PolyKind issue in GHC 7.6.1rc1: How to make a kind a functional dependency?

[Simon Peyton-Jones]

With the code below, I get this error message with HEAD. And that looks right to me, no?
The current 7.6 branch gives the same message printed less prettily.

If I replace the defn of irt with
irt :: a '(i,j) -> Thrist a '(i,j)
irt ax = ax :- Nil

then it typechecks.

Simon


Knett.hs:20:10:
    Couldn't match type `x' with '(i0, k0)
      `x' is a rigid type variable bound by
          the type signature for irt :: a x -> Thrist k a x at Knett.hs:19:8
    Expected type: Thrist k a x
      Actual type: Thrist k a '(i0, k0)
    In the expression: ax :- Nil
    In an equation for `irt': irt ax = ax :- Nil
simonpj at cam-05-unx:~/tmp$


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

module Knett where

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

irt :: a x -> Thrist a x
irt ax = ax :- Nil


From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-bounces at haskell.org] On Behalf Of Edward Kmett
Sent: 31 August 2012 03:38
To: glasgow-haskell-users at haskell.org
Subject: PolyKind issue in GHC 7.6.1rc1: How to make a kind a functional dependency?

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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