Re: [GHC] #8634: Relax functional dependency coherence check ("liberal coverage condition")

28 May 2015

      #8634: Relax functional dependency coherence check ("liberal coverage condition")
-------------------------------------+-------------------------------------
        Reporter:  danilo2           |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  high              |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.7
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:  #1241, #2247,     |                Blocking:
  #8356, #9103, #9227                |  Differential Revisions:  Phab:D69
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Comment (by atnnn):

 What is the status of this feature?

 I ran into another case where it might be useful:

 {{{#!hs
 {-# LANGUAGE FunctionalDependencies, FlexibleInstances,
     UndecidableInstances, ConstraintKinds, GADTs #-}

 class F a | -> a
 instance (eq ~ (a ~ ()), eq) => F a
 }}}

 GHC considers this illegal, but in practice it satisfies the functional
 dependency.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8634#comment:59
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