[m-users.] Dealing with lists of enums as bitflags in C

[emacstheviking]

I have been hacking a good hour or two and not really made much progress
for something that I thought would have been simpler, but. Ignorance!!

In my SDL2 wrapper I have a list of types that correspond to the various
window creation flags, I am passing them as a list as that's the nicest way
of doing it I think,

sdl_createwindow("Test", 100, 100, 640, 480, [resizable, shown], !IO)

The types are declared as foreign enums in the usual way :-
:- type window_flag
    ---> fullscreen ; opengl ; shown ; resizable ..... (elided)

:- pragma foreign_enum("C", window_flag/0, [
    fullscreen          - "SDL_WINDOW_FULLSCREEN",
    opengl              - "SDL_WINDOW_OPENGL",
    shown               - "SDL_WINDOW_SHOWN",
    resizable           - "SDL_WINDOW_RESIZABLE",
   :...(elided)
]).

By using the --make-target option to poke at the generated code I can see
that it comes into the C code as MR_Word, not MR_Word*thought and then I
tracked down the definitions of the MR_is_list_empty macros etc and
realised pretty quickly that there is no "MR_get_next" as it should be as
simple as pointer arithmetic but so far it has escaped my best efforts. I
have seen that the array is composed as a static array of values from the
MR_TAG_COMMON but I lost the trail at that point. Here is my code:

:- pred sdl_createwindow(string::in, int::in, int::in, int::in, int::in,
list(window_flag)::in, io::di, io::uo) is det.
:- pragma foreign_proc("C",
    sdl_createwindow(Title::in, X::in, Y::in, W::in, H::in, Flags::in,
_IO0::di, _IO::uo),
    [promise_pure,will_not_call_mercury,does_not_affect_liveness],
"
Uint32 flags = 0;
if (Flags) {
    MR_Word* flagPtr = &Flags;
    while(!MR_list_is_empty(*flagPtr)) {
        Uint32 f = MR_list_head(*flagPtr);
        flags |= f;
        printf(""FLAGS: %u\\n"", flags);
        flagPtr++;
    }
}
SDL_Window* wnd = SDL_CreateWindow(Title, X, Y, W, H, flags);
").

I just want to iterate the list of types and OR together the final bit flag
uint32 value and get on with creating the window. This is something that is
going to crop up again and again in various parts of the library. I am
pretty sure I will figure out immediately hitting send.....

Thanks in advance,
Sean.
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