[GHC] #5007: "deriving" seems to ignore class context for a type family

[GHC]

#5007: "deriving" seems to ignore class context for a type family
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  Reporter:  jkff                       |          Owner:  simonpj                                                 
      Type:  bug                        |         Status:  closed                                                  
  Priority:  high                       |      Milestone:  7.2.1                                                   
 Component:  Compiler (Type checker)    |        Version:  7.0.2                                                   
Resolution:  invalid                    |       Keywords:  type families, datatype contexts, type classes, deriving
  Testcase:                             |      Blockedby:                                                          
Difficulty:                             |             Os:  Unknown/Multiple                                        
  Blocking:                             |   Architecture:  Unknown/Multiple                                        
   Failure:  GHC rejects valid program  |  
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Changes (by simonpj):

  * status:  new => closed
  * resolution:  => invalid


Comment:

 The error message is quite right.

 Using a context on a data type declaration, as you are doing, is a mis-
 feature of Haskell.  If you read its specification carefully you'll see
 that it is practically useless. Any program using it is suspicious.
 Certainly, it has absolutely no effect on 'deriving' declarations.

 You can follow the advice in the error message and use a standalone
 deriving declaration, thus
 {{{
 {-# LANGUAGE StandaloneDeriving, FlexibleContexts, UndecidableInstances,
 TypeFamilies #-}

 module T5007 where

 class Foo a where
    data Bar a :: *

 class (Show (Bar a)) => Qux a
 data Xyzzy a = Xyzzy (Bar a)

 deriving instance Show (Bar a) => Show (Xyzzy a)
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5007#comment:2>
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