Purpose of "return" statement in Scala?

Question

Is there any real reason of providing the return statement in Scala? (aside from being more "Java-friendly")

Answer 1

Ignoring nested functions, it is always possible to replace Scala calculations with returns with equivalent calculations without returns. This result goes back to the early days of "structured programming", and is called the structured program theorem, cleverly enough.

With nested functions, the situation changes. Scala allows you to place a "return" buried deep inside series of nested functions. When the return is executed, control jumps out of all of the nested functions, into the the innermost containing method, from which it returns (assuming the method is actually still executing, otherwise an exception is thrown). This sort of stack-unwinding could be done with exceptions, but can't be done via a mechanical restructuring of the computation (as is possible without nested functions).

The most common reason you actually would want to return from inside a nested function is to break out of an imperative for-comprehension or resource control block. (The body of an imperative for-comprehension gets translated to a nested function, even though it looks just like a statement.)

for(i<- 1 to bezillion; j <- i to bezillion+6){
if(expensiveCalculation(i, j)){
   return otherExpensiveCalculation(i, j)
}

withExpensiveResource(urlForExpensiveResource){ resource =>
// do a bunch of stuff
if(done) return
//do a bunch of other stuff
if(reallyDoneThisTime) return
//final batch of stuff
}

Answer 2

It is provided in order to accommodate those circumstances in which it is difficult or cumbersome to arrange all control flow paths to converge at the lexical end of the method.

While it is certainly true, as Dave Griffith says, that you can eliminate any use of return, it can often be more obfuscatory to do so than to simply cut execution short with an overt return.

Be aware, too, that return returns from methods, not function (literals) that may be defined within a method.

Answer 3

Here is an example

This method has lots of if-else statements to control flow, because there is no return (that is what I came with, you can use your imagination to extend it). I took this from a real life example and modified it to be a dummy code (in fact it is longer than this):

Without Return:

 def process(request: Request[RawBuffer]): Result = {
      if (condition1) {
        error()
      } else {
        val condition2 = doSomethingElse()
        if (!condition2) {
          error()
        } else {
          val reply = doAnotherThing()
          if (reply == null) {
            Logger.warn("Receipt is null. Send bad request")
            BadRequest("Coudln't receive receipt")
          } else {
            reply.hede = initializeHede()
            if (reply.hede.isGood) {
              success()
            } else {
              error()
            }
          }
        }
      }
  }

With Return:

  def process(request: Request[RawBuffer]): Result = {
      if (condition1) {
        return error()
      }

      val condition2 = doSomethingElse()
      if (!condition2) {
        return error()
      }

      val reply = doAnotherThing()

      if (reply == null) {
        Logger.warn("Receipt is null. Send bad request")
        return BadRequest("Coudln't receive receipt")
      }

      reply.hede = initializeHede()
      if (reply.hede.isGood)
        return success()

      return error()
  }

To my eyes, the second one is more readable and even manageable than the first one. The depth of indentation (with well formatted code) goes deep and deep if you don't use a return statement. And I don't like it :)