That's true. I think it also ties into best practices such as hexagonal architecture, DDD, and the "functional core, imperative shell" idea. You design the core of your program as data and pure functions, and then call that part from parts of the program that use IO. Maybe something like when scheme is used for teaching could work, with different "language levels", where some part disallow IO and reference cells. Or maybe a different file extension that's checked by the compiler to be pure. This could be a great way to teach functional programming but also software engineering.