while programming some Scala I realized the following:

Map[Int, Option[Int]]().updated(3, None).get(3)

returns

Some(None)

instead of the expected

None

This seems very counterintuitive. Is this expected behavior or is this a bug.

I'm on Scala 2.13.8 by the way.

CodePudding user response：

It's very much expected behavior. Option is a "data structure" like any other, so there's no special handling of it from the compiler's side. If you associate a None with a key in that map, then there's an entry there and Map will report it.

If you want the behavior you expected, then either:

1. Don't store values as Options, but instead use the plain Int type, or
2. flatten the result at the end: get(3).flatten.

A similar thing will happen if you put Options inside Options — there's no automagic flattening:

scala> Option(None)
val res0: Option[None.type] = Some(None)

scala> .flatten
val res1: Option[Nothing] = None

CodePudding user response：

Yes, it is expected behavior as you can clearly see from the signature of Map.get: def get(key: K): Option[V] .

In your case, V is Option[Int], so the result of get is Option[Option[Int]].

It is kinda unusual to have Option as value in a Map, in most cases, it is redundant, because you could just not have an entry in the map at all rather than storing None.

One use cases I can think of where it would be useful is implementing some kind of a cache: in that case, None would mean, the entry for the key does not exit, while missing key would indicate that it has not been checked.