[GHC] #8500: Default associated type instances not instantiated

4 Nov 2013

      #8500: Default associated type instances not instantiated
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       Reporter:  goldfire          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 I have these modules:

 {{{
 {-# LANGUAGE TypeFamilies #-}

 module A where

 class C a where
   type F a
   type F a = [a]
 }}}

 {{{
 {-# LANGUAGE TypeFamilies #-}

 module B where

 import A

 instance C Int where
   type F Int = Double

 instance C Bool
 }}}

 When I say `ghc A.hs B.hs`, all works just fine. But, if I say `ghc A.hs`
 and then `ghc B.hs`, I get this:

 {{{
 B.hs:8:8:
     Conflicting family instance declarations:
       F Int -- Defined at B.hs:8:8
       F a1 -- Defined in ‛B’
 }}}

 For some reason, when GHC reads in default type instances from an
 interface file, it doesn't instantiate them properly when they get used.

 This was tested on 7.7.20131031.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8500>
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