[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-4.8.0.0/docs/Foreign-Ptr.html#t:FunPtr
[2] https://hackage.haskell.org/package/base-4.8.0.0/docs/Foreign-Marshal-Utils.html#v:with
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?
>
>
> DEFINITION
>
> 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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