[Haskell-beginners] which typefor this FFI call

[Sylvain Henry]

On 26/06/2017 08:18, Sylvain Henry wrote:
> Hi,
>
> I would use IORef and StablePtr:
>
> import Foreign.StablePtr
> import Foreign.Ptr
> import Data.IORef
>
> type H5Iterate = Hid -> CString -> Ptr () -> Ptr () -> IO ()
>
> -- see 
> https://wiki.haskell.org/Foreign_Function_Interface#Function_pointers
> foreign import ccall "wrapper" mkOp :: H5Iterate -> FunPtr (H5Iterate)
>
> foreign import ccall "H5Lvisit" h5iterate :: Hid -> Index -> Order -> 
> FunPtr H5Iterate -> Ptr () -> IO ()
>
> main = do
>   state <- newIORef []
>   statePtr <- newStablePtr state
>
>   let
>     callback :: H5Iterate
>     callback hid name infoptr dataptr = do
>         -- get the state
>         stRef <- deRefStablePtr (castPtrToStablePtr dataptr)
>         st <- readIORef stRef
>         -- compute the new state
>         newSt <- ...
>         -- store the new state
>         writeIORef stRef newSt
>
>   h5iterate gid idttyp order (mkOp callback) (castStablePtrToPtr 
> statePtr)
>
Don't forget to: freeStablePtr statePtr

>   -- retrieve the final state
>   finalState <- readIORef state
>
>
> - Sylvain
>
>
> On 24/06/2017 15:47, PICCA Frederic-Emmanuel wrote:
>> Hello
>>
>> I would like to create a binding for this function [1].
>>
>> herr_t H5Literate( hid_t group_id, H5_index_t index_type, 
>> H5_iter_order_t order, hsize_t *idx, H5L_iterate_t op, void *op_data )
>>
>> My "only" problem is with the op and op_data part.
>> Here the C op signature[2]
>>
>> op ->  herr_t (*H5L_iterate_t)( hid_t g_id, const char *name, const 
>> H5L_info_t *info, void *op_data)
>>
>> What I would like to do is to create an Haskell function which allows 
>> to return what is contained under the op_data.
>> What I understand from this is that I give the H5Literate function an 
>> op function which is executed for each group of my hdf5 file.
>> So the op method is called with the releavant parameters.
>> The op_data can be use to accumulate things during the traversal.
>>
>> H5Literate :: Hid -> Index  -> Order -> Maybe HSize -> H5Iterate -> _ 
>> -> _
>>
>> type H5Iterate = Hid -> ByteString -> Info -> _ -> _
>>
>> So it seems to me that I need to use a State inorder to do the 
>> accumulation.
>>
>> For exemple,I want to collect each group names and accumulate then in 
>> a list.
>>
>> Somy questioniswhat should be the signature of both function.
>>
>> I have also the fealing that the return type MUST have a 
>> Storableinstance.
>>
>>
>> Thanks for your help
>>
>> Frederic
>>
>>
>> [1] https://support.hdfgroup.org/HDF5/doc/RM/RM_H5L.html#Link-Iterate
>> [2] https://support.hdfgroup.org/HDF5/doc/RM/RM_H5L.html#Link-Visit
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>
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