[GHC] #2895: Implement the "Class System Extension" proposal

[GHC]

#2895: Implement the "Class System Extension" proposal
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Reporter:  porges            |          Owner:                  
    Type:  feature request   |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  6.10.1            |       Severity:  normal          
Keywords:  proposal          |       Testcase:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
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 See: http://haskell.org/haskellwiki/Class_system_extension_proposal

 Executive summary:
    1.  Class and instance declarations would allow method implementations
 to be given for any methods in the class or any ancestor class.
    2. Whenever an instance declaration is visible there would always be a
 full set of instance declarations for all ancestor classes, by
 supplementing the set of explicitly given instance declarations that are
 visible in a module by automatically generated implicit instance
 declarations.
    3. The most specific method implementation would always be chosen (ie
 prefer an explicit instance method over a class method and prefer a
 subclass method to a superclass method)
    4. Modules would only export explicit instance declarations

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2895>
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