Memoisation with state using Scala

FibMemo1.scala

// Fixes memorization bug from code in 
// http://blog.tmorris.net/posts/memoisation-with-state-using-scala/index.html
// Also adds little debugging tool to allow one to see how state evolves.
// returns the memorisation map, to help see that it has been altered.
object FibMemo1 {
  type Memo = Map[BigInt, BigInt]

  def fibmemo1(n: BigInt, debug: Boolean = false): (BigInt,Memo) = {
    def fibmemoR(z: BigInt, memo: Memo): (BigInt, Memo) =
      if(z <= 1)
        (z, memo)
      else memo get z match {
        case None => {
          val (r, memo0) = fibmemoR(z - 1, memo)
          val (s, memo1) = fibmemoR(z - 2, memo0)
          val res = r+s
          if (debug) println(z+"->"+res)
          (res, memo1 + Pair(z,res)) //<- the 
        }
        case Some(v) => (v, memo)
      }

    fibmemoR(n, Map())
  }
}