Re: [GHC] #11534: Allow class associated types to reference functional dependencies

3 Feb 2016

      #11534: Allow class associated types to reference functional dependencies
-------------------------------------+-------------------------------------
        Reporter:  ekmett            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.3
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  FunctionalDependencies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I'm doubtful.  Remember,
  * An associated type declaration is just convenient notation for a top-
 level declaration
  * Those default declarations are just an abbreviation for writing it out
 at every insttance

 With that in mind, what would we make of
 {{{
 instance D a b => C [a] [b] where {}
 }}}
 From a fundep point of view this is fine, provided `D` has a fundep.
 Expanding the default decl gives
 {{{
 instance D a b => C [a] [b] where
   type Bar [a] = [b]
 }}}
 which makes no sense.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11534#comment:1
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