If I define the following

{-# LANGUAGE FunctionalDependencies, GADTs, KindSignatures, MultiParamTypeClasses, PolyKinds, RankNTypes, TypeOperators, DefaultSignatures, DataKinds, FlexibleInstances, UndecidableInstances #-}

module Indexed.Test where

class IMonad (m :: (k -> *) -> k -> *) where 

  ireturn :: a x -> m a x

infixr 5 :-

data Thrist :: ((i,i) -> *) -> (i,i) -> * where

  Nil :: Thrist a '(i,i)

  (:-) :: a '(i,j) -> Thrist a '(j,k) -> Thrist a '(i,k)

instance IMonad Thrist where

  ireturn a = a :- Nil

Then 'ireturn' complains (correctly) that it can't match the k with the kind (i,i). The reason it can't is because when you look at the resulting signature for the MPTC it generates it looks like

class IMonad k m where

  ireturn :: a x -> m a x

However, there doesn't appear to be a way to say that the kind k should be determined by m. 

I tried doing:

class IMonad (m :: (k -> *) -> k -> *) | m -> k where 

  ireturn :: a x -> m a x

Surprisingly (to me) this compiles and generates the correct constraints for IMonad:

ghci> :set -XPolyKinds -XKindSignatures -XFunctionalDependencies -XDataKinds -XGADTs

ghci> class IMonad (m :: (k -> *) -> k -> *) | m -> k where ireturn :: a x -> m a x

ghci> :info IMonad

class IMonad k m | m -> k where

  ireturn :: a x -> m a x

But when I add 

ghci> :{

Prelude| data Thrist :: ((i,i) -> *) -> (i,i) -> * where

Prelude|   Nil :: Thrist a '(i,i)

Prelude|   (:-) :: a '(i,j) -> Thrist a '(j,k) -> Thrist a '(i,k)

Prelude| :}

and attempt to introduce the instance, I crash:

ghci> instance IMonad Thrist where ireturn a = a :- Nil

ghc: panic! (the 'impossible' happened)

  (GHC version 7.6.0.20120810 for x86_64-apple-darwin):

lookupVarEnv_NF: Nothing

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Moreover, attempting to compile them in separate modules leads to a different issue. 

Within the module, IMonad has a type that includes the kind k and the constraint on it from m. But from the other module, :info shows no such constraint, and the above code again fails to typecheck, but upon trying to recompile, when GHC goes to load the IMonad instance from the core file, it panicks again differently about references to a variable not present in the core.

Is there any way to make such a constraint that determines a kind from a type parameter in GHC 7.6 at this time?

I tried the Kind hack used in GHC.TypeLits, but it doesn't seem to be applicable to this situation.

-Edward

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