We once wrote a text editor as an exercise in FP, the key handler function had to return the area of the screen that needed repainting but also proof that this

We once wrote a text editor as an exercise in FP, the key handler function had to return the area of the screen that needed repainting but also proof that this area was sufficient to catch all the changes, I think we were given some library to do the dirty work of actually drawing it to the screen so I don't know how nasty that was but our key handling code was quite simplistic!