[GHC] #11067: Spurious superclass cycle error with type equalities

7 Nov 2015

      #11067: Spurious superclass cycle error with type equalities
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           Reporter:  oerjan         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 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):
 {{{#!haskell
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TypeFamilies #-}

 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’
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11067
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