For idempotency you literally just want f(state) = f(f(state)). Whether you achieve this by just doing the same thing twice (no external effects) or doing the thing exactly once (if you do have side effects) is not important.
But if you have side effects and need something to happen exactly once it seems a lot more useful to communicate this, rather than pretending you did the thing.
For idempotency you literally just want f(state) = f(f(state)). Whether you achieve this by just doing the same thing twice (no external effects) or doing the thing exactly once (if you do have side effects) is not important.
But if you have side effects and need something to happen exactly once it seems a lot more useful to communicate this, rather than pretending you did the thing.