[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 Iceland_jack):

 [http://cs.brynmawr.edu/~rae/papers/2016/thesis/eisenberg-thesis-draft.pdf
 Eisenberg]:

 > A ''data family'' defines a family of datatypes. An example shows best
 how this works:
 >
 > {{{#!hs
 > data family Array a -- compact storage of elements of type a
 > data instance Array Bool = MkArrayBool ByteArray
 > data instance Array Int  = MkArrayInt (Vector Int)
 > }}}
 >
 > With such a definition, we can have a different runtime representation
 for `Array Bool` than we do for `Array Int`, something not possible with
 more traditional parameterized types.

 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]

 {{{#!hs
 newtype instance Array Int# :: TYPE PtrRepUnlifted where
   MkInt# :: ByteArray# -> Array Int#

 newtype instance Array (##) :: TYPE IntRep where
   MkInt# :: Int# -> Array (##)
 }}}

 I don't really understand representation polymorphism, may suffer from
 same problem as #11471.

 ----

 {{{#!hs
 --   WORKS FINE
 data family Array (a :: TYPE rep) :: Type

 --   × × × FAILS × × ×
 --
 -- Unexpected type ‘Int#’
 data Array Int#
 -- Should this fail?
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12369#comment:5>
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