what does this scala code do?

Question

I am still new to Scala. I found this code inside Spray framework.

I am trying to understand what the method signature means:

 /**
 * Completes the request using the given arguments.
 */
def complete: (⇒ ToResponseMarshallable) ⇒ StandardRoute = marshallable ⇒ new StandardRoute {
  def apply(ctx: RequestContext): Unit = ctx.complete(marshallable)
}

What does the => operator do here? I thought it was used for creating a closure? What is the assignment to marshallable being done here? why is it done outside the brackets?

A java version of this code would be nice to see.

Answer 1

So.

The first thing to notice is that complete is declared with a def, so it is a method, but the parentheses that look like they might be method arguments come after the colon :.

So they are not in fact method arguments. complete is a no-argument method!

Then what's up with the parens after the colon?

(⇒ ToResponseMarshallable) ⇒ StandardRoute is the return type of the no-arg method complete. The function complete returns a function that accepts an expression of type ToResponseMarshallable and returns a StandardRoute.

OK. But then what's up with the arrow before ToResponseMarshallable in the function signature (⇒ ToResponseMarshallable) ⇒ StandardRoute?

In a function signature, an ⇒ prior to the type name signifies a "call-by-name" argument. Following java, Scala by default evaluates all expressions in an argument list, and then evaluates the method body, substituting the evaluated values for the variable. But Scala also supports call-by-name semantics, in which an expression is substituted unevaluated into a function, and re-executed every time that it is named. complete returns a function whose single argument has call-by-name semantics.

Following the equals sign is just the implementation of this method. The expression beginning with marshallable ⇒ is an anonymous function, or function literal, or lambda (all names for basically the same thing). marshallable ⇒ "hello" would be a function that accepts (by name) a ToResponseMarshallable and returns the String "hello". But we need a method that accepts a ToResponseMarshallable (by name) and retuns a StandardRoute. So we have marshallable ⇒<expression that evaluates to a StandardRoute>

That <expression that evaluates to a StandardRoute> is just an anonymous inner class, an implementation of StandardRoute that overrides its apply method.

And we're done! We (hopefully) understand all the pieces, and what this declaration means.