GADTs and pattern matching

19 Jun 2013

      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?

Thanks,
Francesco

GADTs and pattern matching

Francesco Mazzoli