Re: precedence have both ways

From	anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups	comp.compilers
Subject	Re: precedence have both ways
Date	2013-05-31 10:54 +0000
Organization	Institut fuer Computersprachen, Technische Universitaet Wien
Message-ID	<13-05-023@comp.compilers> (permalink)
References	<13-05-010@comp.compilers> <13-05-018@comp.compilers> <13-05-020@comp.compilers>

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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/

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Articles/books that discuss separating the context-free part of a language from the context-sensitive part? "Costello, Roger L." <costello@mitre.org> - 2013-05-17 20:57 +0000
  Re: Articles/books that discuss separating the context-free part of a language from the context-sensitive part? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-05-18 00:18 +0000
    Re: Articles/books that discuss separating the context-free part of a language from the context-sensitive part? Gene Wirchenko <genew@telus.net> - 2013-05-19 18:02 -0700
      Re: Articles/books that discuss separating the context-free part of a language from the context-sensitive part? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-05-20 03:17 +0000
  Re: Articles/books that discuss separating the context-free part of a language from the context-sensitive part? Aleksey Demakov <ademakov@gmail.com> - 2013-05-18 16:52 +0400
  Re: Articles/books that discuss separating the context-free part of a language from the context-sensitive part? anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-20 10:53 +0000
    Re: Articles/books that discuss separating the context-free part of a language from the context-sensitive part? Gene Wirchenko <genew@telus.net> - 2013-05-20 23:24 -0700
    precedence have both ways jgk@panix.com (Joe keane) - 2013-05-30 23:21 -0400
      Re: precedence have both ways glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-05-31 05:10 +0000
      Re: precedence have both ways anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2013-05-31 10:54 +0000

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