[GHC] #14887: Explicitly quantifying a kind variable causes a type family to fail to typecheck

[GHC]

#14887: Explicitly quantifying a kind variable causes a type family to fail to
typecheck
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  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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Comment (by RyanGlScott):

 A slight twist on this is if you leave out the type family equations. For
 instance:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# OPTIONS_GHC -fprint-explicit-kinds #-}
 module Bug where

 import Data.Kind
 import Data.Proxy

 type family Foo1             (e :: Proxy (a :: k)) :: Type where {}
 type family Foo2 (k :: Type) (e :: Proxy (a :: k)) :: Type where {}
 }}}

 `Foo1` typechecks, but `Foo2` does not:

 {{{
 $ ghci Bug.hs -fprint-explicit-kinds
 GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:10:1: error:
     • These kind and type variables: k (e :: Proxy k a)
       are out of dependency order. Perhaps try this ordering:
         k (a :: k) (e :: Proxy k a)
       NB: Implicitly declared kind variables are put first.
     • In the type family declaration for ‘Foo2’
    |
 10 | type family Foo2 (k :: Type) (e :: Proxy (a :: k)) :: Type where {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 I have no idea why GHC is complaining about the scoping order here—that
 looks well-scoped to me!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14887#comment:1>
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