Scala Monad For-Comprehension

monad.scala

/** A for comprehension ready Monad */
case class Focomo[A](value: A) {
  self =>
  /** satisfies monadic binding of a function f that returns B */
  def map[B](f: A => B): Focomo[B] = { 
    println("map!"); 
    Focomo(f(value)) 
  }
  /** satisfies monadic binding of a function f that returns Focomo[B] */
  def flatMap[B](f: A => Focomo[B]): Focomo[B] = { 
    println("flatMap!"); 
    f(value) 
  }
  /** expect this to be called in a `Unit` for comprehension */
  def foreach[U](f: A => U): Unit = { 
    println("foreach!"); 
    f(value) 
  }
  /** for comprehension's `if` statements trigger this. 
   *  In a more useful monad, the result of the application 
   *  of a value to function f would determine the a special subclass
   *  of the monad or other either-or behavior.
   */
  def filter(f: A => Boolean): Focomo[A] = { 
    println("filter!"); 
    this
  }
  /** provides a delegate handler for calls to #withFilter */
  class WithFilter(p: A => Boolean) {
    println("with filter!")
    def map[B](f: A => B): Focomo[B] = self.filter(p).map(f)
    def flatMap[B](f: A => Focomo[B]): Focomo[B] = self.filter(p).flatMap(f)
    def foreach[U](f: A => U): Unit = self.filter(p).foreach(f)
    def withFilter(q: A => Boolean): WithFilter =
      new WithFilter(x => p(x) && q(x))
  }
  /** called with conditional statement in for comprehension */
  def withFilter(p: A => Boolean): WithFilter = new WithFilter(p)
}