[GHC] #12766: Allow runtime-representation polymorphic data families

[GHC]

#12766: Allow runtime-representation polymorphic data families
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12369            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Explore how ticket:12369#comment:3, ticket:12369#comment:4 fit into this.

 TODO How `Free` fits with [https://github.com/ghc-proposals/ghc-
 proposals/pull/30 Levity-Polymorphic Type Classes], can we define
 **something** like this

 {{{#!hs
 newtype instance Free (k :: TYPE rep -> Constraint) -> (TYPE rep -> TYPE
 rep) where
   Free0 :: (forall q. k q => (p -> q) -> q) -> Free k p
 }}}

 TODO or like this

 {{{#!hs
 newtype instance Fix (f :: TYPE rep -> TYPE rep) :: TYPE rep where
   In :: f (Fix f) -> Fix f
 }}}

 And would they be useful

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12766#comment:2>
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