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

19 Nov 2015

      #11067: Spurious superclass cycle error with type equalities
-------------------------------------+-------------------------------------
        Reporter:  oerjan            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------

Old description:
...
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’
}}}
New description:

 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 #-}
 {-# 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’
 }}}

--

Comment (by thomie):

 Your example seemed to be missing a `FlexibleContexts` pragma, so I added
 it.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11067#comment:6
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