Is it possible to specify an anonymous function's return type, in Scala?

问题

I know you can create an anonymous function, and have the compiler infer its return type:

val x = () => { System.currentTimeMillis }

Just for static typing's sake, is it possible to specify its return type as well? I think it would make things a lot clearer.

回答1:

In my opinion if you're trying to make things more clear it is better to document the expectation on the identifier x by adding a type annotation there rather than the result of the function.

val x: () => Long = () => System.currentTimeMillis

Then the compiler will ensure that the function on the right hand side meets that expectation.

回答2:

val x = () => { System.currentTimeMillis } : Long

回答3:

Fabian gave the straightforward way, but some other ways if you like micromanaging sugar include:

val x = new (() => Long) {
  def apply() = System.currentTimeMillis
}

or

val x = new Function0[Long] {
  def apply() = System.currentTimeMillis
}

or even

val x = new {
  def apply(): Long = System.currentTimeMillis
}

since in most situations it makes no difference if it descends from Function, only whether it has an apply.

来源：https://stackoverflow.com/questions/2088524/is-it-possible-to-specify-an-anonymous-functions-return-type-in-scala