[Tainnor]

One benefit of describing state change as a pure data structure, instead of directly executing it, is that you may now write functions that run these state changes in different ways.

E.g. you may have a dry-run function that doesn't actually change the state but only describes what it would do. Or you could have a function that generates a verbose log of all steps executed. Etc.

That doesn't necessarily work for IO, as that gets special treatment by the runtime, but you can do it with your own types.

[anchpop]

Yeah! That's why I'm excited for the effect system to land in OCaml. In general I think effect systems are more user friendly than monads and it makes the choice of how to handle the effects more explicit.