Error when deriving Typeable for associated type

Bas van Dijk v.dijk.bas at
Mon Jan 30 20:26:42 CET 2012


Given the following program:

{-# LANGUAGE DeriveDataTypeable, TypeFamilies #-}

import Data.Typeable

class C a where
    data T a :: *

data MyType1 = MyType1 deriving Typeable
data MyType2 = MyType2 deriving Typeable

instance C MyType1 where
    data T MyType1 = A1 deriving (Typeable)

instance C MyType2 where
    data T MyType2 = A2 deriving (Typeable)

I get the following unexpected error:

    Duplicate instance declarations:
      instance Typeable1 T -- Defined at TF_Data.hs:12:34
      instance Typeable1 T -- Defined at TF_Data.hs:15:34

When looking at the output of -ddump-deriv I see that the following
instance is generated:

instance Typeable1 T where ...

I would have expected that the following instances were generated instead:

instance Typeable1 (T MyType1) where ...
instance Typeable1 (T MyType2) where ...

Is this a bug in GHC?



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