Josef Sveningsson wrote:
> On Thu, 21 Feb 2002, Mark Wotton wrote:
>
> > Hi,
> >
> > I'm trying out some combinatorial parsers, and I ran into a slightly
> > inelegant construction. To parse a sequence of things, we have a function
> > like
> >
> > pThen3 :: (a->b->c->d) -> Parser a -> Parser b -> Parser c -> Parser d
> > pThen3 combine p1 p2 p3 toks =
> > [(combine v1 v2 v3, toks3) | (v1, toks1) <- p1 toks,
> > (v2, toks2) <- p2 toks1,
> > (v3, toks3) <- p3 toks2]
> >
> > The problem with this is that this structure has to be duplicated for
> > pThen2, pThen4, and so on. These other forms are very similar to pThen3,
> > but there seems to be no way to capture this in Haskell's type system, as
> > the combine function has a different signature for each pThenX. (This is
> > actually the first time the Haskell type system has got in my way rather
> > than helping.) Is there a way around this problem?
> >
> Yes there is a way around this problem. You can use multi parameter type
> classes to create (and give a type to) a function such as pThenX.
Or, in Standard Haskell you can do something like this:
infixr `then2`
infixr `thenn`
then2:: Parser b -> Parser c -> ((b,c)->d) -> Parser d
then2 p1 p2 comb toks = [(comb (a, b), rest) | (a, r1) <- p1 toks,
(b, rest) <- p2 r1]
thenn:: Parser a b -> ((t->d) -> Parser a d) -> ((b,t)->d) -> Parser a d
thenn p1 cp2 comb toks = [(cmb, rest) | (a, r1) <- p1 toks,
(cmb, rest) <- cp2 (\t->comb (a,t)) r1]
and use like this
(p1 `thenn` p2 `thenn` p3 `then2` p4) (\(a,(b,(c,d))) -> whatever)
I'm not sure if you can get rid of the `then2`, but is seems
quite servicable even so.
Jón
--
Jón Fairbairn [EMAIL PROTECTED]
31 Chalmers Road [EMAIL PROTECTED]
Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!)
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