[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, #14268    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Now that #14268 is implemented, we can work around this issue in a much
 less hacky way:

 {{{#!hs
 type family BaseType (k :: forall a. a ~> Type) (x :: b) :: Type where
   forall (k :: forall a. a ~> Type) x. BaseType k x = (@@) k x
 }}}

 That being said, the way that I originally thought this type family should
 have been defined:

 {{{#!hs
 type family BaseType (k :: forall a. a ~> Type) (x :: b) :: Type where
   BaseType (k :: forall a. a ~> Type) x = (@@) k x
 }}}

 Still does not kind-check:

 {{{
 $ ghc/inplace/bin/ghc-stage2 Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:22:13: error:
     • Expected kind ‘forall a. a ~> *’, but ‘k’ has kind ‘a0 ~> *’
     • In the first argument of ‘BaseType’, namely
         ‘(k :: forall a. a ~> Type)’
       In the type family declaration for ‘BaseType’
    |
 22 |   BaseType (k :: forall a. a ~> Type) x = (@@) k x
    |             ^
 }}}

 This feels like it ought to Just Work™, though. Should we keep this ticket
 open until this is possible?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11719#comment:23>
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