At Wed, 19 Jun 2013 11:25:49 +0100, Francesco Mazzoli wrote:
Hi list,
I had asked this on haskell-cafe but this looks particularly fishy, so posting here in case it is an issue:
{-# LANGUAGE GADTs #-}
data Foo v where Foo :: forall v. Foo (Maybe v)
-- Works foo1 :: Foo a -> a -> Int foo1 Foo Nothing = undefined foo1 Foo (Just x) = undefined
-- This works foo2 :: a -> Foo a -> Int foo2 x Foo = foo2' x
foo2' :: Maybe a -> Int foo2' Nothing = undefined foo2' (Just x) = undefined
-- This doesn't! foo3 :: a -> Foo a -> Int foo3 Nothing Foo = undefined foo3 (Just x) Foo = undefined
So it looks like constraints in pattern matching only propagate forwards. Is this intended?
shachaf on #haskell pointed out that matches get desugared into a series of ‘case’s matching the argument successively, and given that the behaviour above makes sense. Francesco