[GHC] #14514: Higher-Rank Kinds work in ADT but not GADT

[GHC]

#14514: Higher-Rank Kinds work in ADT but not GADT
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 I suppose the answer is that
 [https://ghc.haskell.org/trac/ghc/ticket/14352#comment:1 “GHC never infers
 a higher-rank kind”].

 The correct way of writing this is explicitly quantifying over the kind of
 `ext` when writing it as a GADT (as is done here:
 [https://github.com/goldfirere/dependent-db/blob/master/Basics.hs#L134
 TypeRepX])

 {{{#!hs
 data Exp :: WithAnyTag -> Type where
   Var :: forall (ext::WithAnyTag). ext VarTag -> Exp ext
   -- ..
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14514#comment:1>
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