Hello I am having problems with GHC infering functional dependencies related types in a too conservative fashion.
class Imp2 a b | a -> b instance Imp2 (Foo a) (Wrap a)
newtype Wrap a = Wrap { unWrap :: a } data Foo a = Foo data Proxy (cxt :: * -> *)
foo :: Imp2 (ctx c) d => Proxy ctx -> (forall a b. (Imp2 (ctx a) b) => a -> b) -> c -> d foo p f x = f x
The type of "foo (undefined :: Proxy Foo)" is inferred as "forall c. (forall a b. (Imp2 (Foo a) b) => a -> b) -> c -> Wrap c" which shows the outmost functional dependence is working fine. ctx is carried to the inner Imp2. However "foo (undefined :: Proxy Foo) Wrap" will fail complaining that Couldn't match the rigid variable `b' against `Wrap a' `b' is bound by the polymorphic type `forall a b. (Imp2 (ctx a) b) => a -> b' at <interactive>:1:0-32 Expected type: a -> b Inferred type: a -> Wrap a In the second argument of `foo', namely `Wrap' My guess is that GHC cannot see that the functional dependency guarantees that there are no instances which make the inferred type invalid. Any solutions to this problem? - Einar Karttunen