EB2-EX3 Bonelli

#lang racket
(require eopl/eopl)

(define NaturalNumber? (lambda (x) (and (integer? x) (>= x 0))))

(define-datatype Number Number?
  (n (x NaturalNumber?))
  )

(define-datatype E E?
  (Num (datum Number?))
  (+Node (left E?) (right E?))
  (-Node (left E?) (right E?))
  (parenNode (contents E?))
  )

(define (Number->string num)
  (cases Number num
    (n (x) (number->string x))))

(define (E-string e)
  (cases E e
    (+Node (left right) (string-append (E-string left) " + " (E-string right)))
    (-Node (left right) (string-append (E-string left) " - " (E-string right)))
    (parenNode (contents) (string-append "(" (E-string contents) ")"))
    (Num (n) (Number->string n))))

;1 + (4 - 7)
(E-string (+Node (Num (n 1)) (parenNode (-Node (Num (n 4)) (Num (n 7))))))