[Haskell-cafe] Operator precedence and associativity with Polyparse

[Christian Maeder]

Am 26.10.2011 01:49, schrieb Tom Hawkins:
>>> Can someone provide guidance on how handle operator precedence and
>>> associativity with Polyparse?
>>
>> Do you mean parsing something like "1 + 2 * 3" ?  I don't think
>> there's any real difference in using Polyparse vs Parsec for this,
>> except for doing "p `orElse` q" rather than "try p<|>  q".
>
> Actually, I was looking for something equivalent to
> Text.ParserCombinators.Parsec.Expr.buildExpressionParser.  I suppose I
> should learn how Parsec implements this under the hood.

I would do it as described under chainl1 in
http://hackage.haskell.org/packages/archive/parsec2/1.0.0/doc/html/Text-ParserCombinators-Parsec-Combinator.html

I believe Parsec.Expr cannot handle a prefix operator (i.e. unary minus) 
properly, that has lower precedence than an infix operator (i.e. "^" 
power). If it can parse "-x^2" as "-(x^2)" then if cannot parse "x^ -2" 
as "x^(-2)".

Cheers Christian

>
> -Tom