[GHC] #5462: Deriving clause for arbitrary classes

[GHC]

#5462: Deriving clause for arbitrary classes
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    Reporter:  simonpj           |        Owner:              
        Type:  feature request   |       Status:  new         
    Priority:  normal            |    Milestone:              
   Component:  Compiler          |      Version:  7.2.1       
    Keywords:                    |     Testcase:              
   Blockedby:                    |   Difficulty:              
          Os:  Unknown/Multiple  |     Blocking:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
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Comment(by dterei):

 Replying to [comment:9 simonpj]:
 > But I'm not sure whether that is what you meant.

 Yes that is what I meant. Basically GHC should set a list of requirements
 that must be met (chosen so GHC has enough information to correctly
 derive) and if a class meets that you can put it in a deriving clause.
 Otherwise if you put it in a deriving clause you'll get an error message
 like you currently do.

 The proposed DERIVE pragma is the same as this anyway except I guess you
 could throw an error earlier and maybe a little more informative if you
 encounter a class that has the pragma but doesn't meet the requirements.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5462#comment:10>
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