[GHC] #12369: data families shouldn't be required to have return kind *, data instances should

[GHC]

#12369: data families shouldn't be required to have return kind *, data instances
should
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        Reporter:  ekmett            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 Replying to [comment:5 Iceland_jack]:
 >
 > The part about **runtime representation** made me wonder if you could
 define
 >
 > {{{#!hs
 > --   × × × FAILS × × ×
 > data family Array a :: TYPE rep
 > }}}
 >
 > I dunno about the instances, maybe using
 [https://ghc.haskell.org/trac/ghc/wiki/UnliftedDataTypes#Proposal3:Allownewtypesoverunliftedtypes
 Proposal 3: Allow newtypes over unlifted types]

 While there's nothing very wrong about the data family definition, the
 instances would be a problem, because we don't yet have a way to make
 user-written unlifted types. It would indeed be useful to have this, but
 we don't. So I think this kind of construct is best off waiting until we
 solve the simpler case of normal unlifted datatypes before we try unlifted
 data families.

 > ----
 >
 > {{{#!hs
 > --   WORKS FINE
 > data family Array (a :: TYPE rep) :: Type
 >
 > --   × × × FAILS × × ×
 > --
 > -- Unexpected type ‘Int#’
 > data Array Int#
 > -- Should this fail?
 > }}}

 You just want to say `data instance Array Int#`, which indeed works fine.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12369#comment:6>
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