Example of Kleisli composition of Either

gistfile1.scala

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

  type EEither[+T] = Either[String, T]
  
  def toDouble(s: String): EEither[Double] = allCatch.either(s.toDouble).fold(_.toString.left, _.right)
  def sqrt(d: Double): EEither[Double] = if (d >= 0) math.sqrt(d).right else "sqrt(%s) is too complex for me".format(d).left
  
  val composed = Kleisli(toDouble _) >==> sqrt
  
  println(composed("2"))
  println(composed("-2"))
  println(composed("hi"))
}

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