[m-users.] IO argument clobbering

Hi,

I am trying to pass a predicate to a rendering loop so the loop can manage the system but call back each frame to get the application to render its state.

I have this as the callback definition:


:- type callback == (pred(world, world, io, io)).
:- inst callback == (pred(in, out, di, uo) is det).

:- pred run_loop(
    callback::in, world::in, world::out, io::di, io::uo
) is det.

My call site code looks like this, nothing spectacular:

:- pred app_loop(world::in, world::out, io::di, io::uo) is det.

app_loop(!W, !IO) :-
 % do stuff
 .

The game loop set up and call is this:

    mk_world(60, World, !IO),
    run_loop(app_loop, World, _, !IO),
    close_window(!IO).

The idea was that, at that point, the 'control' of the application is managed by my emerging framework, dare I call it that, and I wanted to be able to pass in various callbacks, starting with basic frame rendering, the implementation code is this:

run_loop(AppHandler, !W, !IO) :-
    window_should_close(X, !IO),
    (
        X = yes,
        trace_info("Stopped by user", [], !IO)
    ;
        X = no,
           :
           :
        %%%%%%%%%%
        % RENDER %
        %%%%%%%%%%
        begin_drawing(!IO),
193:        AppHandler(!W, !IO),
194:    end_drawing(!IO),
195:
196:    run_loop(AppHandler, !W, !IO)
    ).

The compiler error centres around line 196, which I wasn't expecting, of course!

ui.m:196: In clause for `run_loop(in, in, out, di, uo)':
ui.m:196:   mode error in conjunction. The next 3 error messages indicate
ui.m:196:   possible causes of this error.
ui.m:196:   
ui.m:193:   In clause for `run_loop(in, in, out, di, uo)':
ui.m:193:   in argument 1 (i.e. the predicate term) of higher-order predicate
ui.m:193:   call:
ui.m:193:   mode error: variable `AppHandler' has instantiatedness `ground',
ui.m:193:   expecting higher-order pred inst of arity 4.
ui.m:196:   
ui.m:194:   In clause for `run_loop(in, in, out, di, uo)':
ui.m:194:   in argument 1 of call to predicate `raylib.end_drawing'/2:
ui.m:194:   unique-mode error: the called procedure would clobber its argument,
ui.m:194:   but variable `STATE_VARIABLE_IO_29' is still live.
ui.m:196:   
ui.m:196:   In clause for `run_loop(in, in, out, di, uo)':
ui.m:196:   in argument 4 of call to predicate `ui.run_loop'/5:
ui.m:196:   unique-mode error: the called procedure would clobber its argument,
ui.m:196:   but variable `STATE_VARIABLE_IO_30' is still live.


I don't understand what it means by 'still live'. I am threading the !IO world state through this run loop, passing it into the callback code, but on the tail-back into the next iteration, well, I don't understand what the actual issue is.

Seeking explanation!

Thanks again,.
Sean.

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