Re: [Haskell-cafe] Partial pattern matching

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.