#lang scheme/base
(require web-server/private/request-structs
         net/url)
(provide start)

(define (start req)
  (let* ([uri (request-uri req)]
         [qry (url-query uri)])
    (cond
      [(assoc 'second qry)
       => (lambda (a-pair)
            `(html (head (title "Answer Page"))
                   (body
                    (h1 "Answer Page")
                    (p ,(format "The answer is: ~a"
                                (+ (string->number (cdr a-pair))
                                   (string->number (cdr (assoc 'first qry)))))))))]
      [(assoc 'first qry)
       => (lambda (a-pair)
            `(html (head (title "Second Page"))
                   (body
                    (h1 "Second Page")
                    (form ([action ,(url->string uri)]
                           [method "get"] [enctype "application/x-www-form-urlencoded"])
                          "Enter the second number to add: "
                          (input ([type "hidden"] [name "first"] [value ,(cdr a-pair)]))
                          (input ([type "text"] [name "second"] [value ""]))
                          (input ([type "submit"] [name "enter"] [value "Enter"]))))))]
      [else
       `(html (head (title "Hello"))
              (body
               (h1 "Hello World!")
               (form ([action ,(url->string uri)]
                      [method "get"] [enctype "application/x-www-form-urlencoded"])
                     "Enter the first number to add: "
                     (input ([type "text"] [name "first"] [value ""]))
                     (input ([type "submit"] [name "enter"] [value "Enter"])))))])))