[GHC] #14795: Data type return kinds don't obey the forall-or-nothing rule

12 Feb 2018

      #14795: Data type return kinds don't obey the forall-or-nothing rule
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
  (Type checker)                     |
           Keywords:  TypeInType     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Originally noticed [https://github.com/ghc-proposals/ghc-
 proposals/pull/103#issuecomment-364562974 here]. GHC accepts this:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeInType #-}

 import Data.Kind

 data Foo :: forall a. a -> b -> Type where
   MkFoo :: a -> Foo a b
 }}}

 Despite the fact that `Foo`'s return kind is headed by an explicit
 `forall` which does not quantify `b`.

 The users' guide doesn't explicitly indicate that the `forall`-or-nothing
 rule should apply to data type return kinds, but goldfirere believes that
 not doing so is an [https://github.com/ghc-proposals/ghc-
 proposals/pull/103#issuecomment-364670215 inconsistent design], so I'm
 opening this ticket to track this.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14795
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