id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
4530,Deriving Data for existentially quantified types,mitar,,"I cannot derive Data for:

{{{
data EqBox where
  EB :: forall e . (Data e, Eq e) => e -> EqBox
}}}

I am getting such error:

{{{
   Ambiguous type variable `e' in the constraints:
      (Eq e) arising from a use of `EB' at Test.hs:11:1-28
      (Data e) arising from a use of `k' at Test.hs:11:1-28
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `z', namely `EB'
    In the first argument of `k', namely `z EB'
    In the expression: k (z EB)
    When typechecking the code for  `gunfold'
      in a standalone derived instance for `Data EqBox':
      To see the code I am typechecking, use -ddump-deriv
}}}",bug,closed,normal,7.4.1,Compiler,7.1,invalid,,mmitar@…,Unknown/Multiple,Unknown/Multiple,None/Unknown,,,,,