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
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