Re: [GHC] #14331: Overzealous free-floating kind check causes deriving clause to be rejected

13 Oct 2017

      #14331: Overzealous free-floating kind check causes deriving clause to be rejected
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.2.2
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  deriving/should_compile/T14331
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I don't understand your reasoning in comment:16. After all, we don't
 reject this:

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

 import Data.Proxy

 data Foo a = Foo (Proxy (a :: k))
 }}}

 After all, `k` is implicitly bound by the datatype `D`, so everything
 works out. The same situation applies in:

 {{{#!hs
 data D a = D deriving (C (a :: k))
 }}}

 Once again, `k` is implicitly bound by `D`, so `k` and the kind of `a` are
 one and the same.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14331#comment:17
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