[Haskell-cafe] Parse Error ( Parsec )

[mukesh tiwari]

Hi Mateusz,
Thank you. Now it's working fine by using 'try' function.

-Mukesh Tiwari


On Sun, Jan 12, 2014 at 1:02 AM, Mateusz Kowalczyk
<fuuzetsu at fuuzetsu.co.uk>wrote:

> On 11/01/14 19:12, mukesh tiwari wrote:
> > Hello Cafe,
> >  I am trying to write a parser for propositional logic[1]. It's working
> > fine for every input except equivalence ( <=> ).
> >
> >
> > *Main> calculator "a=>b"
> > Imp (Lit 'a') (Lit 'b')
> > *Main> calculator "a<=b"
> > Red (Lit 'a') (Lit 'b')
> > *Main> calculator "a<=>b"
> > *** Exception: failed to parse
> >
> >
> > I think, the reason is parser taking equivalence ( <=> ) as reduction (
> <=
> > ) and next character is '>'  so it is parse error . If I remove both
> > implication and reduction then equivalence is  working fine.
> >
> > *Main> calculator "a<=>b"
> > Eqi (Lit 'a') (Lit 'b')
> >
> > Could some please tell me how to solve this problem.  I also tried fixity
> > declaration but got this error
> > LogicPraser.hs:12:10:
> >     The fixity signature for `<=>' lacks an accompanying binding
> >
> > -Mukesh Tiwari
> >
> >
> > [1] http://logic.stanford.edu/classes/cs157/2010/notes/chap02.html
> > [snip]
>
> Hi,
>
> I have not studied your code but if the problem is what you describe
> it, you should try with back-tracking so that the parser can retry
> when it fails. I believe Parsec offers the ‘try’ function for this.
>
> Regarding your ‘infix <=>’, of course that would not work. It's a
> Haskell declaration, not something Parsec does.
>
> You're getting an error because you're saying that ‘<=>’ has left fixity
> of 9 but then you aren't giving a definition for ‘<=>’. The "<=>" you're
> parsing has nothing to do with this. Haskell sees the fixity
> declaration and then doesn't see you defining the ‘<=>’ operator
> anywhere so it complains.
>
> --
> Mateusz K.
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