Question

Hi I’m getting $$ has no return type when I run the following Bison:

%{
#include <stdio.h>
#include <string.h>

void yyerror(const char *str)
{
    fprintf(stderr,"error: %s\n",str);
}

int yywrap()
{
    return 1;
}

main()
{
    yyparse();
}

%}
%start query
%token AND OR GREATER LESS COLON TO

%union 
{
        int number;
        char *string;
}

%token <number> VALUE
%token <string> WORD

%%

    query: /* empty */
        | query expression { }
        ;

    expression:
        term
        |expression AND term {$$=$1}
        |expression OR term {}
        ;

    term:
        WORD { printf("Term:%s\t",$1);}
        | VALUE
        ;

Following is my flex:

%{
#include <stdio.h>
#include <string.h>
#include "y.tab.h"
%}
%%
":"                     return COLON; 
"and"|"&"|"&&"          return AND; 
"or"|"|"|"||"           return OR; 
".."                    return TO;
">"                     return GREATER; 
"<"                     return LESS; 
\n                      /* ignore end of line */;
\t                      /* ignore end of line */;

%%

What does this error mean? How can I fix this?

Answer 1

Its because you’re missing the declaration %type <...> expression in the first part of your grammar, where ... is some type declared in your %union statement. Without this, you can’t access $$ in any expression action. Also, if you set $$ in ANY action for a non-terminal, you need to set it in all actions for that non-terminal.

In general, you need to decide for all non-terminals whether they produce a value that can be used in rules that use that non-terminal or not. If they do produce such a value, they need a %type declaration and every action needs to set $$ to something. If they do not, then no action can use $1 (or whatever) to access the non-existent value.

You need the same for all terminals, except for terminals, you use %token instead of %type, and you need to set yylval in your lexer, as that’s the equivalent of $$.

In your particular example, you’ve dealt with the terminals but ignored the non-terminals. Depending on exactly what you’re trying to do, you probably want a %type for both expression and term, but may not want one for query. This means that every term and expression rule needs an action that sets $$