I'm surprised at the preference of GreenCard over H/Direct. H/Direct was designed as an evolution of GreenCard based on experience with it, and has a strong foundation, as described in ICFP '98.
Perhaps I should admit a certain bias: GreenCard is partly my creation so I have a soft spot for it... H/Direct would be the tool of choice if you happen to have IDL descriptions of the Mac graphical API. Given Microsoft's interest in the Mac, it is possible that these already exist. Lacking an IDL description, my tool of choice would be to use the FFI directly and use a suitable ffi library to take care of the tedium (and/or to use c2hs) GreenCard would probably never have come into existence if the current ffi specification had already existed. -- Alastair Reid