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

[GHC]

#8634: Relax functional dependency coherence check ("liberal coverage condition")
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              Reporter:  danilo2     |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:  7.10.1
              Priority:  high        |           Version:  7.7
             Component:  Compiler    |          Keywords:
            Resolution:              |  Operating System:  Unknown/Multiple
Differential Revisions:  Phab:D69    |   Type of failure:  None/Unknown
          Architecture:              |         Test Case:
  Unknown/Multiple                   |          Blocking:
            Difficulty:  Unknown     |
            Blocked By:              |
       Related Tickets:  #1241,      |
  #2247, #8356, #9103, #9227         |
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Comment (by danilo2):

 @diatchki please do not base your opinion on the examples above - they are
 a little old and of course, they do not obey some basic principles.
 The idea with `-XDysfunctionalDependencies` is just to lift both the
 Paterson Conditions and the Coverage Condition - something
 `-XDysfunctionalDependencies` claims  to do (according to documentation),
 but does not (as simonpj noticed above). When using this extension you can
 just give hints to typechecker and compile programs like the one I've
 posted on https://phabricator.haskell.org/D69 :

 {{{#!haskell
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE DysfunctionalDependencies #-}

 class CTest a b | a -> b where
    ctest :: a -> b

 data X = X

 instance Monad m => CTest X (m Int) where

 ctest _ = return 5

 main = print (ctest X :: [Int]) -- [5]
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:19>
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