Anonymous function as argument in scala

Question

In Play 2.3, the Action.async method has the signature

final def async(block: ⇒ Future[Result]): Action[AnyContent]

I did not figure out the meaning of => Future[Result], is it an anonymous function? Then shouldn't it be () => Future[Result]?

Answer 1

This is a call by name not by value as usual. It means, the argument, here block is of Type Future[Result] and it is lazy evaluated when needed not instantly on function call.

Answer 2

It is a "By-name parameter":

• Call by name vs call by value in Scala, clarification needed

• By-name parameter vs anonymous function