Hi there,

There's no difference. See http://hackage.haskell.org/packages/archive/base/4.0.0.0/doc/html/src/Data-Data.html#line-1034

You can also see many other instances there.


Cheers,
Pedro

On Sun, Jan 18, 2009 at 10:50, haihualin <haihualin@163.com> wrote:
Hi,

Does some one know how to define gunfold on recursive data struction like List?

The ghc doc only give the example for non-recursive data like below.

 data T a b = C1 a b | C2 deriving (Typeable, Data)

GHC will generate an instance that is equivalent to

 instance (Data a, Data b) => Data (T a b) where
    gfoldl k z (C1 a b) = z C1 `k` a `k` b
    gfoldl k z C2       = z C2

    gunfold k z c = case constrIndex c of
                        1 -> k (k (z C1))
                        2 -> z C2

    toConstr (C1 _ _) = con_C1
    toConstr C2       = con_C2

    dataTypeOf _ = ty_T

 con_C1 = mkConstr ty_T "C1" [] Prefix
 con_C2 = mkConstr ty_T "C2" [] Prefix
 ty_T   = mkDataType "Module.T" [con_C1, con_C2]



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