Scala Cookbook

3.18. Creating Your Own Control Structures

Problem

You want to define your own control structures to improve the Scala language, simplify your own code, or create a DSL for others to use.

Solution

The creators of the Scala language made a conscious decision not to implement some keywords in Scala, and instead implemented functionality through Scala libraries. This was demonstrated in Recipe 3.5, which showed that although the Scala language doesn’t have break and continue keywords, you can achieve the same functionality through library methods.

As a simple example of creating what appears to be a control structure, imagine for a moment that for some reason you don’t like the while loop and want to create your own whilst loop, which you can use like this:

package foo

import com.alvinalexander.controls.Whilst._

object WhilstDemo extends App {

  var i = 0
  whilst (i < 5) {
    println(i)
    i += 1
  }

}

To create your own whilst control structure, define a function named whilst that takes two parameter lists. The first parameter list handles the test condition—in this case, i < 5—and the second parameter list is the block of code the user wants to run.

You could implement this as a method that’s just a wrapper around the while operator:

// 1st attempt
def whilst(testCondition: => Boolean)(codeBlock: => Unit) {
  while (testCondition) {
    codeBlock
  }
}

But a more interesting approach is to implement the whilst method without calling while. This is shown in a complete object here:

package com.alvinalexander.controls ...