#5863: Deriving Typeable for data families
---------------------------------+------------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
---------------------------------+------------------------------------------
Bas points out that `deriving Typeable` for data families isn't right at
all.
{{{
{-# LANGUAGE DeriveDataTypeable, TypeFamilies #-}
import Data.Typeable
class C a where
data T a :: *
instance C Int where
data T Int = A1 deriving (Typeable)
instance C Bool where
data T Bool = A2 deriving (Typeable)
}}}
gives this error:
{{{
TF_Data.hs:12:34:
Duplicate instance declarations:
instance Typeable1 T -- Defined at TF_Data.hs:12:34
instance Typeable1 T -- Defined at TF_Data.hs:15:34
}}}
The real problem is that we should have only one instance for `Typeable
T`. We should only allow
{{{
deriving instance Typeable1 T
}}}
and prohibit `deriving Typeable` on `data instance` declarations.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5863>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
