[m-users.] 64 bit subtraction, compiler sees 'int'

Hi,

I am getting this error:

agame.m:158: In clause for predicate `game_loop'/8:
agame.m:158:   in unification of variable `Diff'
agame.m:158:   and term `(Now - Then)':
agame.m:158:   type error in arguments of functor `-'/2.
agame.m:158:   Argument 1 (Now) has type `int64',
agame.m:158:   expected type was `int'.
agame.m:158:   Argument 2 (Then) has type `int64',
agame.m:158:   expected type was `int'.
** Error making `Mercury/cs/agame.c'.

I have this predicate, an FFI, to get the current 64 bit tick counter:

:- pred sdl_getticks64(int64::out, io::di, io::uo) is det.

:- pragma foreign_proc("C", sdl_getticks64(Ticks::out, _IO0::di, _IO::uo),
    [promise_pure,will_not_call_mercury,does_not_affect_liveness],
"
    Ticks = SDL_GetTicks64();
").

I assign it like this in the setup code

    sdl_getticks64(Now, !IO),
    game_loop(Window, Renderer, Now, Now, !GameState, !IO),


ad then in the game loop I calculate the difference but I don't see how I've failed to convey to the compiler the use of 64 bits!

        :- pred game_loop(sdl_window::in, sdl_renderer::in, int64::in, int64::in,
            game_state::in, game_state::out, io::di, io::uo) is det.

    157: game_loop(W, R, Now, Then, !State, !IO) :-
==> 158:    Diff = (Now-Then),
            io.format("Now: %li, Then: %li, Diff: %li\n",
                [ i64(Now), i64(Then), i64(Diff) ], !IO),


I can't see where it thinks the expected type was 'int'. Is there a different library I should use, I looked but couldn't really find anything in 'int' or 'math'.

Thanks,
Sean.

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