Cont.scala

package cont

import scala.util.Try

abstract class Cont[A] {
  def run[R](cont: A => R): R
  def map[B](fn: A => B): Cont[B] = {
    val self = this
    new Cont[B] {def run[R](cont: B => R) = self.run(cont compose fn)}
  }
  def flatMap[B](fn: A => Cont[B]): Cont[B] = {
    val self = this
    new Cont[B] {def run[R](cont: B => R) = self.run(a => fn(a).run(cont))}
  }
  def get = run(identity)
}

object Constraint {
  def choose[A](as: A*):Cont[A] = new Cont[A] {
    override def run[R](cont: A => R): R = {
      doChoose(cont, as.toList)
    }

    def doChoose[R](cont: A => R, values: List[A]):R = {
      if(values.isEmpty) throw new RuntimeException("No choice is valid")
      Try(cont(values.head)).getOrElse(doChoose(cont, values.tail))
    }
  }
  def check(bool: => Boolean): Unit = {
    if(!bool) throw new RuntimeException("Constraint fails")
  }
}


object PythagoreanTriples {
  import Constraint._
  def main(args: Array[String]) = {
    println((for(
      a <- choose(1,2,3,4,5);
      b <- choose(1,2,3,4,5);
      c <- choose(1,2,3,4,5)
    ) yield {
      check(a > b)
      check((a*a + b*b) == c*c)
      (a,b,c)
    }).get)
  }
}