Some of us today had an idea how to repair Edward Kmett's regrettably unsound Data.Constraint.Forall module. The method works fine in some cases, but seems to occasionally trigger a spurious superclass cycle error.

In the cases I've seen so far, it seems to happen when a class is defined with a Forall superclass, where that Forall itself has as parameter another class, that itself has a type equality superclass.

Example file (a bit larger than necessary to show how a similar example without a type equality ''doesn't'' give an error):

{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleContexts #-}

import Data.Monoid
import GHC.Exts (Constraint)

type family Skolem (p :: k -> Constraint) :: k
type family SkolemF (p :: k2 -> Constraint) (f :: k1 -> k2) :: k1

-- | A quantified constraint
type Forall (p :: k -> Constraint) = p (Skolem p)
type ForallF (p :: k2 -> Constraint) (f :: k1 -> k2) = p (f (SkolemF p f))

-- These work
class ForallF Monoid t => Monoid1 t
instance ForallF Monoid t => Monoid1 t

class ForallF Monoid1 t => Monoid2 t
instance ForallF Monoid1 t => Monoid2 t

-- Changing f a ~ g a to, (Ord (f a), Ord (g a)), say, removes the error
class (f a ~ g a) => H f g a
instance (f a ~ g a) => H f g a

-- This one gives a superclass cycle error.
class Forall (H f g) => H1 f g
instance Forall (H f g) => H1 f g

And the resulting error:

Test.hs:31:1:
    Cycle in class declaration (via superclasses):
      H1 -> Forall -> H -> H
    In the class declaration for ‘H1’

Test.hs:31:1:
    Cycle in class declaration (via superclasses):
      H1 -> Forall -> H -> H
    In the class declaration for ‘H1’