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From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.compilers
Subject: Re: precedence have both ways
Date: Fri, 31 May 2013 10:54:00 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
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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/