Menu Test

#lang racket

(define (prompt st)
  (display st)
  (flush-output)
  (read-line))

(define (nth n xs)
  (car (drop xs  n )))

(define (create-menu . args)
  (cons 'menu args))


(define (add-to-menu item menu)
  (cons 'menu (cons item (cdr menu))))

(define (runmenu menu)
  (let loop [(xs (cdr menu)) (i 1)]
    (if (null? xs)
      (list)
      (begin
        (display i)
        (display ". ")
        (display (cadar xs))
        (newline)
        (loop (cdr xs) (+ 1 i)))))
  (let [(selection (guard-get-num "> "))]
    (if (integer? selection)
      (if (and (<= selection (length menu)) (> selection 0))
        (car (nth selection menu))
        (begin
          (display "Invalid Selection\n")
          (runmenu menu)))
      (begin
        (display "Invalid Selection\n")
        (runmenu menu)))))

(define (guard-get-num st)
  (let [(n (string->number (prompt st)))]
    (if (not (number? n))
      (begin (display "Not a valid number.\n")
             (guard-get-num st))
      n)))

(define (gen-get-nums f)
  (λ () (f
           (guard-get-num "Number 1? ")
           (guard-get-num "Number 2? "))))

(define (quit)
  (display "Goodbye!\n")
  (exit))

((runmenu
   (create-menu
     (list (gen-get-nums +) "Add two numbers")
     (list (gen-get-nums *) "Multiply two numbers")
     (list (gen-get-nums /) "Divide two numbers")
     (list (gen-get-nums -) "Subtract two numbers")
     (list quit "Quit"))))