I mean, there is no way to write a firstCoord function so that it would work, for example, on '\y -> P 42 y' and yield 42. Except for this one: firstCoord proj = case (proj undefined) of P x y -> x However, this requires proj to be non-strict in its remaining argument. But this will actually work if you pass "P x" to it for some x, because it *is* non-strict in the remaining argument. 2009/3/9 Eugene Kirpichov :

P x is indistinguishable neither in compile-time nor in run-time from the value \y -> P x y.

And pattern matching and equality on functions is, of course, undecidable.

2009/3/9 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?

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-- Eugene Kirpichov Web IR developer, market.yandex.ru

-- Eugene Kirpichov Web IR developer, market.yandex.ru

Yes of course, P x is a function, and you can't pattern match against functions, I knew that. How silly of me, I could have guessed that myself. On Mon, Mar 9, 2009 at 5:43 PM, Daniel Fischer 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.