Monads.scala

// The main interface of a Monad.
trait Monad[M[_]] {
  def unit[A](a: A): M[A]
  def flatMap[A, B](fa: M[A])(f: A => M[B]): M[B]
}


// Maybe and its concrete 'implementations'.
sealed trait Maybe[+A]
case class Just[+A](a: A) extends Maybe[A]
case object Nothing extends Maybe[Nothing]


// Companion object, which implements the Monad interface.
object Maybe extends Monad[Maybe] {
  def unit[A](a: A): Maybe[A] = Just(a)
  def flatMap[A, B](fa: Maybe[A])(f: A => Maybe[B]): Maybe[B] = fa match {
    case Just(a) => f(a)
    case Nothing => Nothing
  }
}


object Monads {
  def main(args: Array[String]): Unit = {
    val before = List(Just(1), Just(2), Nothing, Just(4), Nothing, Just(6))
    val after  = before map { m => Maybe.flatMap(m) { x => Maybe.unit(x * 60) } }

    println(before) // output: List(Just(1), Just(2), Nothing, Just(4), Nothing, Just(6))
    println(after)  // output: List(Just(60), Just(120), Nothing, Just(240), Nothing, Just(360))
  }
}