prism/examples/prism-bison.html

<h2>Comments</h2>
<pre><code>// Single-line comment
/* Multi-line
comment */</code></pre>

<h2>C prologue and Bison declarations</h2>
<pre><code>%{
  #include &lt;stdio.h>
  #include &lt;math.h>
  int yylex (void);
  void yyerror (char const *);
%}

%define api.value.type {double}
%token NUM
%union { char *string; }
%%
%%</code></pre>

<h2>Grammar rules</h2>
<pre><code>%%
exp:
  NUM           { $$ = $1;           }
| exp exp '+'   { $$ = $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    */
;

$@1: %empty { a(); };
$@2: %empty { c(); };
$@3: %empty { d(); };
exp: $@1 "b" $@2 $@3 "e" { f(); };
%%</code></pre>

<h2>Full example</h2>
<pre><code>/* Mini Calculator */
/* calc.y */

%{
#include "heading.h"
int yyerror(char *s);
int yylex(void);
%}

%union{
  int		int_val;
  string*	op_val;
}

%start	input

%token	&lt;int_val>	INTEGER_LITERAL
%type	&lt;int_val>	exp
%left	PLUS
%left	MULT

%%

input:		/* empty */
		| exp	{ cout &lt;&lt; "Result: " &lt;&lt; $1 &lt;&lt; endl; }
		;

exp:		INTEGER_LITERAL	{ $$ = $1; }
		| exp PLUS exp	{ $$ = $1 + $3; }
		| exp MULT exp	{ $$ = $1 * $3; }
		;

%%

int yyerror(string s)
{
  extern int yylineno;	// defined and maintained in lex.c
  extern char *yytext;	// defined and maintained in lex.c

  cerr &lt;&lt; "ERROR: " &lt;&lt; s &lt;&lt; " at symbol \"" &lt;&lt; yytext;
  cerr &lt;&lt; "\" on line " &lt;&lt; yylineno &lt;&lt; endl;
  exit(1);
}

int yyerror(char *s)
{
  return yyerror(string(s));
}</code></pre>