Reader compose Option

ReaderOption.scala

case class ~=>[T, A](f: T => A) {
  def map[B](g: A => B): T ~=> B =
    ~=>(f andThen g)

  def flatMap[B](g: A => T ~=> B): T ~=> B =  
    ~=>(t => g(f(t)).f(t))
}

object ~=> {
  // unit: A => F[A]
  def constant[T, A](a: A): T ~=> A =
    ~=>(_ => a)
}

object T {
  import ~=>._
  // F[A] => (A => F[B]) => F[B]
  // let F = T ~=> Option[_]
  //                      (F           [A])=> (A => F           [B])=>F           [B]
  def flatMap2[T, A, B](x: T ~=> Option[A], f: A => T ~=> Option[B]): T ~=> Option[B] =
    for {
      optiona <- x
      optionb <- optiona match {
                   case None => ~=>.constant[T, Option[B]](None)
                   case Some(a) => f(a)
                 }
    } yield optionb
}