scala – 在翻译中使用单子的要点是什么？

object simpleInterpreter {

  case class M[A](value: A) {
    def bind[B](k: A => M[B]): M[B] =  k(value)
    def map[B](f: A => B): M[B] =  bind(x => unitM(f(x)))
    def flatMap[B](f: A => M[B]): M[B] = bind(f)
  }

  def unitM[A](a: A): M[A] = M(a)

  def showM(m: M[Value]): String = m.value.toString();

  type Name = String

  trait Term;
  case class Var(x: Name) extends Term
  case class Con(n: int) extends Term
  case class Add(l: Term,r: Term) extends Term
  case class Lam(x: Name,body: Term) extends Term
  case class App(fun: Term,arg: Term) extends Term

  trait Value
  case object Wrong extends Value {
   override def toString() = "wrong"
  } 
  case class Num(n: int) extends Value {
    override def toString() = n.toString()
  }
  case class Fun(f: Value => M[Value]) extends Value {
    override def toString() = "<function>"
  }

  type Environment = List[Pair[Name,Value]]

  def lookup(x: Name,e: Environment): M[Value] = e match {
    case List() => unitM(Wrong)
    case Pair(y,b) :: e1 => if (x == y) unitM(b) else lookup(x,e1)
  }

  def add(a: Value,b: Value): M[Value] = Pair(a,b) match {
    case Pair(Num(m),Num(n)) => unitM(Num(m + n))
    case _ => unitM(Wrong)
  }

  def apply(a: Value,b: Value): M[Value] = a match {
    case Fun(k) => k(b)
    case _ => unitM(Wrong)
  }

  def interp(t: Term,e: Environment): M[Value] = t match {
    case Var(x) => lookup(x,e)
    case Con(n) => unitM(Num(n))
    case Add(l,r) => for (val a <- interp(l,e);
               val b <- interp(r,e);
               val c <- add(a,b))
                      yield c
    case Lam(x,t) => unitM(Fun(a => interp(t,Pair(x,a) :: e)))
    case App(f,t) => for (val a <- interp(f,e);
               val b <- interp(t,e);
               val c <- apply(a,b))
              yield c
  }

  def test(t: Term): String = 
    showM(interp(t,List()))

  val term0 = App(Lam("x",Add(Var("x"),Var("x"))),Add(Con(10),Con(11)))
  val term1 = App(Con(1),Con(2))

  def main(args: Array[String]) {
    println(test(term0))
    println(test(term1))
  }
}

这里的单体计算有什么用途/优点？事实上,M只不过是一个身份的monad.这仅仅是为了给出一个单一语法的例子,还是有一个重要的作用？