Hello, I'm working on a new design of monad-control[1]. However I get a type error I don't understand. Here's an isolated example: {-# LANGUAGE UnicodeSyntax, RankNTypes, TypeFamilies #-} class MonadTransControl t where type St t ∷ * → * liftControl ∷ Monad m ⇒ (Run t → m α) → t m α restore ∷ Monad o ⇒ St t γ → t o γ type Run t = ∀ n β. Monad n ⇒ t n β → n (St t β) foo :: (Monad m, MonadTransControl t) => (Run t -> m α) -> t m α foo f = liftControl f Type checking 'foo' this gives the following error: Couldn't match expected type `Run t' with actual type `Run t' Expected type: Run t -> m α Actual type: Run t -> m α In the first argument of `liftControl', namely `f' In the expression: liftControl f When I remove the type annotation of 'foo' the program type checks. But when I ask ghci the type of 'foo' it tells me it's the same type:
:t foo foo :: (Monad m, MonadTransControl t) => (Run t -> m α) -> t m α
Is this a bug in GHC? I'm using ghc-7.2.1. Regards, Bas [1] http://hackage.haskell.org/package/monad-control