You don't need to know category theory. You just need to understand a few things that came from it, like `Maybe`, which is the most obviously useful and trivially easy monad. Just forget about the fact that it's a monad and any general rules about monads and just learn how to use `Maybe`.

From there, input handling is just a state machine. Easy to draw on paper as a graph.

> Part of the benefits with modelling a program on paper should be to make communication easier. And to require people you communicate with to have knowledge in category theory to understand your design fells silly.

This is an odd complaint, because you can also say: requiring people you communicate with to have knowledge in (state machines | graphs | `if` statements | ...) to understand your design feel silly.