[GHC] #8090: MetaKinds - PolyKinds generalization

[GHC]

#8090: MetaKinds - PolyKinds generalization
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        Reporter:  wvv               |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by goldfire):

 To remove ambiguity, I'll adopt the "many stars" notation. If we assume an
 infinite hierarchy of classifications ("sorts") (and a right-associative
 ::), we have

 {{{
 True :: Bool :: * :: ** :: ***  :: **** :: ...
 }}}

 We currently have mono-kinded type variables {{{(a :: *)}}} and poly-
 kinded type variables {{{(a :: k :: **)}}. If we have the next level,
 {{{(a :: k :: m :: ***)}}}, what useful value could {{{m}}} have other
 than {{{**}}}? My understanding is that, in this scenario, any value other
 than {{{**}}} for {{{m}}} would mean that the kind `k` would be empty --
 that is, `a` wouldn't exist.

 I still don't see where this extension would be helpful. For example, you
 propose this type:

 {{{
 data Foo (a::(b::k)->*)
 }}}

 What would its right-hand side look like?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8090#comment:5>
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