[Haskell-beginners] Re: parsing upto n items with parsec

[Ashish Agarwal]

Right. I took "option" out for some reason. The following definitions have
the desired behavior. I also added a base case to fromTo because otherwise
the call to count would consume m tokens even if n-m < 0, which is probably
not what we'd expect.
upTo :: Int -> GenParser tok st a -> GenParser tok st [a]
upTo n p
    | n <= 0 = return []
    | otherwise = option [] $ liftM2 (:) p (upTo (n-1) p)

fromTo :: Int -> Int -> GenParser tok st a -> GenParser tok st [a]
fromTo m n p
    | n-m < 0 = return []
    | otherwise = liftM2 (++) (count m p) (upTo (n-m) p)

Thanks!


On Mon, Oct 19, 2009 at 12:55 PM, Chaddaï Fouché
<chaddai.fouche at gmail.com>wrote:

> On Mon, Oct 19, 2009 at 6:22 PM, Ashish Agarwal <agarwal1975 at gmail.com>
> wrote:
> > The semantics of (upTo n p) should be to parse at most n tokens, but if
> less
> > than n tokens are available that should still be a successful parse. And
> the
> > next token should be the first one upTo failed on.
> > I attempted to use the "try" parser in various locations but that doesn't
> > seem to help, or maybe I'm using it incorrectly.
>
>
> First, you should probably use pattern matching rather than if for
> your base case/recursive case distinction, it's clearer (at least most
> Haskellers seems to think it is) :
>
> > upTo 0 p = return []
>
> second, option was conceived for these case where you want to try a
> parser and returns something if it fail :
>
> > upTo n p = option [] $ liftM2 (:) p (upTo (n-1) p)
>
> Even then, be careful of putting a try before any multi-token parser
> that could fail harmlessly during the upTo.
>
> --
> Jedaï
>
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