[Haskell-beginners] FFI foreignPtr construction

[Sylvain Henry]

Hi,

You can try something like this (not tested):

data Geo = Geo -- this represents the HklGeometry C structure (for clarity)

newtype Geometry = Geometry (ForeignPtr Geo) deriving (Show, Storable)

newGeometry :: Factory -> IO Geometry
newGeometry f = do -- avoid unsafePerformIO
                  geometry <- c_hkl_factory_create_new_geometry f
                  Geometry <$> newForeignPtr c_hkl_geometry_free geometry

foreign import ccall safe "hkl.h hkl_factory_create_new_geometry"
  c_hkl_factory_create_new_geometry :: Factory
                                    -> IO (Ptr Geo)

foreign import ccall safe "hkl.h &hkl_geometry_free"
  c_hkl_geometry_free :: FunPtr (Ptr Geo -> IO ())

2016-01-27 13:50 GMT+01:00 PICCA Frederic-Emmanuel <
frederic-emmanuel.picca at synchrotron-soleil.fr>:

> Hello,
>
> since the last time I think that I understand how to manage Ptr, now I
> woud like to masterize the ForeignPtr in order to let haskell managed the
> life of my C objects.
> So I try to create a Geoemtry object like this.
>
> -- data Geometry
> newtype Geometry = Geometry (Ptr Geometry) deriving (Show, Storable)
>
> newGeometry :: Factory -> ForeignPtr Geometry
> newGeometry f = unsafePerformIO $ do
>                   geometry <- c_hkl_factory_create_new_geometry f
>                   newForeignPtr c_hkl_geometry_free geometry
>
> foreign import ccall safe "hkl.h hkl_factory_create_new_geometry"
>   c_hkl_factory_create_new_geometry :: Factory
>                                     -> IO (Geometry)
>
> foreign import ccall safe "hkl.h &hkl_geometry_free"
>   c_hkl_geometry_free :: FunPtr (Geometry -> IO ())
>
>
>
> the C signature are
>
> HKLAPI HklGeometry *hkl_factory_create_new_geometry(const HklFactory
> *self) HKL_ARG_NONNULL(1);
>
> HKLAPI void hkl_geometry_free(HklGeometry *self) HKL_ARG_NONNULL(1);
>
>
> But when I try to compile this code, I get this error message
>
>
> 1 of 1] Compiling Hkl.C            ( src/Hkl/C.hs, dist/build/Hkl/C.o )
>
> src/Hkl/C.hs:51:33:
>     Couldn't match type ‘Geometry’ with ‘Ptr Geometry’
>     Expected type: GHC.ForeignPtr.FinalizerPtr Geometry
>       Actual type: FunPtr (Geometry -> IO ())
>     In the first argument of ‘newForeignPtr’, namely
>       ‘c_hkl_geometry_free’
>     In a stmt of a 'do' block:
>       newForeignPtr c_hkl_geometry_free geometry
>
> src/Hkl/C.hs:51:53:
>     Couldn't match expected type ‘Ptr Geometry’
>                 with actual type ‘Geometry’
>     In the second argument of ‘newForeignPtr’, namely ‘geometry’
>     In a stmt of a 'do' block:
>       newForeignPtr c_hkl_geometry_free geometry
>
> I do not understand what is wrong in my code
>
> thanks if you can help
>
> Frederic
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