[GHC] #12384: Type family not reduced, again

12 Jul 2016


      #12384: Type family not reduced, again
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           Reporter:  kosmikus       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Linux
       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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 This one looks similar to #12381, but unfortunately, it does not seem to
 be fixed in HEAD:
 {{{#!hs
 {-# LANGUAGE TypeInType, TypeFamilies, FlexibleInstances #-}

 import GHC.Types

 type family F (a :: Type) :: Type

 class C a where
   type D (a :: Type) :: F a

 instance (F a ~ Bool) => C a where
 type D a = True
 }}}

 This yields (in 8.0.1 and 8.1.20160709):
 {{{
 Constraint.hs:11:14: error:
     • Expected kind ‘F a’, but ‘'True’ has kind ‘Bool’
     • In the type ‘True’
       In the type instance declaration for ‘D’
       In the instance declaration for ‘C a’
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12384
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