On Mon, Mar 9, 2009 at 5:43 PM, Daniel Fischer <daniel.is.fischer@web.de> wrote:

Am Montag, 9. März 2009 17:30 schrieb Peter Verswyvelen:

> In Haskell, a data constructor can be used partially applied:
> data Pair a b = P a b
>
> f = P 1
>
> however, I cannot do "partial pattern matching", e.g
>
> firstCoord (P x) = x
>
> does not work.
>
> I guess a very important reason must exist why this is the case?

For one, the type. If x :: a, then P x :: b -> Pair a b, so we'd have

firstCoord :: (b -> Pair a b) -> a

But you can pattern-match only on constructors of the appropriate type. P is
not a constructor of (b -> Pair a b) (function types don't have
constructors), so you can't match on a partially applied constructor.