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Scala example source code file (TailCalls.scala)

This example Scala source code file (TailCalls.scala) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Scala tags/keywords

a, a, call, call, done, done, tailcalls, tailrec, tailrec

The Scala TailCalls.scala source code

package scala.util.control

/** Methods exported by this object implement tail calls via trampolining.
 *  Tail calling methods have to return their result using `done` or call the next
 *  method using `tailcall`. Both return a `TailRec` object. The result of evaluating
 *  a tailcalling function can be retrieved from a `Tailrec` value using method result`.
 *  Here's a usage example:
 *  {{{
 *  import scala.util.control.TailCalls._
 *
 *  def isEven(xs: List[Int]): TailRec[Boolean] =
 *    if (xs.isEmpty) done(true) else tailcall(isOdd(xs.tail))
 *
 *  def isOdd(xs: List[Int]): TailRec[Boolean] =
 *   if (xs.isEmpty) done(false) else tailcall(isEven(xs.tail)) 
 *
 *  isEven((1 to 100000).toList).result
 *  }}}
 */
object TailCalls {

  /** This class represents a tailcalling computation.
   */
  abstract class TailRec[+A] {
    /** Returns the result of the tailcalling computation
     */
    def result: A = {
      def loop(body: TailRec[A]): A = body match {
        case Call(rest) => loop(rest())
        case Done(result) => result
      }
      loop(this)
    }
  }

  /** Internal class representing a tailcall */
  protected case class Call[A](rest: () => TailRec[A]) extends TailRec[A]

  /** Internal class representing the final result return from a tailcalling computation */
  protected case class Done[A](override val result: A) extends TailRec[A]

  /** Performs a tailcall
   *  @param rest  the expression to be evaluated in the tailcall
   *  @return a `TailRec` object representing the expression `rest`
   */
  def tailcall[A](rest: => TailRec[A]): TailRec[A] = new Call(() => rest)

  /** Used to return final result from tailcalling computation
   *  @param  `result` the result value
   *  @return a `TailRec` object representing a computation which immediately returns `result`
   */
  def done[A](result: A): TailRec[A] = new Done(result)
  
}


Other Scala examples (source code examples)

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