[GHC] #15593: QuantifiedConstraints: trouble with type family

[GHC]

#15593: QuantifiedConstraints: trouble with type family
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints,             |
  ConstraintKinds                    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# Language KindSignatures, TypeFamilies, QuantifiedConstraints,
 FlexibleInstances #-}

 import Data.Kind

 data TreeF a b = T0 | T1 a | T2 b b

 -- from Data.Reify
 class MuRef (a :: Type) where
   type DeRef a :: Type -> Type

 class    (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree
 instance (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree
 }}}

 fails with

 {{{
 $ ~/code/qc-ghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-ghci
 351.hs
 GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( 351.hs, interpreted )

 351.hs:12:10: error:
     • Couldn't match type ‘DeRef (tree xx)’ with ‘TreeF xx’
         arising from the superclasses of an instance declaration
     • In the instance declaration for ‘MuRef1 tree’
    |
 12 | instance (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Failed, no modules loaded.
 Prelude>
 }}}

 ----

 What I want to write:

 {{{#!hs
 {-# Language KindSignatures, TypeFamilies, QuantifiedConstraints,
 FlexibleInstances, DataKinds, TypeInType, MultiParamTypeClasses,
 UndecidableInstances, AllowAmbiguousTypes #-}

 import Data.Kind

 -- from Data.Reify
 class MuRef (a :: Type) where
   type DeRef a :: Type -> Type

 type T   = Type
 type TT  = T -> T
 type TTT = T -> TT

 class    (forall xx. (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx)) => MuRef1
 (f :: TT) (deRef1 :: TT -> TTT)
 instance (forall xx. (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx)) => MuRef1
 (f :: TT) (deRef1 :: TT -> TTT)
 }}}

 where I am trying to capture
 [https://hackage.haskell.org/package/folds-0.7.4/docs/src/Data-Fold-
 Internal.html MuRef1 & DeRef1]

 {{{#!hs
 class MuRef1 (f :: TT) where
   type DeRef1 f :: TTT
   muRef1 :: proxy (f a) -> Dict (MuRef (f a), DeRef (f a) ~ DeRef1 f a)
 }}}

 ----

 Workarounds: I think splitting the class alias & quantification does the
 job (I haven't tested it but it compiles), I want to know if the first two
 programs are meant to compile or not

 {{{#!hs
 {-# Language KindSignatures, TypeFamilies, QuantifiedConstraints,
 FlexibleInstances, DataKinds, TypeInType, MultiParamTypeClasses,
 UndecidableInstances, AllowAmbiguousTypes, FlexibleContexts #-}

 -- ..

 class    (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx) => MuRef1_ f deRef1 xx
 instance (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx) => MuRef1_ f deRef1 xx

 class    (forall xx. cls xx) => Forall cls
 instance (forall xx. cls xx) => Forall cls

 class    Forall (MuRef1_ f deRef1) => MuRef1 f deRef1
 instance Forall (MuRef1_ f deRef1) => MuRef1 f deRef1
 }}}

 or as a regular type/constraint synonym (at the loss of partial
 application)

 {{{#!hs
 type MuRef1 f deRef1 = (forall xx. (MuRef (f xx), DeRef (f xx) ~ deRef1 f
 xx))
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15593>
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