[Haskell-cafe] FFI Ptr Question

Aldo Davide aldodavide at gmx.com
Sun Apr 5 15:12:16 UTC 2015

Sorry but I am having trouble understanding your post, but here's some information you might find useful:

* If you want to pass a callback to a C function, you need to use a FunPtr. In particular, you need to create a "wrapper" foreign import that will allow you to convert a haskell function to a FunPtr that can be then passed to C and called by C. The docs on FunPtr [1] explain how to do this.

* Ptr is not a data constructor, its a type constructor, so e.g. `Ptr CInt` is a valid type, but `Ptr 4` is not a valid expression. If you want to store a value in the heap and then create a pointer to it, you can use the `with` function [2]. So e.g. in `with 5 $ \p -> ...`, p is a `Ptr CInt`. Remember that `Ptr`s are to useful for passing callbacks though.

[1] https://hackage.haskell.org/package/base-
[2] https://hackage.haskell.org/package/base-

Michael Jones, wrote:
> I am having trouble figuring out how to pass callbacks to C.
> Given the definition below it is not clear how to define a function and pass it. I tried:
> getNumberOfRows :: CInt 
> getNumberOfRows = 1
>   table <- wxcGridTableCreate getNumberOfRows…
> but the compiler barks at me:
> src/MainGui.hs:577:32: Not in scope: data constructor ‘Ptr’
> even through I do the imports:
> import Foreign
> import Foreign.C
> There is also a _obj as the first value of wxcGridTableCreate and the callbacks, and I am not sure how to make a this* for it. I would have throughout that wxcGridTableCreate would return a Ptr.
> The goal is to pass haskell functions into wxcGridTableCreate so the grid can call back to haskell.
> Can someone show an example of just this one function, how to wrap a Ptr around it, and what to do with _obj?
> wxcGridTableCreate :: Ptr  a -> Ptr  b -> Ptr  c -> Ptr  d -> Ptr  e -> Ptr  f -> Ptr  g -> Ptr  h -> Ptr  i -> Ptr  j -> Ptr  k -> Ptr  l -> Ptr  m -> Ptr  n -> Ptr  o -> Ptr  p -> Ptr  q ->  IO (WXCGridTable  ())
> wxcGridTableCreate _obj _EifGetNumberRows _EifGetNumberCols _EifGetValue _EifSetValue _EifIsEmptyCell _EifClear _EifInsertRows _EifAppendRows _EifDeleteRows _EifInsertCols _EifAppendCols _EifDeleteCols _EifSetRowLabelValue _EifSetColLabelValue _EifGetRowLabelValue _EifGetColLabelValue 
>   = withObjectResult $
>     wx_ELJGridTable_Create _obj  _EifGetNumberRows  _EifGetNumberCols  _EifGetValue  _EifSetValue  _EifIsEmptyCell  _EifClear  
>     _EifInsertRows  _EifAppendRows  _EifDeleteRows  _EifInsertCols  _EifAppendCols  _EifDeleteCols  
>     _EifSetRowLabelValue  _EifSetColLabelValue  _EifGetRowLabelValue  _EifGetColLabelValue  
> foreign import ccall "ELJGridTable_Create" wx_ELJGridTable_Create :: Ptr  a -> Ptr  b -> Ptr  c -> Ptr  d -> Ptr  e -> Ptr  f -> Ptr  g -> Ptr  h -> Ptr  i -> Ptr  j -> Ptr  k -> Ptr  l -> Ptr  m -> Ptr  n -> Ptr  o -> Ptr  p -> Ptr  q -> IO (Ptr (TWXCGridTable ()))
> typedef int   _cdecl (*TGridGetInt)(void* _obj);
> typedef int   _cdecl (*TGridIsEmpty)(void* _obj, int row, int col);
> typedef void* _cdecl (*TGridGetValue)(void* _obj, int row, int col);
> typedef void  _cdecl (*TGridSetValue)(void* _obj, int row, int col, void* val);
> typedef void  _cdecl (*TGridClear)(void* _obj);
> typedef int   _cdecl (*TGridModify)(void* _obj, int pos, int num);
> typedef int   _cdecl (*TGridMultiModify)(void* _obj, int num);
> typedef void  _cdecl (*TGridSetLabel)(void* _obj, int idx, void* val);
> typedef void* _cdecl (*TGridGetLabel)(void* _obj, int idx);
> EWXWEXPORT(void*,ELJGridTable_Create)(void* self,void* _EifGetNumberRows,void* _EifGetNumberCols,void* _EifGetValue,void* _EifSetValue,void* _EifIsEmptyCell,void* _EifClear,void* _EifInsertRows,void* _EifAppendRows,void* _EifDeleteRows,void* _EifInsertCols,void* _EifAppendCols,void* _EifDeleteCols,void* _EifSetRowLabelValue,void* _EifSetColLabelValue,void* _EifGetRowLabelValue,void* _EifGetColLabelValue)
> {
> 	return (void*)new ELJGridTable (self,
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