[GHC] #2456: For higher kinds, instance declarations need quantification in the context

[GHC]

#2456: For higher kinds, instance declarations need quantification in the context
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 Reporter:  ronwalf          |          Owner:          
     Type:  feature request  |         Status:  reopened
 Priority:  normal           |      Milestone:  _|_     
Component:  Compiler         |        Version:  6.9     
 Severity:  normal           |     Resolution:          
 Keywords:                   |     Difficulty:  Unknown 
 Testcase:                   |   Architecture:  x86     
       Os:  MacOS X          |  
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Changes (by simonpj):

  * summary:  Generics compilation problem: "FATAL:Symbol _XJv_srt already
              defined." => For higher kinds, instance
              declarations need quantification in the context
  * type:  bug => feature request
  * milestone:  => _|_

Comment:

 However, concerning your comment above, I had missed the fact that `Expr`
 has kind `(*->*) -> *`.  What you ''really'' want to say is:
 {{{
 deriving instance (forall a. Data a => Data (f a)) => Data (Expr f)
 }}}
 Notice that the context of the instance has a nested `forall`.  This has
 been a long-standing want; see
 [http://research.microsoft.com/%7Esimonpj/Papers/derive.htm Derivable type
 classes], Section 7.

 So I'll retitle this ticket as a feature request for such a feature. But
 don't hold your breath.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2456#comment:7>
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