Wrapping a DLL import in a module in F#

Question

I'm attempting to create a "wrapper" module for some windows api functions from user32.dll. I'm still learning F# so I'm rather fuzzy on how inheritance and polymorphism works in F# and how to apply that to this situation.

I have this module:

module MouseControl =         
    [<DllImport( "user32.dll", CallingConvention = CallingConvention.Cdecl )>]
    extern void ShowCursor(bool show)

    [<DllImport( "user32.dll", CallingConvention = CallingConvention.Cdecl )>]
    extern void internal mouse_event(int flags, int dX, int dY, int buttons, int extraInfo)
    let MouseEvent(flags, dX, dY, buttons, extraInfo) = mouse_event(flags, dX, dY, buttons, extraInfo)

My goal is to be able to "hide" the mouse_event function from other code that uses this module and instead expose that function as MouseEvent. With this code both mouse_event and MouseEvent are currently available to code that calls this module. How do I hide mouse_event where it's private to the module?

Answer 1

In your code sample, you already marked the mouse_event function as internal - so in principle, you should just need to mark it as private. However, it looks that F# compiler is ignoring visibility annotations on extern members, so the easiest option is to put them in a nested module and hide the whole nested module:

module MouseControl =         
  module private Imported = 
    [<DllImport( "user32.dll", CallingConvention = CallingConvention.Cdecl )>]
    extern void mouse_event(int flags, int dX, int dY, int buttons, int extraInfo)

  let MouseEvent(flags, dX, dY, buttons, extraInfo) = 
    Imported.mouse_event(flags, dX, dY, buttons, extraInfo)

The module Imported is now visible only inside the MouseControl module. From the outside, you cannot access anything inside MouseControl.Imported.