[GHC] #6024: Allow defining kinds alone, without a datatype

[GHC]

#6024: Allow defining kinds alone, without a datatype
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 Reporter:  dreixel           |          Owner:                         
     Type:  feature request   |         Status:  new                    
 Priority:  normal            |      Component:  Compiler (Type checker)
  Version:  7.5               |       Keywords:                         
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple       
  Failure:  None/Unknown      |       Testcase:                         
Blockedby:                    |       Blocking:                         
  Related:                    |  
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 Sometimes we want to define a kind alone, and we are not interested in the
 datatype. In principle having an extra datatype around is not a big
 problem, but the constructor names will be taken, so they cannot be used
 somewhere else. A contrived example:
 {{{
 data Code = Unit | Prod Code Code

 data family Interprt (c :: Code) :: *
 data instance Interprt Unit       = Unit1
 data instance Interprt (Prod a b) = Prod1 (Interprt a) (Interprt b)
 }}}
 We're only interested in the constructors of the data family `Interprt`,
 but we cannot use the names `Unit` and `Prod` because they are
 constructors of `Code`.

 The suggestion is to allow defining:
 {{{
 data kind Code = Unit | Prod Code Code
 }}}
 Such that `Code` is a kind, and not a type, and `Unit` and `Prod` are
 types, and not constructors.

 Note that using "data kind" instead of just "kind" means the word "kind"
 does not have to be a reserved keyword.

 You could also think you would want to have datatypes that should not be
 promoted:
 {{{
 data K

 data type T = K
 }}}
 But I don't see a need for this, as the fact that the `K` constructor is
 promoted to a type does not prevent you from having a datatype named `K`.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6024>
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