You don’t even have to ignore io: input is a coeffect which is passed to your otherwise pure function as an argument and output is directly derived from your returned works state. That is, yes there’s some side effects happening before and after each iteration but it can be relatively cleanly separated. In my toy engine, I do this: input is fed in and rendering is driven by the output. My game logic isn’t a pure function (it’s a task graph that reads and writes from event queues and internally maintained task state, but input is fed into the start of the task graph and rendering is driven by tasks at the end), but it conceptually could be as all the statefulness is encapsulated and internal.