Scala -> Function

Question

Looking at the following Scala in Depth code, what does handle -> callback mean?

trait Observable {
  type Handle
  def observe(callback: this.type => Unit): Handle = {
    val handle = createHandle(callback)
    callbacks += (handle -> callback)
    handle

  [code omitted]
}

Answer 1

-> ultimately comes from scala.ArrowAssoc . scala.Predef also defines:

@inline implicit def any2ArrowAssoc[A](x: A): ArrowAssoc[A] = new ArrowAssoc(x)

which causes an implicit conversion of Any to an ArrowAssoc. This has the effect of adding ArrowAssoc's -> method to all objects (because all objects inherit from Any).

As to what -> does , it merely returns a Tuple2 of the two parameters. Or to put it another way, (handle -> callback) is more or less identical to (handle, callback) .

Although you've deleted the definition, callbacks is probably a mutable.Map object. += adds (or updates) an entry in said map associating handle to callback . So basically, this appears to be adding the newly-created handle and the callback to a map of callbacks.

Answer 2

Handle is an "abstract type". It must be defined in a concrete subclass.

callback is a function that takes a (this.type) and returns nothing (Unit).

The -> syntax is just alternate syntax for a tuple:

(3 -> "abcd") is the same as (3, "abcd")