[GHC] #10114: Kind mismatches with AnyK in rank-2 types

[GHC]

#10114: Kind mismatches with AnyK in rank-2 types
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        Reporter:  cam               |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by simonpj):

 Some notes for myself (work in progress).  At the moment, for type `T`
 above we get
 {{{
 -- Without PolyKinds
 type T = forall (f :: AnyK -> *) (a :: AnyK). f a

 -- With PolyKinds
 type T k = forall (f :: AnyK -> k) (a :: AnyK). f a
 }}}
 Trouble is that we are not generalising over the kind of `a`.  I think we
 want:
 {{{
 -- Without PolyKinds
 type T = forall (f :: * -> *) (a :: *). f a

 -- With PolyKinds
 type T k1 k2 = forall (f :: k1 -> k2) (a :: k1). f a
 }}}
 But note the strange kind of `T` in `PolyKinds` case: `T :: forall k1 k2.
 k2`.

 I'm pretty sure we do NOT want to kind-generalise "inside", thus:
 {{{
 type T = forall k1 k2 (f :: k1->k2) (a::k1). f a
 }}}
 Reasons:
  * see `Note [Kind generalisation]` in `TcHsType`
  * what kind would `T` have?   `T :: k2` ???
  * more operationally, kind foralls must be implicitly instantiated,
 during kind inference using only `T`'s kind

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10114#comment:1>
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