Re: Using inherited attributes within a yacc grammar

Chris Palmer (crpalmer@undergrad.math.uwaterloo.ca) wrote:
> My question is: Is it possible to safely and cleanly use inherited attributes
> within the yacc grammar?


Depends on what you call cleanly and safely...


... It can be done, and I think any real compiler project using some kind
of yacc will do it. In yacc the way is provided by two things:
1. "Epsilon"-derivations with an action rule, embedded in right sides (not
      only at the end)
2. Looking into the stack with negative indices (e.g. $<type>-2) gives you
      access to "inherited" attributes.


> (get an integer as either x<hex> or <decimal>)


in yacc syntax (forgive typos, it's some years I last used yacc)
your grammar may become something like:


number: base integer { $$ = $2; }
base: "x" { $$ = 16; }
        | { $$ = 10; }
integer: digit
              | integer digit { $$ = $1 * $0 + $2 ;
/* $ 0 refers to left context of rule namely base*/ }


> Granted this is a poor grammar but it does illustrate the concept. Also,
> i'm fully aware that this can be accomplished more easily by the use of a
> global variable. However, i'm wondering if there is a general way to
> make use of inherited attributes within yacc.


If you are interested I can try to dig out my modula2 front end, where I
have made heavy use of negative indices for inherited attributes like
record field offsets.
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