[GHC] #3707: deriving Data, Typeable for empty data decls

[GHC]

#3707: deriving Data, Typeable for empty data decls
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    Reporter:  seanmcl           |       Owner:              
        Type:  feature request   |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  6.10.4            |    Keywords:              
          Os:  Unknown/Multiple  |    Testcase:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
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 Empty data decls are useful for things like phantom types.  However,
 deriving (Data, Typeable) does not work with them, so if you want to use
 them with generics you must do something like the following:

 data PredTag

 instance Typeable PredTag where
   typeOf _ = G.mkTyConApp (G.mkTyCon "PredTag") []

 Surely GHC could derive this for us!

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3707>
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