What foreach(_ => ()) this mean in Scala?

Question

I found this foreach(_ => ()) in a code that uses Monix stream, I am not understanding the meaning. Can someone explain me this?

monix_concurrent_subject.foreach(_ => ())

Answer 1

As urmaul explained in the comments, it depends:

• If the datastructure is eager, it does nothing. Examples are List, Option etc.

• If the datastructure is lazy, it initialises its content. An example is a Stream.

Also as foreach returns nothing, it implies that somewhere will be a Side-Effect in the code before the foreach.

If you check the API (ConcurrentSubject.html#foreach) it states:

foreachL(cb: (O) ⇒ Unit): Task[Unit] Creates a new Task that will consume the source observable, executing the given callback for each element.