#9106: GHC Panic related to functional dependencies - kindFunResult ---------------------------------------+--------------------------- Reporter: yuriy | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Windows | Architecture: x86 Type of failure: Compile-time crash | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: ---------------------------------------+--------------------------- Comment (by darchon): The type family version also works: {{{ {-# LANGUAGE DataKinds, FlexibleInstances, MultiParamTypeClasses, PolyKinds, ScopedTypeVariables, TypeFamilies, TypeOperators, UndecidableInstances #-} module FunctorN where import GHC.TypeLits data Proxy (a :: k) = Proxy type family F n f a where F 0 f a = a F n f a = f (F (n-1) f a) class F n f a ~ fa => FunctorN n f a fa where fmapn :: Proxy n -> Proxy f -> (a -> a) -> fa -> fa instance FunctorN 0 f a a where fmapn _ _ a = a instance ( Functor f, FunctorN (n-1) f a fa, F (n - 1) f a ~ fa , F n f a ~ f fa) => FunctorN n f a (f fa) where fmapn _ pf f = fmap (fmapn (Proxy :: Proxy (n-1)) pf f) test :: Maybe (Maybe (Maybe Int)) test = fmapn (Proxy :: Proxy 3) (Proxy :: Proxy Maybe) (+1) (Just (Just (Just 3))) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9106#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler