[GHC] #12938: Polykinded associated type family rejected on false pretenses

6 Dec 2016

      #12938: Polykinded associated type family rejected on false pretenses
-------------------------------------+-------------------------------------
           Reporter:  goldfire       |             Owner:  goldfire
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.0.1
           Keywords:  TypeInType     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If I say

 {{{
 class HasRep a where
   type Rep a :: TYPE r
 }}}

 I get

 {{{
     • Kind variable ‘r’ is implicitly bound in datatype
       ‘Rep’, but does not appear as the kind of any
       of its type variables. Perhaps you meant
       to bind it (with TypeInType) explicitly somewhere?
       Type variables with inferred kinds: a
     • In the class declaration for ‘HasRep’
 }}}

 This definition should be accepted, though, as `r` is just an invisible
 parameter to the associated type family. (I don't know how ''useful'' this
 is, but it's not bogus.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12938
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