[GHC] #14348: Poly-kinded definitions silently introduce extra type arguments captured by TypeApplications

[GHC]

#14348: Poly-kinded definitions silently introduce extra type arguments captured by
TypeApplications
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        Reporter:  gallais           |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:  invalid           |  TypeApplications
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):

 * status:  new => closed
 * keywords:   => TypeApplications
 * resolution:   => invalid


Comment:

 This is expected behavior. As noted in the
 [https://downloads.haskell.org/~ghc/8.2.1/docs/html/users_guide/glasgow_exts.html?highlight
 =#ghc-flag--XTypeApplications users' guide], GHC determines the order of
 arguments for type applications by doing a stable topological sort on the
 user-written type variables, keeping kind variables before type variables.
 Since `k` is user-written, this means it's available for type application,
 and since it's the kind of `a`, `k` comes before `a`.

 If you don't want `k` to be available for type application, then you can
 do so by not writing it explicitly:

 {{{#!hs
 {-# LANGUAGE GADTs, PolyKinds, ScopedTypeVariables, TypeApplications #-}

 data EQ :: k -> k -> * where
   Refl :: EQ a a

 data Wrap a = Wrap (EQ a a)

 wrap :: forall a. Wrap a
 wrap = Wrap @a Refl
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14348#comment:1>
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