On Fri, Aug 31, 2012 at 8:55 AM, Edward Kmett <ekmett@gmail.com> wrote:

Hrmm. This seems to work manually for getting product categories to work. Perhaps I can do the same thing for thrists.

{-# LANGUAGE PolyKinds, DataKinds, TypeOperators, GADTs, TypeFamilies #-}

module Product where

import Prelude hiding (id,(.))

class Category k where
id :: k a a
(.) :: k b c -> k a b -> k a c

type family Fst (a :: (p,q)) :: p
type instance Fst '(p,q) = p

type family Snd (a :: (p,q)) :: q
type instance Snd '(p,q) = q

data (*) :: (x -> x -> *) -> (y -> y -> *) -> (x,y) -> (x,y) -> * where
(:*) :: x (Fst a) (Fst b) -> y (Snd a) (Snd b) -> (x * y) a b

instance (Category x, Category y) => Category (x * y) where
id = id :* id

(xf :* yf) . (xg :* yg) = (xf . xg) :* (yf . yg)

On Fri, Aug 31, 2012 at 8:44 AM, Edward Kmett <ekmett@gmail.com> wrote:

Hrmm. This seems to render product kinds rather useless, as there is no way to refine the code to reflect the knowledge that they are inhabited by a single constructor. =(

For instance, there doesn't even seem to be a way to make the following code compile, either.

{-# LANGUAGE PolyKinds, DataKinds, TypeOperators, GADTs #-}
module Product where

import Prelude hiding (id,(.))

class Category k where

id :: k a a
(.) :: k b c -> k a b -> k a c

data (*) :: (x -> x -> *) -> (y -> y -> *) -> (x,y) -> (x,y) -> * where
(:*) :: x a b -> y c d -> (x * y) '(a,c) '(b,d)

instance (Category x, Category y) => Category (x * y) where
id = id :* id

(xf :* yf) . (xg :* yg) = (xf . xg) :* (yf . yg)

This all works perfectly fine in Conor's SHE, (as does the thrist example) so I'm wondering where the impedence mismatch comes in and how to gain knowledge of this injectivity to make it work.

Also, does it ever make sense for the kind of a kind variable mentioned in a type not to get a functional dependency on the type?

e.g. should

class Foo (m :: k -> *)

always be

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

?

Honest question. I can't come up with a scenario, but you might have one I don't know.

-Edward

On Fri, Aug 31, 2012 at 5:55 AM, Simon Peyton-Jones <simonpj@microsoft.com> wrote:

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@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@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of Edward Kmett
Sent: 31 August 2012 03:38
To: glasgow-haskell-users@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