Not to mention idempotent code is trivially memoized, allowing you to skip computations you couldn't previously reason about. A function that produces a value i

Not to mention idempotent code is trivially memoized, allowing you to skip computations you couldn't previously reason about. A function that produces a value is always comparable to another value — a function that modifies some state requires that you have a way to observe that the world changed in order to skip doing it again.