[themulticaster]

Your example isn't entirely clear to me - do you mean that “I step forward” and “I try to shoot it” are instructions in sequence? What exactly do you mean with allowing the input to depend on the execution of the mutation instructions?

In any case, what I'm going for is that in both imperative and functional programming languages I would have a function/method like "handleInput" that takes an input and decides what to do with it. The difference would be that in a classical OOP setting handleInput would be a method of your GameState class [1] while in FP handleInput would look something like "Input -> (GameState -> GameState)", i.e. a function that takes an input value and returns a function that transforms the game state in some way (alternatively and equivalently, thanks to associativity/currying: a function that takes an input value and a current game state and returns a new game state).

[1] I know, this obviously is a very contrived example, it'd only work for very simple games. Game programming patterns are interesting but not the focus here.