to-roman.rkt

#lang racket
 (require racket/match)
 (require racket/string)
 
 (define (to-roman x)
   (define (digit digits)
     (match digits
       [(list 1 x _ _) (list x)]
       [(list 2 x _ _) (list x x)]
       [(list 3 x _ _) (list x x x)]
       [(list 4 x y _) (list x y)]
       [(list 5 _ y _) (list y)]
       [(list 6 x y _) (list y x)]
       [(list 7 x y _) (list y x x)]
       [(list 8 x y _) (list y x x x)]
       [(list 9 x _ z) (list x z)]))
   (define (roman x) 
     (cond [(= x 0) '()]
           [(>= x 1000) (cons #\M (roman (- x 1000)))]
           [(>= x 100) (append (digit (list (quotient x 100) #\C #\D #\M))
                               (roman (remainder x 100)))]
           [(>= x 10) (append (digit (list (quotient x 10) #\X #\L #\C))
                              (roman (remainder x 10)))]
           [(>= x 1) (digit (list x #\I #\V #\X))]))
   (list->string (roman x)))