Hi list,
I have stumbled upon a strange annoyance:
{-# LANGUAGE GADTs #-}
data Foo v where
Foo :: Foo (Maybe v)
-- This doesn't work
foo1 :: a -> Foo a -> Int
foo1 Nothing Foo = undefined
foo1 (Just x) Foo = undefined
-- This does
foo2 :: a -> Foo a -> Int
foo2 x Foo = foo2' x
foo2' :: Maybe a -> Int
foo2' Nothing = undefined
foo2' (Just x) = undefined
The first definition fails with the error
Couldn't match expected type `a' with actual type `Maybe t0'
`a' is a rigid type variable bound by
the type signature for foo1 :: a -> Foo a -> Int
at /tmp/foo_flymake.hs:8:9
In the pattern: Nothing
In an equation for `foo1': foo1 Nothing Foo = undefined
Now, GHC can clearly derive and use the type equalities correctly, given
that the second definition works, but why can’t I pattern match
directly?
Thanks,
Francesco
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