[Haskell] How to define gunfold on List?

[José Pedro Magalhães]

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 at 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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