;; 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-advanced-reader.ss" "lang")((modname calc) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #t #t none #f ())))
(require htdp/gui)

(define (make-calculator)
  (local [(define TOTAL 0)
          
          (define WORKING 0)
          
          (define PREV-OP +)
          
          (define total-message
            (make-message (number->string TOTAL)))
          
          ;; op-button : string (num num -> num) -> button
          (define (op-button label OP)
            (make-button label
                         (lambda (evt)
                           (begin
                             (change-total PREV-OP WORKING)
                             (set! PREV-OP OP)
                             true))))
  
          ;; digit-button : num -> button
          (define (digit-button n)
            (make-button (number->string n)
                         (lambda (evt)
                           (add-digit n))))
          
          ;; add-digit : num -> true
          ;;  sets WORKING
          (define (add-digit n)
            (begin
              (set! WORKING (+ n (* WORKING 10)))
              (draw-message total-message (number->string WORKING))))
          
          ;; change-total : (num num -> num) num -> true
          ;;  Sets TOTAL and WORKING
          (define (change-total OP amt)
            (local [(define new-total (OP TOTAL amt))]
              (begin
                (set! TOTAL new-total)
                (set! WORKING 0)
                (draw-message total-message (number->string new-total)))))]
    
    (create-window (list (list total-message)
                         (map digit-button '(7 8 9))
                         (map digit-button '(4 5 6))
                         (map digit-button '(1 2 3))
                         (map digit-button   '(0))
                         (list (op-button "+" +)
                               (op-button "-" -)
                               (op-button "*" *)
                               (op-button "/" /))))))

(make-calculator)
(make-calculator)