[GHC] #7688: :kind should do kind generalisation

[GHC]

#7688: :kind should do kind generalisation
---------------------------------+------------------------------------------
    Reporter:  simonpj           |       Owner:                  
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:  7.8.1           
   Component:  Compiler          |     Version:  7.6.2           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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 Suppose you load this into GHCi:
 {{{
 {-# LANGUAGE PolyKinds #-}
 module Test where

 data Proxy (t::k) = Proxy
 }}}
 Now you use `:kind` to see the kind of `Proxy`:
 {{{
 ghci> :k Proxy
 Proxy :: * -> *
 }}}
 It is true that `Proxy` has that kind but it has a more polymorphic kind,
 and that's the one we'd like to see.

 The trouble is this. `Proxy` really does have a polymorphic kind.
 However, `:kind` takes a '''type''' (not a type constructor) as its
 argument, so you can say
 {{{
         :kind (Maybe Int)
 }}}
 for example.  So as usual we instantiate `Proxy` with fresh kind variables
 -- '''BUT''' `:kind` does not do kind generalisation, so we get a
 monomorphic instance.  (And we default unbound kind variables to `*`.

 Conclusion: `:kind` should do kind-generalisation.

 See also #7586

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7688>
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