Re: precedence have both ways

jgk@panix.com (Joe keane) writes:
>One person designs a language 'C1' with precedence:
>
> [higher]
> ...
> |
> &
> <, <=, >, <=
> ==, !=
> ...
> [lower]
>
>Some other person designs a language 'C2' with precedence:
>
> [higher]
> ...
> <, <=, >, <=
> ==, !=
> |
> &
> ...
> [lower]
>
>They're the same besides that.
>
>To compromise, they make a new language 'C3' such that a program is
>valid C3 if it is valid C1 and valid C2 -and- both ways mean the same.
>
>Is there some good way to do this -in the grammar-?


Sure, you don't want | & to asssociate with < <= > >= == != (I find
the remaining associations questionable, but let's stick with your
example). So write the grammar in a way that makes the wanted
associations explicit and does not contain the unwanted associations:


boolexpr: orexpr
                | ineqexpr ;
orexpr: orexpr '|' andexpr
            | andexpr ;
andexpr: andexpr '&' lower
              | lower ;
ineqexpr: eqexpr ineqop eqexpr
                | eqexpr ;
eqexpr: lower eqop lower
            | lower ;
...
term: '(' highest ')'
        | ... ;


(This assumes that a<b<c and a==b==c is not wanted; also, resolving
conflicts is left as an exercise).


- anton
--
M. Anton Ertl
anton@mips.complang.tuwien.ac.at
http://www.complang.tuwien.ac.at/anton/

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