[GHC] #13814: Unable to resolve instance for polykinded superclass constraint on associated-type-family.

[GHC]

#13814: Unable to resolve instance for polykinded superclass constraint on
associated-type-family.
-------------------------------------+-------------------------------------
        Reporter:  isovector         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  polykinds,
                                     |  type families
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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Replying to [comment:3 simonpj]:
 > The real problem is that `Back` is too polymorphic.  If it had kind `k
 -> k` we'd probably be fine.

 Ah, you're right. This program //does// typecheck, as expected:

 {{{#!hs
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE PolyKinds        #-}
 {-# LANGUAGE TypeFamilies     #-}

 module Test where

 class Back t

 class Back (FrontBack t) => Front t where
   type FrontBack (t :: k) :: k

 instance Back Bool

 instance Front Int where
   type FrontBack Int = Bool
 }}}

 > Maybe we should complain about the unresolved kind variables in the
 class decl?

 That sounds like a good approach, yes. The error message might be
 confusing since it mentions an inferred kind variable, but I'd argue that
 that error would be far clearer than the current one, which is rather
 enigmatic.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13814#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler