[GHC] #13105: Allow type families in RuntimeReps

[GHC]

#13105: Allow type families in RuntimeReps
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           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Conal Elliott provided me with this puzzle:

 {{{
 type family RepRep a ∷ RuntimeRep

 class HasRep a where
   type Rep a ∷ TYPE (RepRep a)
   repr ∷ a → Rep a
   abst ∷ Rep a → a

 type instance RepRep Int = IntRep

 instance HasRep Int where
   type Rep Int = Int#
   abst n = I# n
   repr (I# n) = n
 }}}

 I think we should accept. However, doing so (even with my solution to
 #12809 complete) is hard, because we frequently consult a kind in order to
 determine a runtime representation. When that kind is `TYPE (RepRep Int)`,
 the code generator throws up its arms in despair.

 The solution here is either to teach the code generator how to normalise
 types (requiring propagating the `FamInstEnvs`) or to do a whole-program
 transformation at some point (zonker? desugarer? maybe we can wait until
 !CorePrep or even unarisation?) to change ids whose types have kinds like
 `TYPE (RepRep Int)` into ids with types with kinds like `TYPE IntRep`.

 But I don't want to let this hold me up at the moment, so I'm posting here
 as a reminder to revisit this problem.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13105>
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