How Does One Make Scala Control Abstraction in Repeat Until?

Question

I am Peter Pilgrim. I watched Martin Odersky create a control abstraction in Scala. However I can not yet seem to repeat it inside IntelliJ IDEA 9. Is it the IDE?

Answer 1

You don't need the 2nd pair of braces, the usage should be:

repeatLoop (x) until (cond) //or...
repeatLoop {x} until {cond}

And not:

repeatLoop {x} { until(cond) } //EXTRA PAIR OF BRACES

The error means that Scala thinks you are trying to call a method with a signature something like:

def repeatLoop(x: => Unit)(something: X) //2 parameter lists

And can find no such method. It is saying "repeatLoop(body)" does not take parameters. A full code listing for the solution probably looks something a bit more like:

object Control0 {
  def repeatLoop(body: => Unit) = new Until(body)

  class Until(body: => Unit) {
    def until(cond: => Boolean) {
      body;
      val value: Boolean = cond;

      if (value) repeatLoop(body).until(cond)
    }
  }


  def main(args: Array[String]) {
    var y: Int = 1
    println("testing ... repeatUntil() control structure")
    repeatLoop {
      println("found y=" + y)
      y += 1
    }.until(y < 10)
  }
}

There are two useful observations to make here:

1. The solution is not tail-recursive and will result in a StackOverflowError for long iterations (try while (y < 10000))
2. The until seems the wrong way round to me (it would be more natural to stop when the condition becomes true, not carry on while it is true).