#7256: Missing dataCast1 and dataCast2 methods in Data.Data instances
---------------------------------+------------------------------------------
Reporter: dreixel | Owner: dreixel
Type: bug | Status: infoneeded
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Changes (by dreixel):
* status: new => infoneeded
Comment:
Here's the fix:
{{{
diff --git a/Data/Data.hs b/Data/Data.hs
index 0a29668..cb2d894 100644
--- a/Data/Data.hs
+++ b/Data/Data.hs
@@ -1275,27 +1275,27 @@ instance (Data a, Data b, Data c, Data d, Data e,
Data f
------------------------------------------------------------------------------
-instance Typeable a => Data (Ptr a) where
+instance (Data a, Typeable a) => Data (Ptr a) where
toConstr _ = error "Data.Data.toConstr(Ptr)"
gunfold _ _ = error "Data.Data.gunfold(Ptr)"
dataTypeOf _ = mkNoRepType "GHC.Ptr.Ptr"
-
+ dataCast1 x = gcast1 x
------------------------------------------------------------------------------
-instance Typeable a => Data (ForeignPtr a) where
+instance (Data a, Typeable a) => Data (ForeignPtr a) where
toConstr _ = error "Data.Data.toConstr(ForeignPtr)"
gunfold _ _ = error "Data.Data.gunfold(ForeignPtr)"
dataTypeOf _ = mkNoRepType "GHC.ForeignPtr.ForeignPtr"
-
+ dataCast1 x = gcast1 x
------------------------------------------------------------------------------
-- The Data instance for Array preserves data abstraction at the cost of
-- inefficiency. We omit reflection services for the sake of data
abstraction.
-instance (Typeable a, Data b, Ix a) => Data (Array a b)
+instance (Typeable a, Data a, Data b, Ix a) => Data (Array a b)
where
gfoldl f z a = z (listArray (bounds a)) `f` (elems a)
toConstr _ = error "Data.Data.toConstr(Array)"
gunfold _ _ = error "Data.Data.gunfold(Array)"
dataTypeOf _ = mkNoRepType "Data.Array.Array"
-
+ dataCast2 x = gcast2 x
}}}
Note however that this requires adding additional `Data` constraints to
these instances; perhaps this is the reason why these methods were omitted
in the first place. So fixing this might break client code that uses these
instances. Nonetheless, they are required for the correct behavior of
SYB's `ext/1/2` functions.
Shall I go ahead and push the patch?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7256#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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