#4530: Deriving Data for existentially quantified types
---------------------------------+------------------------------------------
Reporter: mitar | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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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
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4530>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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