[Haskell-cafe] Associative prefix operators in Parsec

Christian Maeder Christian.Maeder at dfki.de
Mon Mar 12 10:55:14 CET 2012

Am 08.03.2012 17:16, schrieb Troels Henriksen:
> Christian Maeder<Christian.Maeder at dfki.de>  writes:
>> The simplest solution is to parse the prefixes yourself and do not put
>> it into the table.
>> (Doing the infixes "&" and "|" by hand is no big deal, too, and
>> possibly easier then figuring out the capabilities of
>> buildExpressionParser)
> Is there another solution?  My post was a simplified example to showcase
> the problem; in general I would prefer to use a function to build the
> expression parser.  I could just write my own that does not have this
> problem, and in fact, I already have, I just wanted to know whether
> Parsec could be wrangled into shape.

Yes, it certainly could do better. The code for prefix and postfix 
currently looks like:

                termP      = do{ pre  <- prefixP
                              ; x    <- term
                              ; post <- postfixP
                              ; return (post (pre x))

This supports (only) one prefix or postfix (or both), where the prefix 
binds stronger than the postfix (although, they have equal precedence).

Problem 1: "- - 5" is not supported

Another problem are prefix or postfix operators that bind weaker than 
infixes, like infix "^" and prefix "-".

Problem 2: "1 ^ -2" is rejected, although no other parsing is possible.

(The same would apply to a weakly binding postfix operator following the 
left argument: "4+ ^ 5". This would even need some look ahead to find 
out, if + is not an infix and ^ a prefix operator)

(Haskell features these problems, too.)

Maybe the special case of repeated prefixes could be solved by putting 
in the prefix entry twice into the table for the 
currentbuildExpressionParser, but considering possibly equal symbols for 
prefix, postfix or infix symbols seems quite difficult, although only an 
ambiguity needs to be reported.

Cheers C.

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