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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