[GHC] #12045: Visible kind application

[GHC]

#12045: Visible kind application
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        Reporter:  Iceland_jack      |                Owner:  Iceland_jack
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  ​Phab:D2216
       Wiki Page:                    |
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Comment (by Iceland_jack):

 [research.microsoft.com/en-us/um/...f/promotion-tldi12-sub.pdf Giving
 Haskell a Promotion]:

 {{{#!hs
 data Mu :: forall k. ((k -> Type) -> (k -> Type)) -> (k -> Type) where
   Roll :: f (Mu f) a -> Mu f a

 data VecF :: Type -> (Nat -> Type) -> (Nat -> Type) where
   VFNil  :: VecF a f Zero
   VFCons :: a -> f n -> VecF a f (Succ n)
 }}}

 Applying `type Vec a n = Mu (VecF a) n` instantiates `k` to `Nat` in `Mu`,
 so this should work:

 {{{
 >>> :kind Mu
 Mu :: ((k -> Type) -> k -> Type) -> k -> Type

 >>> :kind Mu @Nat
 Mu @Nat :: ((Nat -> Type) -> Nat -> Type) -> Nat -> Type

 >>> :kind forall a. Mu @Nat (VecF a)
 forall a. Mu @Nat (VecF a) :: Nat -> Type
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12045#comment:6>
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