[GHC] #9106: GHC Panic related to functional dependencies - kindFunResult

[GHC]

#9106: GHC Panic related to functional dependencies - kindFunResult
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        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:
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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>
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