Re: [GHC] #15079: GHC HEAD regression: cannot instantiate higher-rank kind

23 Apr 2018

      #15079: GHC HEAD regression: cannot instantiate higher-rank kind
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 A more complicated example that also typechecks in 8.2.2 and 8.4.2, but
 not in GHC HEAD:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}

 module Bug where

 import           Data.Kind
 import qualified Data.Type.Equality as Eq
 import           GHC.Exts (Any)

 infixl 4 :==
 -- | Heterogeneous Leibnizian equality.
 newtype (a :: j) :== (b :: k)
   = HRefl { hsubst :: forall (c :: forall (i :: Type). i -> Type). c a ->
 c b }

 newtype Flay :: (forall (i :: Type). i -> i -> Type)
              -> forall (j :: Type). j -> forall (k :: Type). k -> Type
 where
   Flay :: forall (p :: forall (i :: Type). i -> i -> Type)
                  (j :: Type) (k :: Type) (a :: j) (b :: k).
           { unflay :: p a (MassageKind j b) } -> Flay p a b

 type family MassageKind (j :: Type) (a :: k) :: j where
   MassageKind j (a :: j) = a
   MassageKind _ _        = Any

 fromLeibniz :: forall a b. a :== b -> a Eq.:~: b
 fromLeibniz f = unflay $ hsubst f $ Flay Eq.Refl
 }}}
 {{{
 $ ~/Software/ghc/inplace/bin/ghc-stage2 Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:29:42: error:
     • Kind mismatch: cannot unify (p0 :: forall i. i -> i -> *) with:
         (Eq.:~:) :: forall k. k -> k -> *
       Their kinds differ.
       Expected type: p0 k a (MassageKind k a)
         Actual type: a Eq.:~: a
     • In the first argument of ‘Flay’, namely ‘Eq.Refl’
       In the second argument of ‘($)’, namely ‘Flay Eq.Refl’
       In the second argument of ‘($)’, namely ‘hsubst f $ Flay Eq.Refl’
    |
 29 | fromLeibniz f = unflay $ hsubst f $ Flay Eq.Refl
    |                                          ^^^^^^^
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15079#comment:2
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler