ConstraintKinds and default associated empty constraints

Simon Peyton-Jones simonpj at
Fri Dec 23 10:42:25 CET 2011

it’s a bug.  I’m fixing it.


From: glasgow-haskell-users-bounces at [mailto:glasgow-haskell-users-bounces at] On Behalf Of Edward Kmett
Sent: 22 December 2011 17:03
To: Bas van Dijk
Cc: glasgow-haskell-users at
Subject: Re: ConstraintKinds and default associated empty constraints

On Wed, Dec 21, 2011 at 6:45 PM, Bas van Dijk <v.dijk.bas at<mailto:v.dijk.bas at>> wrote:
I'm playing a bit with the new ConstraintKinds feature in GHC
7.4.1-rc1. I'm trying to give the Functor class an associated
constraint so that we can make Set an instance of Functor. The
following code works but I wonder if the trick with: class Empty a;
instance Empty a, is the recommended way to do this:

{-# LANGUAGE ConstraintKinds, TypeFamilies, FlexibleInstances #-}

import GHC.Prim (Constraint)

import Prelude hiding (Functor, fmap)

import           Data.Set (Set)
import qualified Data.Set as S (map, fromList)

class Functor f where
    type C f :: * -> Constraint
    type C f = Empty

    fmap :: (C f a, C f b) => (a -> b) -> f a -> f b

class Empty a; instance Empty a

instance Functor Set where
    type C Set = Ord
    fmap =

instance Functor [] where
    fmap = map

testList = fmap (+1) [1,2,3]
testSet  = fmap (+1) (S.fromList [1,2,3])

Cheers and thanks for a great new feature!


This is the same solution I wound up with in

Adding an argument to the family would work but is somewhat unsatisfying as it mucks with polymorphic recursive use of the dictionary, and with placing constraints on constraints, so I prefer to keep as few arguments as possible.

You can go farther with Functor by using polymorphic kinds and indexing the source and destination Category as well as the class of objects in the category.

I should probably write up what I've done with this, but doing so lets you have real product and coproduct Category instances, which were previously not possible (a fact which in part drove me to write all the semigroupoid code i have on hackage.

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