7.3.9.2 Inspecting the Request

The Hello World program above is a general greeter, responding to all URIs. To make a more exclusive greeter, we need to inspect the request object, and conditionally produce different results. So let’s load up the request, response, and URI modules, and do just that.

(use-modules (web server)) ; you probably did this already
(use-modules (web request)
             (web response)
             (web uri))

(define (request-path-components request)
  (split-and-decode-uri-path (uri-path (request-uri request))))

(define (hello-hacker-handler request body)
  (if (equal? (request-path-components request)
              '("hacker"))
      (values '((content-type . (text/plain)))
              "Hello hacker!")
      (not-found request)))

(run-server hello-hacker-handler)

Here we see that we have defined a helper to return the components of the URI path as a list of strings, and used that to check for a request to /hacker/. Then the success case is just as before – visit http://localhost:8080/hacker/ in your browser to check.

You should always match against URI path components as decoded by split-and-decode-uri-path. The above example will work for /hacker/, //hacker///, and /h%61ck%65r.

But we forgot to define not-found! If you are pasting these examples into a REPL, accessing any other URI in your web browser will drop your Guile console into the debugger:

<unnamed port>:38:7: In procedure module-lookup:
<unnamed port>:38:7: Unbound variable: not-found

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]> 

So let’s define the function, right there in the debugger. As you probably know, we’ll want to return a 404 response.

;; Paste this in your REPL
(define (not-found request)
  (values (build-response #:code 404)
          (string-append "Resource not found: "
                         (uri->string (request-uri request)))))

;; Now paste this to let the web server keep going:
,continue

Now if you access http://localhost/foo/, you get this error message. (Note that some popular web browsers won’t show server-generated 404 messages, showing their own instead, unless the 404 message body is long enough.)

This document was generated on February 3, 2012 using texi2html 5.0.