When I tried to do something like:
{-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-}
class Test a where type TestMonad a :: * -> * from :: a b -> TestMonad a b to :: TestMonad a b -> a b
data Testable a b = Testable (a b)
instance (Test a, Functor (TestMonad a)) => Functor (Testable a) where f `fmap` Testable v = Testable $! (to . fmap f . from) v
It asks for adding UndecidableInstances as: test.hs:11:0: Constraint is no smaller than the instance head in the constraint: Functor (TestMonad a) (Use -XUndecidableInstances to permit this) In the instance declaration for `Functor (Testable a)' What is undecidable? a is bound so TestMonad a should be bound so Functor (TestMonad a) should be valid. Is it a bug/missing feature in ghc or do I fail to see something? Regards