> No, it's easy to get right in any specific case. It's iterating over a dang list, it's the first thing you learn how to do as a programmer. It's only hard to get right if you're trying to solve it for every use case with a single interface.

Isn't any correct implementation of any function that retains any only the elements that pass some predicate going to have to do basically the same thing that reject! does, or else be slow (creating a new list instead of mutating in place)?

I guess there's a somewhat simpler implementation that would work for some use cases (those in which order is not important) that just swaps in the last element instead of removing. But if maintaining order is important the complexity is similar.

I think that what would happen if Ruby didn't include reject! is that most programmers would just needlessly create new arrays all the time, since that's easier to write.