Re: [GHC] #14887: Explicitly quantifying a kind variable causes a telescope to fail to kind-check (was: Explicitly quantifying a kind variable causes a type family to fail to typecheck)

22 Apr 2018

      #14887: Explicitly quantifying a kind variable causes a telescope to fail to kind-
check
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * keywords:  TypeFamilies, TypeInType => TypeInType

Comment:

 As it turns out, this is not at all specific to type families. The same
 phenomenon occurs in data types:

 {{{#!hs
 data Foo1             (e :: Proxy (a :: k)) :: Type -- typechecks
 data Foo2 (k :: Type) (e :: Proxy (a :: k)) :: Type -- doesn't typecheck
 }}}

 Or even plain old functions:

 {{{#!hs
 foo1 :: forall (e :: Proxy (a :: k)).
         Int -- typechecks
 foo1 = 42

 foo2 :: forall (k :: Type) (e :: Proxy (a :: k)).
         Int -- doesn't typecheck
 foo2 = 42
 }}}

 So I'm guessing that any type variable telescope suffers from this issue.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14887#comment:6
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