[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:6 simonpj]:
 > We can't allow just any old class in a deriving clause!

 I don't understand this issue. Why do we need to annotate certain classes
 as being allowed in a deriving clause? The annotation doesn't add any
 extra information (except a boolean to say its allowed) so GHC must have
 everything it needs to derive. Why would I want to write a class and all
 the supporting code to allow it to be derived but not add the DERIVABLE
 pragma?

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5462#comment:7>
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