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- Info file bison.info, produced by Makeinfo, -*- Text -*- from input
file bison.texinfo.
This file documents the Bison parser generator.
Copyright (C) 1988, 1989, 1990 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License" and
"Conditions for Using Bison" are included exactly as in the original,
and provided that the entire resulting derived work is distributed
under the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License", "Conditions for Using Bison" and this permission notice may
be included in translations approved by the Free Software Foundation
instead of in the original English.
File: bison.info, Node: Top, Next: Introduction, Prev: (DIR), Up: (DIR)
This manual documents version 1.16 of Bison.
* Menu:
* Introduction::
* Conditions::
* 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:: Debugging Bison parsers that parse wrong.
* Invocation:: How to run Bison (to produce the parser source file).
* Table of Symbols:: All the keywords of the Bison language are explained.
* Glossary:: Basic concepts are explained.
* Index:: Cross-references to the text.
File: bison.info, Node: Introduction, Next: Conditions, Prev: Top, Up: Top
Introduction
************
"Bison" is a general-purpose parser generator that converts a
grammar description for an LALR(1) context-free grammar into a C
program to parse that grammar. Once you are proficient with Bison,
you may 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 programming in order to use Bison or to understand
this manual.
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 primarily by Robert Corbett; Richard Stallman made
it Yacc-compatible. This edition corresponds to version 1.16 of Bison.
File: bison.info, Node: Conditions, Next: Copying, Prev: Introduction, Up: Top
Conditions for Using Bison
**************************
Bison grammars can be used only in programs that are free software.
This is in contrast to what happens with the GNU C compiler and the
other GNU programming tools.
The reason Bison is special is that the output of the Bison
utility--the Bison parser file--contains a verbatim copy of a sizable
piece of Bison, which is the code for the `yyparse' function. (The
actions from your grammar are inserted into this function at one
point, but the rest of the function is not changed.)
As a result, the Bison parser file is covered by the same copying
conditions that cover Bison itself and the rest of the GNU system: any
program containing it has to be distributed under the standard GNU
copying conditions.
Occasionally people who would like to use Bison to develop
proprietary programs complain about this.
We don't particularly sympathize with their complaints. The
purpose of the GNU project is to promote the right to share software
and the practice of sharing software; it is a means of changing
society. The people who complain are planning to be uncooperative
toward the rest of the world; why should they deserve our help in
doing so?
However, it's possible that a change in these conditions might
encourage computer companies to use and distribute the GNU system. If
so, then we might decide to change the terms on `yyparse' as a matter
of the strategy of promoting the right to share. Such a change would
be irrevocable. Since we stand by the copying permissions we have
announced, we cannot withdraw them once given.
We mustn't make an irrevocable change hastily. We have to wait
until there is a complete GNU system and there has been time to learn
how this issue affects its reception.
File: bison.info, Node: Copying, Next: Concepts, Prev: Conditions, Up: Top
GNU GENERAL PUBLIC LICENSE
**************************
Version 2, June 1991
Copyright (C) 1989, 1991 Free Software Foundation, Inc.
675 Mass Ave, Cambridge, MA 02139, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
Preamble
========
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How to Apply These Terms to Your New Programs
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interest in the program `Gnomovision'
(which makes passes at compilers) written
by James Hacker.
SIGNATURE OF TY COON, 1 April 1989
Ty Coon, President of Vice
This General Public License does not permit incorporating your
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library, you may consider it more useful to permit linking proprietary
applications with the library. If this is what you want to do, use
the GNU Library General Public License instead of this License.
File: bison.info, Node: Concepts, Next: Examples, Prev: Copying, Up: Top
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.
* 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, Prev: Concepts, Up: Concepts
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.
Not all context-free languages can be handled by Bison, only those
that are LALR(1). In brief, this means that it must be possible to
tell how to parse any portion of an input string with just a single
token of look-ahead. Strictly speaking, that is a description of an
LR(1) grammar, and LALR(1) involves additional restrictions that are
hard to explain simply; but it is rare in actual practice to find an
LR(1) grammar that fails to be LALR(1). *Note Mysterious
Reduce/Reduce Conflicts: Mystery Conflicts, for more information on
this.
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 (x) /* identifier, open-paren, */
/* identifier, close-paren */
int x; /* keyword `int', identifier, semicolon */
{ /* 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
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 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.
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 Rules::.
File: bison.info, Node: Semantic Values, Next: Semantic Actions, Prev: Grammar in Bison, Up: Concepts
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 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: Bison Parser, Prev: Semantic Values, Up: Concepts
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: Bison Parser, Next: Stages, Prev: Semantic Actions, Up: Concepts
Bison Output: the Parser File
=============================
When you run Bison, you give it a Bison grammar file as input. The
output is a C source file that parses the language described by the
grammar. This file is called a "Bison parser". 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 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 Lexical::.
The Bison parser 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 Interface::.
Aside from the token type names and the symbols in the actions you
write, all variable and function names used in the Bison parser file
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.
File: bison.info, Node: Stages, Next: Grammar Layout, Prev: Bison Parser, Up: Concepts
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
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
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
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:
%{
C DECLARATIONS
%}
BISON DECLARATIONS
%%
GRAMMAR RULES
%%
ADDITIONAL C CODE
The `%%', `%{' and `%}' are punctuation that appears in every Bison
grammar file to separate the sections.
The C declarations 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.
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 additional C code can contain any C code you want to use.
Often the definition of the lexical analyzer `yylex' goes here, plus
subroutines called by the actions in the grammar rules. 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
Examples
********
Now we show and explain three sample programs written using Bison: a
reverse polish notation calculator, an algebraic (infix) notation
calculator, and a multi-function calculator. All three have been
tested under BSD Unix 4.3; each produces a usable, though limited,
interactive desk-top calculator.
These examples are simple, but Bison grammars for real programming
languages are written the same way.
You can copy these examples out of the Info file and 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.
* 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, Prev: Examples, Up: Examples
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 input files.
* Menu:
* Decls: Rpcalc Decls. Bison and C declarations for rpcalc.
* Rules: Rpcalc Rules. Grammar Rules for rpcalc, with explanation.
* Input: Rpcalc Input. Explaining the rules for `input'.
* Line: Rpcalc Line. Explaining the rules for `line'.
* Expr: Rpcalc Expr. Explaining the rules for `expr'.
* Lexer: Rpcalc Lexer. The lexical analyzer.
* Main: Rpcalc Main. The controlling function.
* Error: Rpcalc Error. The error reporting function.
* Gen: Rpcalc Gen. Running Bison on the grammar file.
* Comp: Rpcalc Compile. Run the C compiler on the output code.
File: bison.info, Node: Rpcalc Decls, Next: Rpcalc Rules, Prev: RPN calc, Up: RPN calc
Declarations for `rpcalc'
-------------------------
Here are the C and Bison declarations for the reverse polish
notation calculator. As in C, comments are placed between `/*...*/'.
/* Reverse polish notation calculator. */
%{
#define YYSTYPE double
#include <math.h>
%}
%token NUM
%% /* Grammar rules and actions follow */
The C declarations section (*note C Declarations::.) contains two
preprocessor directives.
The `#define' directive defines the macro `YYSTYPE', thus
specifying the C data type for semantic values of both tokens and
groupings (*note Value Type::.). The Bison parser will use whatever
type `YYSTYPE' 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.
The `#include' directive is used to declare the exponentiation
function `pow'.
The second section, Bison declarations, provides information to
Bison about the token types (*note Bison Declarations::.). Each
terminal symbol that is not a single-character literal must be
declared here. (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 Input, Prev: Rpcalc Decls, Up: RPN Calc
Grammar Rules for `rpcalc'
--------------------------
Here are the grammar rules for the reverse polish notation
calculator.
input: /* empty */
| input line
;
line: '\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
;
exp: NUM { $$ = $1; }
| exp exp '+' { $$ = $1 + $2; }
| exp exp '-' { $$ = $1 - $2; }
| exp exp '*' { $$ = $1 * $2; }
| exp exp '/' { $$ = $1 / $2; }
/* Exponentiation */
| exp exp '^' { $$ = pow ($1, $2); }
/* Unary minus */
| exp 'n' { $$ = -$1; }
;
%%
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 `|' punctuator
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.
File: bison.info, Node: Rpcalc Input, Next: Rpcalc Line, Prev: Rpcalc Rules, Up: RPN Calc
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 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
write the comment `/* empty */' in it.
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
file.
File: bison.info, Node: Rpcalc Line, Next: Rpcalc Expr, Prev: Rpcalc Input, Up: RPN Calc
Explanation of `line'
.....................
Now consider the definition of `line':
line: '\n'
| exp '\n' { printf ("\t%.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 `$$'.
As a consequence, the semantic value associated with the `line' is
uninitialized (its value will be unpredictable). This would be a bug
if that value were ever used, but we don't use it: once rpcalc has
printed the value of the user's input line, that value is no longer
needed.
File: bison.info, Node: Rpcalc Expr, Next: Rpcalc Lexer, Prev: Rpcalc Line, Up: RPN Calc
Explanation of `expr'
.....................
The `exp' grouping has several rules, one for each kind of
expression. The first rule handles the simplest expressions: those
that are just numbers. The second handles an addition-expression,
which looks like two expressions followed by a plus-sign. The third
handles subtraction, and so on.
exp: NUM
| exp exp '+' { $$ = $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 '-' { $$ = $1 - $2; } ;
...
Most of the rules have actions that compute the value of the
expression in terms of the value of its parts. For example, in the
rule for addition, `$1' refers to the first component `exp' and `$2'
refers to the second one. The third component, `'+'', has no
meaningful associated semantic value, but if it had one you could
refer to it as `$3'. When `yyparse' recognizes a sum expression using
this rule, the sum of the two subexpressions' values is produced as
the value of the entire expression. *Note Actions::.
You don't have to give an action for every rule. When a rule has no
action, Bison by default copies the value of `$1' into `$$'. 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 whitespace 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.
|
