[GHC] #10815: Need more kind inference in associated type instances

31 Aug 2015

      #10815: Need more kind inference in associated type instances
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              Reporter:  goldfire    |             Owner:  goldfire
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
  (Type checker)                     |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 When I say

 {{{
 {-# LANGUAGE DataKinds, PolyKinds, TypeFamilies #-}

 module Bug where

 import Data.Proxy

 class kproxy ~ 'KProxy => C (kproxy :: KProxy k) where
   type F (a :: k)

 instance C ('KProxy :: KProxy Bool) where
   type F a = Int
 }}}

 I get

 {{{
     Type indexes must match class instance head
     Found ‘k’ but expected ‘Bool’
     In the type instance declaration for ‘F’
     In the instance declaration for ‘C (KProxy :: KProxy Bool)’
 }}}

 But the kind of `a` should really be known to be `Bool`.

 This ticket is broken out from comment:3:ticket:9063, which was evidently
 not addressed when fixing that ticket.

 Patch coming soon.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10815
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