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:Fun... [2] https://hackage.haskell.org/package/base-4.8.0.0/docs/Foreign-Marshal-Utils.... 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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