Kata Roman Numerals en Scheme Racket idiomático (funcional)

num-roman.rkt

;; -*- coding: utf-8; mode: scheme -*-
;; $Id: num-roman.rkt,v 1.14 2012/02/16 04:02:35 alcides_fp Exp $
#lang racket

(provide numeral)

(define (numeral digito posicion)
  (let ((matriz-numerales
         #(#("" "I" "II" "III" "IV" "V" "VI" "VII" "VIII" "IX")
           #("" "X" "XX" "XXX" "XL" "L" "LX" "LXX" "LXXX" "XC")
           #("" "C" "CC" "CCC" "CD" "D" "DC" "DCC" "DCCC" "CM")
           #("" "M" "MM" "MMM"))))
    (vector-ref (vector-ref matriz-numerales posicion) digito) ) )
;;------------------------------------------------------------------------
(provide dec->list)

(define (dec->list numero)
  "Obtiene lista de cifras de un número base 10 en orden inverso"
  (let loop ([cantidad numero][retval '()])
    (if (= cantidad 0) retval
        (loop (quotient cantidad 10)
              (append retval (list (remainder cantidad 10))) ) ) ) )
;;------------------------------------------------------------------------
(provide num-roman)

(define (num-roman arabigo)
  (let ([cifras (dec->list arabigo)])
    (for/fold ([retval ""])
              (((digito posicion)(in-indexed cifras)))
      (string-append (numeral digito posicion) retval) ) ) )
;;------------------------------------------------------------------------