[GHC] #11719: Cannot use higher-rank kinds with type families

[GHC]

#11719: Cannot use higher-rank kinds with type families
-------------------------------------+-------------------------------------
        Reporter:  ocharles          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc2
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  dependent/should_compile/T11719
      Blocked By:                    |             Blocking:
 Related Tickets:  #13913            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Is this another consequence of this limitation? Consider the following
 program:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 import Data.Kind

 type family Sing :: k -> Type

 data SBool :: Bool -> Type where
   SFalse :: SBool False
   STrue  :: SBool True
 type instance Sing = SBool

 sFalse :: Sing False
 sFalse = SFalse
 }}}

 This typechecks without issue. However, if you change the following line
 of code:

 {{{#!hs
 type family Sing :: k -> Type
 }}}

 To become this:

 {{{#!hs
 type family Sing :: forall k. k -> Type
 }}}

 Then it stops typechecking:

 {{{
 $ /opt/ghc/8.6.1/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:18:10: error:
     • Couldn't match type ‘SBool’ with ‘Sing’
       Expected type: Sing 'False
         Actual type: SBool 'False
     • In the expression: SFalse
       In an equation for ‘sFalse’: sFalse = SFalse
    |
 18 | sFalse = SFalse
    |          ^^^^^^
 }}}

 I find this extremely surprising: I would have thought that GHC could
 treat `type family Sing :: k -> Type` and `type family Sing :: forall k. k
 -> Type` identically without the need for any fancy higher-rank business
 (like what the original program in this ticket requires).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11719#comment:13>
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