how scala folding works?

Question

I am trying to understand the following code but can't.

it is supposed to create a child actor for an Event if it does not exist, otherwise says that the Event exist as it as an associated child actor.

context.child(name).fold(create())(_ => sender() ! EventExists)

But the fold here does not make sense to me. If the context.child is emtpty we get the creation and i understand that. However if there is children we are still going to create why ?

Answer 1

Akka's child returns an Option

As you can see from Option's scaladoc:

fold[B](ifEmpty: ⇒ B)(f: (A) ⇒ B): B Returns the result of applying f to this scala.Option's value if the scala.Option is nonempty. Otherwise, evaluates expression ifEmpty.

Or making it more clear:

This (fold) is equivalent to scala.Option map f getOrElse ifEmpty.

So the first parameter of fold is lazy (call-by-name) and evaluates only if Option is empty. The second parameter (a function) is called only if Option is not empty.

Experiment:

scala> Some(0).fold({println("1");1}){_ => println("2"); 2}
2
res0: Int = 2

scala> None.fold({println("1");1}){_ => println("2"); 2}
1
res1: Int = 1

Here's some readings about:

https://kwangyulseo.com/2014/05/21/scala-option-fold-vs-option-mapgetorelse/

And some critics of that approach:

http://www.nurkiewicz.com/2014/06/optionfold-considered-unreadable.html

But in Option.fold() the contract is different: folding function takes just one parameter rather than two. If you read my previous article about folds you know that reducing function always takes two parameters: current element and accumulated value (initial value during first iteration). But Option.fold() takes just one parameter: current Option value! This breaks the consistency, especially when realizing Option.foldLeft() and Option.foldRight() have correct contract (but it doesn't mean they are more readable).