Example of Kleisli composition of Validation

gistfile1.scala

object KleisliValidation extends App {
  import scalaz._
  import Scalaz._
  import scala.util.control.Exception._

  type EValidation[+T] = Validation[String, T]
  implicit val binding = new Bind[EValidation] {
    def map[A, B](fa: EValidation[A])(f: A => B): EValidation[B] = fa.map(f)
    def bind[A, B](fa: EValidation[A])(f: A => EValidation[B]): EValidation[B] = fa.flatMap(f)
  }
  
  def toDouble(s: String):EValidation[Double] = allCatch.either(s.toDouble).fold(_.toString.fail, _.success)
  def sqrt(d: Double):EValidation[Double] = if (d >= 0) math.sqrt(d).success else "sqrt(%s) is too complex for me".format(d).fail
  
  val composed = Kleisli(toDouble _) >==> (sqrt(_)) 
  
  println(composed("2"))
  println(composed("-2"))
  println(composed("hi"))
}

/*
Success(1.4142135623730951)
Failure(sqrt(-2.0) is too complex for me)
Failure(java.lang.NumberFormatException: For input string: "hi")
*/

BTW this requires scalaz 7

BTW this requires scalaz 7