Re: [wxhaskell-users] Use of Ptr type for calling treeCtrlHitTest

[Takano A. <al...@hy...>]

Hi Bernd,

On Fri, 08 Dec 2006 14:20:15 +0100
Bernd Holzm=FCller <ber...@ic...> wrote:

> I would like to use the function
>=20
> treeCtrlHitTest :: TreeCtrl a -> Point -> Ptr CInt -> IO TreeItem
>=20
> but I cannot find out from the wxHaskell documentation how to create /=20
> use the Ptr argument, which stands for an "out" parameter.

You can create a pointer using functions from Foreign.Marshal.Alloc,
and read/write it using functions from Foreign.Marshal.Storable.

For example, using alloca and peek, you can write a simple wrapper around
treeCtrlHitTest, which returns a pair instead of requiring a Ptr.

myHitTest :: TreeCtrl a -> Point -> IO (TreeItem, CInt)
myHitTest w point =3D alloca $ \ptr -> do
  r <- treeCtrlHitTest w point ptr
  flags <- peek ptr
  return (r, flags)

Regards,
Takano Akio