Roman.scala

import scalaz.std.stream._
import scalaz.std.tuple._
import scalaz.syntax.bifunctor._

def romanize(number: Int) = {
  val numerals = Seq( ( "M", 1000), ("CM", 900)
                    , ( "D", 500) , ("CD", 400)
                    , ( "C", 100) , ("XC", 90)
                    , ( "L", 50)  , ("XL", 40)
                    , ( "X", 10)  , ("IX", 9)
                    , ( "V", 5)   , ("IV", 4)
                    , ( "I", 1)
                    )

  def next(in: Int) = numerals find(_._2 <= in) map (_.rightMap(in - _))
  unfold(number)(next).mkString("")
}

assert(romanize(422) == "CDXXII")