(web server): Trying to define a predicate »request -included-in-nav-bar?«

[Dr. Ludwig Meier]

Hello everybody,

I would like to extend the example given in section 7.3.10.2 of the 
guile-manual. I am trying to write a web handler procedure which shall 
inspect whether or not a request can be found in a list that represents 
items of a navigation bar.

In order to understand what guile’s web module is actually doing I wrote 
this code:


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(use-modules (sxml simple)
             (web uri)
             (web request)
             (web response)
             (web server))

(define *navigation* '("start"
                       "gallery"))

(define *uri-1* (build-uri 'http
                         #:host "localhost"
                         #:port 8080
                         #:path "/start"))

(define *uri-2* (build-uri 'http
                         #:host "localhost"
                         #:port 8080
                         #:path "/gallery"))

(define *uri-foo* (build-uri 'http
                         #:host "localhost"
                         #:port 8080
                         #:path "/foo"))

(define *request-1* (build-request *uri-1*))
(define *request-2* (build-request *uri-2*))
(define *request-foo* (build-request *uri-foo*))

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

(define (request-included-in-nav-bar? request)
  (if (member (car (request-path-components request))
              *navigation*)
      #t #f))

(define (page request)
  (string->symbol
   (string-append "*" (car (request-path-components request)) "*")))

(define (respond-test page)
  (begin
    (display "Appearing in browser:")
    (newline)
    (display page)
    (newline)))

(define (not-found-test request)
  (begin
    (display "Appearing in browser:")
    (newline)
    (display "Page not found: ")
    (display (car (request-path-components request)))
    (newline)))

(define (handler-test request)
  (if (request-included-in-nav-bar? request)
      (respond-test (page request))
      (not-found-test request)))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


The definitons are intended to be a kind of a simulation of a web-server 
and work fine so far, e.g.:

------

scheme@(guile-user)> (request-included-in-nav-bar? *request-1*)
$2 = #t
scheme@(guile-user)> (request-included-in-nav-bar? *request-foo*)
$3 = #f
scheme@(guile-user)> (handler-test *request-2*)
Appearing in browser:
*gallery*
scheme@(guile-user)> (handler-test *request-foo*)
Appearing in browser:
Page not found: foo

-----

In the real world things seem to be different of course... Consider the 
following definitions to be used together with those above and the 
procedures »templatize«, »not-found« and »respond« as defined in section 
7.3.10.2/3:


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define *start*
  '((h1 "Homepage ...")))

(define *gallery*
  '((h1 "Gallery ...")))

(define (homepage request body)
  (if (request-included-in-nav-bar? request)
      (respond (page request))
      (not-found request)))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


Starting the web-server and pointing the browser to localhost:8080/start 
yields an error:

------

scheme@(guile-user)> (run-server homepage)
<unnamed port>:36:14: In procedure request-included-in-nav-bar?:
<unnamed port>:36:14: In procedure car: Wrong type argument in position 
1 (expecting pair): ()

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]> ,bt
In current input:
    142:6  1 (homepage #<<request> method: GET uri: #<<uri> scheme: 
http us…> …)
    36:14  0 (request-included-in-nav-bar? #<<request> method: GET uri: 
#<<u…>)
scheme@(guile-user) [1]> ,q

;;; WARNING (Error handling request wrong-type-arg (car Wrong type 
argument in position ~A (expecting ~A): ~S (1 pair ()) (())))
<unnamed port>:36:14: In procedure request-included-in-nav-bar?:
<unnamed port>:36:14: In procedure car: Wrong type argument in position 
1 (expecting pair): ()

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.

--------

While working fine above, my predicate »request-included-in-nav-bar?« 
now breaks: The expression (request-path-components request) seems to 
evaluate to anything but ("start").  What am I doing wrong?

Thank you very much in advance for your help.

--
Dr. Ludwig Meier