Identity Monad

Identity.scala

package com.jamesmaggs.monads

trait Identity[A] { self =>

  def run: A

  def map[B](f: A => B): Identity[B] =
    new Identity[B] { def run = f(self.run) }

  def flatMap[B](f: A => Identity[B]): Identity[B] =
    new Identity[B] { def run = f(self.run).run }
}

object Identity {
  def apply[A](a: A): Identity[A] = new Identity[A] { def run = a }
}

IdentityTest.scala

package com.jamesmaggs.monads

import org.scalatest.FunSuite

class IdentityTest extends FunSuite {

  val f: Int => Identity[Int] = x => Identity(x + x)
  val g: Int => Identity[Int] = x => Identity(x * 2)

  val a = 7

  test("Left Identity") {
    assert(Identity(a).flatMap(f).run == f(a).run)
  }

  test("Right Identity") {
    assert(Identity(a).flatMap(Identity(_)).run == Identity(a).run)
  }

  test("Associativity") {
    assert(Identity(a).flatMap(f).flatMap(g).run == Identity(a).flatMap(i => f(i).flatMap(g)).run)
  }

}