Bison nonassociative precedence rule - parsing

Question

I am trying to enforce some parsing errors by defining a non-associative precedence.
Here is part of my grammar file:
Comparison :
Value ComparisonOp Value
{
$2->Left($1);
$2->Right($3);
$$ = $2;
}
;
Expressions like 1 = 2 should parse, but expressions like 1 = 2 = 3 are not allowed in the grammar. To accommodate this, I tried to make my operator non associative as follows:
%nonassoc NONASSOCIATIVE
.
.(rest of the grammar)
.
Comparison :
Value ComparisonOp Value %prec NONASSOCIATIVE
{
$2->Left($1);
$2->Right($3);
$$ = $2;
}
;
1 = 2 = 3 still passes, Could someone please tell me what I am doing wrong?

Answer 1

You need to set the associativity of the ComparisonOp token(s). Either
%nonassoc ComparisonOp NONASSOCIATIVE
if ComparisonOp is a token or something like
%nonassoc '=' '<' '>' NOT_EQUAL GREATOR_OR_EQUAL LESS_OR_EQUAL NONASSOCIATIVE
if you have mulitple tokens and ComparisonOp is a rule that expands to any of them
As a concrete example, the following works exactly like you are requesting:
%{
#include <stdio.h>
#include <ctype.h>
int yylex();
void yyerror(const char *);
%}
%nonassoc '=' '<' '>' CMPOP
%left '+' '-' ADDOP
%left '*' '/' '%' MULOP
%token VALUE
%%
expr: expr cmp_op expr %prec CMPOP
| expr add_op expr %prec ADDOP
| expr mul_op expr %prec MULOP
| VALUE
| '(' expr ')'
;
cmp_op: '=' | '<' | '>' | '<' '=' | '>' '=' | '<' '>' ;
add_op: '+' | '-' ;
mul_op: '*' | '/' | '%' ;
%%
int main() { return yyparse(); }
int yylex() {
int ch;
while(isspace(ch = getchar()));
if (isdigit(ch)) return VALUE;
return ch;
}
void yyerror(const char *err) { fprintf(stderr, "%s\n", err); }
so if you are having other problems, try posting an MVCE that shows the actual problem you are having...