Temperature converter

tempconv.rkt

#lang racket/gui

(define (convert-c C)
  (number->string (+ 32 (* C (/ 9 5)))))

(define (convert-f F)
  (number->string (* (- F 32) (/ 5 9))))

(define (update-fahrenheit control event)
  (define tc (send text-celsius get-value))
  (if (eq? tc "")
      (begin
        (send text-fahrenheit set-value "")
        (send text-celsius set-field-background (make-object color% 255 255 255 1)))
      (begin
        (if (number? (string->number tc))
            (begin
              (send text-celsius set-field-background (make-object color% 255 255 255 1))
              (send text-fahrenheit set-value (convert-c (string->number tc))))  
            (begin
              (send text-celsius set-field-background (make-object color% 255 0 0 1))
              (send text-fahrenheit set-value ""))))))

(define (update-celsius control event)
  (define tf (send text-fahrenheit get-value))
  (if (eq? tf "")
      (begin
        (send text-celsius set-value "")
        (send text-fahrenheit set-field-background (make-object color% 255 255 255 1)))
      (begin
        (if (number? (string->number tf))
            (begin
              (send text-fahrenheit set-field-background (make-object color% 255 255 255 1))
              (send text-celsius set-value (convert-c (string->number tf))))  
            (begin
              (send text-fahrenheit set-field-background (make-object color% 255 0 0 1))
              (send text-celsius set-value ""))))))

(define frame (new frame%
                   [label "TempConv"]
                   [width 150]
                   [height 60]))

(define main-pane (new horizontal-pane%
                       [parent frame]
                       [alignment '(center center)]))

(define text-celsius (new text-field%
                          [label #f]
                          [parent main-pane]
                          [callback update-fahrenheit]))

(define label-celsius (new message%
                           [parent main-pane]
                           [label "Celsius ="]))

(define text-fahrenheit (new text-field%
                             [label #f]
                             [parent main-pane]
                             [callback update-celsius]))

(define label-fahrenheit (new message%
                              [parent main-pane]
                              [label "Fahrenheit"]))
 
; Show the frame by calling its show method
(send frame show #t)