[throwawaymaths]

part of function coloring is "not being trivially resolvable". in this case the function coloring boundary is trivially resolvable.

    const pick_a_global_io = ...;

    fn needs_io(io:IO) void {...}

    fn doesnt_take_io() void {
       needs_io(pick_a_global_io);
    }

easy peasy. you've resolved the coloring boundary.

now, if you want to be a library writer, yeah, you have to color your functions if you don't want to be an asshole, but for the 95% use case this is not function coloring.