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

11 Jun 2017

      #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
           Keywords:  polykinds,     |  Operating System:  Unknown/Multiple
  type families                      |
       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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 The following program doesn't compile:

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

 module Test where

 class Back k

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