:
symbol_type make_IDENTIFIER (const std::string&, const location_type&);
symbol_type make_INTEGER (const int&, const location_type&);
symbol_type make_COLON (const location_type&);
symbol_type make_EOF (const location_type&);
which should be used in a scanner as follows.
[a-z]+ return yy::parser::make_IDENTIFIER (yytext, loc);
[0-9]+ return yy::parser::make_INTEGER (text_to_int (yytext), loc);
":" return yy::parser::make_COLON (loc);
<> return yy::parser::make_EOF (loc);
Tokens that do not have an identifier are not accessible: you cannot
simply use characters such as ‘':'’, they must be declared with
‘%token’, including the end-of-file token.
File: bison.info, Node: A Complete C++ Example, Prev: C++ Scanner Interface, Up: C++ Parsers
10.1.7 A Complete C++ Example
-----------------------------
This section demonstrates the use of a C++ parser with a simple but
complete example. This example should be available on your system,
ready to compile, in the directory
“.../share/doc/bison/examples/calc++”. It focuses on the use of Bison,
therefore the design of the various C++ classes is very naive: no
accessors, no encapsulation of members etc. We will use a Lex scanner,
and more precisely, a Flex scanner, to demonstrate the various
interactions. A hand-written scanner is actually easier to interface
with.
* Menu:
* Calc++ --- C++ Calculator:: The specifications
* Calc++ Parsing Driver:: An active parsing context
* Calc++ Parser:: A parser class
* Calc++ Scanner:: A pure C++ Flex scanner
* Calc++ Top Level:: Conducting the band
File: bison.info, Node: Calc++ --- C++ Calculator, Next: Calc++ Parsing Driver, Up: A Complete C++ Example
10.1.7.1 Calc++ — C++ Calculator
................................
Of course the grammar is dedicated to arithmetics, a single expression,
possibly preceded by variable assignments. An environment containing
possibly predefined variables such as ‘one’ and ‘two’, is exchanged with
the parser. An example of valid input follows.
three := 3
seven := one + two * three
seven * seven
File: bison.info, Node: Calc++ Parsing Driver, Next: Calc++ Parser, Prev: Calc++ --- C++ Calculator, Up: A Complete C++ Example
10.1.7.2 Calc++ Parsing Driver
..............................
To support a pure interface with the parser (and the scanner) the
technique of the “parsing context” is convenient: a structure containing
all the data to exchange. Since, in addition to simply launch the
parsing, there are several auxiliary tasks to execute (open the file for
scanning, instantiate the parser etc.), we recommend transforming the
simple parsing context structure into a fully blown “parsing driver”
class.
The declaration of this driver class, in ‘driver.hh’, is as follows.
The first part includes the CPP guard and imports the required standard
library components, and the declaration of the parser class.
#ifndef DRIVER_HH
# define DRIVER_HH
# include
# include