Yacc/Bison, minimize amount by grouping math ops

Question

I am looking at the calc source here http://epaperpress.com/lexandyacc/

I see theses lines in calc.y

| expr '+' expr         { $$ = opr('+', 2, $1, $3); }
| expr '-' expr         { $$ = opr('-', 2, $1, $3); }
| expr '*' expr         { $$ = opr('*', 2, $1, $3); }
| expr '/' expr         { $$ = opr('/', 2, $1, $3); }
| expr '<' expr         { $$ = opr('<', 2, $1, $3); }
| expr '>' expr         { $$ = opr('>', 2, $1, $3); }

Is there a way to group them? so i can write something like the below instead?

| expr mathOp expr         { $$ = opr(mathOp, 2, $1, $3); }
| expr cmpOp  expr         { $$ = opr(cmpOp, 2, $1, $3); }

NOTE: I am using bison.

Why wouldn't you go all the way and combine mathOp/cmpOp as well? — paxdiablo
@pax: cmp ops return bools, math returns the same type it is (think C#. 0!=false). I assume its easier to separate. — user34537
Why think C#? You're using Bison, and therefore C/C++. It's far easier to combine, especially if you're going to be calling the same function anyway. — Chris Lutz

Chris Dodd Chris Dodd · Accepted Answer · 2009-09-22T03:08:12

The problem with grouping them like that is that you lose the precedences on the rules -- you only have one rule that has different precedence depending on which mathop it is, which bison/yacc cannot handle. That said, you CAN group ops of the same precedence level together

expr: expr mulOp expr { $$ = opr($2, 2, $1, $3); } %prec '*'
    | expr addOp expr { $$ = opr($2, 2, $1, $3); } %prec '+'
    | expr relOp expr { $$ = opr($2, 2, $1, $3); } %prec '<'
             :

mulOp: '*' { $$ = '*'; }
     | '/' { $$ = '/'; }
;

Yacc/Bison, minimize amount by grouping math ops

2 Answers