Monads in Scala

monads.sc

trait M[T] {
  def flatMap[U](f: T => M[U]): M[U]
  def unit[T](x: T): M[T]
}

val l = List(1)
l.map(x => x * 2)
l.flatMap(x => List(x * 2))

// left unit
def f(x:Int): List[Int] = List(x * 2)

l flatMap f
f(1)

// right unit
l.flatMap(List(_))

// associativity
def g(x:Int): List[Int] = List(x * 3)

l flatMap f flatMap g
l flatMap f flatMap(x => g(x))
l flatMap(x => f(x)) flatMap g