[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:                    |
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Description changed by isovector:

@@ -10,1 +10,1 @@
- class Back k
+ class Back t
@@ -12,2 +12,2 @@
- class Back (FrontBack k) => Front k where
-   type FrontBack k :: k'
+ class Back (FrontBack t) => Front t where
+   type FrontBack t :: k

New description:

 The following program doesn't compile:

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

 module Test where

 class Back t

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

 instance Back Bool

 instance Front Int where
   type FrontBack Int = Bool

 }}}

 with the error message:

 {{{
     • No instance for (Back (FrontBack Int))
         arising from the superclasses of an instance declaration
     • In the instance declaration for ‘Front Int’
 }}}


 The example successfully compiles if the kind annotation on FrontBack is
 removed.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13814#comment:1>
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