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,          


        

Answer 1

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

Answer 2

You can do it in 2 ways:

• At lex stage define recognition of operators and provide terminal symbol (in you syntax mathOp) with value of operator '+', '-' ...

• Using mathOp as nonterminal you can return some associated value:

  mathOp : '+' { $$ = '+'; } | '-' { $$ = '-'; } ...

Then usage will look like (pay attention to $2):

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

may be you would like to define more complicated mathOp then use %type