Reader Monad + Validation

foo.scala

import scalaz._
import Scalaz._

object Foo {
  type Result[A] = Validation[String,A]
  type Reader[A] = Int => Result[A]

  implicit val readerBind: Bind[Reader] = new Bind[Reader] {
    def map[A,B](fa: Reader[A])(f: A=>B) : Reader[B] = x => fa(x) map f
    def bind[A,B](fa: Reader[A])(f: A => Reader[B]) : Reader[B] = {
      x => 
        fa(x) match { 
          case Success(a) => f(a)(x)
          case Failure(f) => Failure(f)
        }
    }
  }
  implicit def resultMonad = Validation.validationMonad

  def add(x: Int): Reader[Int] = y => (x+y+3).success
  def str(x: Int): Reader[String] = y => (x*y).toString.success

  def composeThem(a: Int) : Reader[String] = add(a).flatMap(str)

  def main(args: Array[String]) = {
    println(composeThem(1)(2)) // should be (1 + 2 + 3) * 2 as a string
  }
}