;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-intermediate-lambda-reader.ss" "lang")((modname mini-dr) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))
(require htdp/gui)
(require "string.rkt")

;; An expr is either
;;  - num
;;  - (list '+ expr expr)
;;  - (list '- expr expr)

;; eval-expr : expr -> num
;(define (eval-expr x)
;  (cond
;    [(number? x) ... x ...]
;    [(symbol=? '+ (first x)) ... (eval-expr (second x)) ... (eval-expr (third x)) ...]
;    [(symbol=? '- (first x)) ... (eval-expr (second x)) .... (eval-expr (third x)) ...]))
(define (eval-expr x)
  (cond
    [(number? x) x]
    [(symbol=? '+ (first x)) (+ (eval-expr (second x)) (eval-expr (third x)))]
    [(symbol=? '- (first x)) (- (eval-expr (second x)) (eval-expr (third x)))]))

(check-expect (eval-expr 4) 4)
(check-expect (eval-expr '(- 9 8)) 1)
(check-expect (eval-expr '(+ (- 9 1) (- 8 2))) 14)

;; ----------------------------------------

(define (read-eval-print evt)
  (draw-message result-msg
                (to-string
                 (eval-expr
                  (from-string
                   (text-contents expr-field))))))

(define expr-field 
  (make-text "Expr:"))
(define eval-button
  (make-button "Evaluate" read-eval-print))
(define result-msg
  (make-message "__________________________"))

;; Like `big-bang' --- so don't include it in
;; handin!
(create-window (list (list expr-field)
                     (list eval-button)
                     (list result-msg)))