> On the other hand, you could make my function append to a string and then return the string. Then it wouldn't have side effects
State mutation is a side effect. If you mean it builds it up locally, it would then not have nonlocal side effects (e.g., it would have a pure functional interface even though it has an imperative implementation). But to do that, you'd have construction and mutation overhead in the code, whereas with functional idioms / comprehensions, you would not.
This is especially the case when (as is often the case in practice) you are taking collection datastructures and producing collection structures to work with.
State mutation is a side effect. If you mean it builds it up locally, it would then not have nonlocal side effects (e.g., it would have a pure functional interface even though it has an imperative implementation). But to do that, you'd have construction and mutation overhead in the code, whereas with functional idioms / comprehensions, you would not.
This is especially the case when (as is often the case in practice) you are taking collection datastructures and producing collection structures to work with.