[Haskell-cafe] how to call a callback in a haskell dll

My program creates a Windows dll. The calling program calls a function with strings as parameters. Works fine. I want to add a extra parameter. This extra parameter is a callback: a function Int -> Int: stdcall. In Haskell I want to call this function. So I have to convert the pointer to a (formal) Haskell function. In the examples for the FFI, Haskell provides the callback. I need an example or documentation. Kees