I really like Python's design here, if we are not talking about full on language features to enforce immutability, where functions that mutate never return a value and are named with verbs (e.g. 'sort()'), while functions that don't mutate return their value and are named with adjectives (e.g. 'sorted()'). This feels natural - mutations are actions, while pure functions are descriptions.

The only real downside is the lack of return values mean you can't chain mutations, but personally that never bothered me.

Coming from PHP, we’re used to it. Half the methods have $haystack, $needle, and the other half use them in the other order.