'Handmade' implementation of DO-WHILE loop in Scala

Scala is wery powerfool language. Almost every scala feature may be implemented using only built in language features. I want to demonstrate how to implementat a loop.

Scala already has do-while loop, that is why I'm using repeat-until naming in the example.

We want to implement something like this:

repeat {
  // do some stuff
} until (condition)

To achive this goal we have to declare a function repeat which will repeat some actions. The second part is the function until. It will have condition as an argument to contorol the loop. Finally we need to wire both the functions. To be able to do that we'll declare until function as a method inside the object returned by repeat function. Sounds complicated, let's take a look on the code:

def repeat(command: => Unit) = {
  new {
    def until(condition: => Boolean): Unit = {
      command
      if (condition)
        until(condition)
      else ()
    }
  }
}

As you can see we have anonymous object inside function body and one more functions inside. That means that the object will be returned by repeat so we can invoke until directly after repeat invocation. The function body itself is pretty simple. First we invoke command and then depending on condition we decide if we need to terminate or we need to make one more recursive call.

Let's see how it works on example:

var i = 0
repeat {
  println("Iteration #" + i)
  i = i + 1
} until (i < 5)

The output as you may expect will be:

Iteration #0
Iteration #1
Iteration #2
Iteration #3
Iteration #4

How to run the code

1. You need to have Scala SDK installed on your machine

2. Run Scala console

$ scala

3. Run the code

scala> :load loop.scala