A very simple Writer monad written in Scala.

Writer.scala

class Writer[A](runWriter: (A, String)) {

	def apply(x: (A, String)): Writer[A] = new Writer[A](x)

	def run = runWriter

	def flatMap[B](x: A => Writer[B]): Writer[B] = {
		val (a: A, b: String) = runWriter
		val (y: B, z: String) = x(a).run
		new Writer[B]((y, b + z))
	}

	def map[B](f: A => B): Writer[B] = {
		val (a: A, b: String) = runWriter
		val (y: B, z: String) = f(a)
		new Writer[B]((y, z))
	}

}