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José Pedro Magalhães | 17 Oct 2012 10:18
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Adding dataCast1 and dataCast2 methods to the Data instances of Ptr, ForeignPtr, and Array

Hello all,

This email is to draw your attention to http://hackage.haskell.org/trac/ghc/ticket/7256. Missing dataCast1 and dataCast2 methods in Data instances results in incorrect behaviour when using SYB's ext1/ext2 functions. The proposed fix adds the methods, but it requires adding additional Data constraints to these instances. Here's the relevant part of the diff:

-instance Typeable a => Data (Ptr a) where
+instance (Data a, Typeable a) => Data (Ptr a) where
 
-instance Typeable a => Data (ForeignPtr a) where
+instance (Data a, Typeable a) => Data (ForeignPtr a) where

-instance (Typeable a, Data b, Ix a) => Data (Array a b)
+instance (Typeable a, Data a, Data b, Ix a) => Data (Array a b)

Does anyone find this change problematic?


Cheers,
Pedro

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