[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:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * related:  #13913 => #13913, #14268


Comment:

 I've found a workaround for the original issue in this ticket after much
 persistence. The trick is to associate `BaseType` with a type class:

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

 Then, define a flexible instance like so, using an explicit `forall` to
 give `k` the appropriate higher-rank kind:

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

 That's it! Now you can use `BaseType` like you'd expect:

 {{{
 λ> data ProxySym0 :: forall a. a ~> Type
 λ> type instance ProxySym0 @@ x = Proxy x
 λ> :kind! BaseType ProxySym0 Bool
 BaseType ProxySym0 Bool :: *
 = Proxy Bool
 λ> :kind! BaseType ProxySym0 Maybe
 BaseType ProxySym0 Maybe :: *
 = Proxy Maybe
 λ> :kind! BaseType ProxySym0 'Just
 BaseType ProxySym0 'Just :: *
 = Proxy 'Just
 }}}

 It's a bit gnarly, but this is the best that you can do until #14268 is
 implemented.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11719#comment:22>
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