racket calculator

#lang racket/gui

(define-struct state (n1 n2 op))
(define my-state (make-state null null null))

(define (add-num s num)
  (cond
    [(null? (state-n1 s)) (set! my-state (make-state (string->number num) null null))]
    [else (set! my-state (make-state (state-n1 s) (string->number num) (state-op s)))]
    ))

(define (add-op s op)
  (cond
    [(not (null? (state-n1 s))) (set! my-state (make-state (state-n1 s) (state-n2 s) op))]))

(define (calculate)
  (if (and (not(null? (state-n1 my-state))) (not(null? (state-n2 my-state))) (not(null? (state-op my-state))))
  (send editor insert (number->string ((state-op my-state) (state-n1 my-state) (state-n2 my-state))))
  "error"))

(define frame (new frame%

                   ; Do not allow the frame to be resized.
                   [stretchable-width #f]
                   [stretchable-height #f]
                   [label "Calculator"]
                   [min-width 200]
                   [min-height 150]
                   ))
(define my-font (make-object font% 26 'modern))
(define text (new text-field%
                  [label ""]
                  [min-height 50]
                  [min-width 258]
                  [parent frame]
                  [font my-font]
                  [enabled #f]))

(define editor (send text get-editor))

(define (clear-text)
  (send editor delete 0 (string-length (send editor get-text))))



(send editor set-padding
      225 ;left padding
      0 ;top padding
      0 ;right padding
      0 ;bottom padding
      )


(define hcontainer (new horizontal-panel% [parent frame]))
(define num-buttons (new vertical-panel% [parent hcontainer]))
(define equal-clear (new vertical-panel% [parent hcontainer]))


(define third-row (new horizontal-panel% [parent num-buttons]))
(define second-row (new horizontal-panel% [parent num-buttons]))
(define first-row (new horizontal-panel% [parent num-buttons]))
(define zero-row (new horizontal-panel% [parent num-buttons]))


(define zero (new button%
                 [label "0"]
                 [parent zero-row]
                 [min-width 104]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "0"))]
                 ))


(define decimal (new button%
                 [label "."]
                 [parent zero-row]
                 [min-width 50]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "."))]
                 ))



(define one (new button%
                 [label "1"]
                 [parent first-row]
                 [min-width 50]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "1"))]))

(define two (new button%
                 [label "2"]
                 [parent first-row]
                 [min-width 50]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "2"))]
                 ))

(define three (new button%
                 [label "3"]
                 [parent first-row]
                 [min-width 50]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "3"))]
                 ))

(define four (new button%
                 [label "4"]
                 [parent second-row]
                 [min-width 50]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "4"))]
                 ))

(define five (new button%
                 [label "5"]
                 [parent second-row]
                 [min-width 50]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "5"))]
                 ))

(define six (new button%
                 [label "6"]
                 [parent second-row]
                 [min-width 50]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "6"))]
                 ))

(define seven (new button%
                 [label "7"]
                 [parent third-row]
                 [min-width 50]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "7"))]
                 ))

(define eight (new button%
                 [label "8"]
                 [parent third-row]
                 [min-width 50]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "8"))]
                 ))

(define nine (new button%
                 [label "9"]
                 [parent third-row]
                 [min-width 50]
                 [min-height 50]
                 [callback (lambda (b e) (send editor insert "9"))]
                 ))

(define plus (new button%
                  [label "+"]
                  [parent zero-row]
                  [min-width 50]
                  [min-height 50]
                  [callback (lambda (b e) (add-num my-state (send editor get-text))
                              (add-op my-state +)
                              (clear-text))]))

(define clear (new button%
                   [label "Clr"]
                   [parent equal-clear]
                   [min-width 50]
                   [min-height 104]
                   [callback (lambda (b e) (clear-text)
                               (set! my-state (make-state null null null)))]))

(define equals (new button%
                    [label "="]
                    [parent equal-clear]
                    [min-width 50]
                    [min-height 104]
                    [callback (lambda (b e) (add-num my-state (send editor get-text))
                                (clear-text)
                                (calculate))]))

(define minus (new button%
                   [label "-"]
                   [parent first-row]
                   [min-width 50]
                   [min-height 50]))
(define times (new button%
                   [label "*"]
                   [parent second-row]
                   [min-width 50]
                   [min-height 50]))

(define division (new button%
                      [label "/"]
                      [parent third-row]
                      [min-width 50]
                      [min-height 50]))

; Display the frame
(send frame show #t)