You can mix several tasks in a single threaded loop, e.g. by using idle functions (as I explained here).

So with a GUI event loop, when some external event happens (e.g. the user presses a button), the callback could add an idle function into the loop.

Then it is important that the idle function gives back the control to the event loop quite quickly (typically, in a few milliseconds). It could add itself again to the loop to continue some processing.

Of course, the concept of continuations is relevant, and you can see an idle-function re-adding itself as some hand-written continuation-passing style

See also this answer for more references.

You can mix several tasks in a single threaded loop, e.g. by using idle functions (as I explained here).

So with a GUI event loop, when some external event happens (e.g. the user presses a button), the callback could add an idle function into the loop.

Then it is important that the idle function gives back the control to the event loop quite quickly (typically, in a few milliseconds). It could add itself again to the loop to continue some processing.

Of course, the concept of continuations is relevant, and you can see an idle-function re-adding itself as some hand-written continuation-passing style

See also this answer for more references.