Java for example has Map.computeIfAbsent. This function returns a nullable value and does not need 2 layers of if absence to work. I would personally argue that exposing the 2 levels of absence is exposing an implementation details, but I'll accept this as a valid usage of double optionals.
computeIfAbsent doesn't need a second kind of absence because it can never return absence. That's fine if that API is a good fit for your use case, but it isn't always (there's a reason Map.get exists as well as Map.computeIfAbsent).
> I would personally argue that exposing the 2 levels of absence is exposing an implementation details
Probably you wouldn't want to return Option<Option<X>> (or Nullable) to outside callers - maybe you want to convert None to one kind of domain-meaningful error and Some(None) to a different kind of domain-meaningful error, maybe you want to take some different codepaths to respond to "recover" from the different kinds of absence.
But it's extremely valuable to be able to compose together existing libraries that might use absence to mean something and have them just do the right thing rather than always having to worry about the edge cases where one has a kind of absence that's subtly different from the other's kind of absence. I mean fundamentally you can't ever assume that a random third-party function in Java is safe to call with null, because many of them aren't. But you also can't ever assume that a random third-party function won't return null, because some of them do. So even to just compose two functions you've got to check their docs and think about the behaviour of this special value, and it's just all so avoidable.
