Atom Feeds With CHICKEN


I do rarely eat out, but when I do, I visit Cologne’s nicest burger restaurant. They regularly come up with a new “Burger Of The Week”, but I dislike visiting their website over and over again. Later I did find out that it’s actually grabbing said weekly offer from their Facebook feed. No wonder it errors out for me once in a while, just to display an outdated image instead… This made for a perfect excuse to write some more CHICKEN, so here’s a quick walkthrough of the code.

First comes the scraping part. http-client, html-parser, uri-common and sxpath are used for fetching the sources, parsing them and selecting the relevant data. The only stumbling block besides learning XPath was making sense of Facebook’s HTML. For some reason I receive the pages inside a comment, therefore I need to find the most recent post involving the burger in a comment:

(define (fetch-fragment)
  (let* ((base-url "")
         (document (with-input-from-request base-url #f html->sxml))
         (comment-string ((sxpath "string(//comment()[contains(.,'BURGER')])")
         (comment (call-with-input-string comment-string html->sxml)))
    (car ((sxpath "//div[contains(@class,'userContentWrapper') and contains(string(),'BURGER')]")

Only then I can parse the comment again for its metadata:

(define (burger-metadata fragment)
  (let* ((timestamp ((sxpath "number(//abbr/@data-utime)") fragment))
         (description ((sxpath "string(//div[contains(@class,'userContent')]/p)")
         (image ((sxpath "string(//div[contains(@class,'uiScaledImageContainer')]/img/@src)")
         (base-url "")
         (relative-permalink ((sxpath "string(//a/abbr/../@href)") fragment))
         (path (uri-path (uri-reference relative-permalink)))
         (permalink (uri->string (update-uri (uri-reference base-url)
                                             path: path))))
    (list timestamp description image permalink)))

I did consider inventing my own format for storing the scraped data, but went with SQLite instead for simplicity’s sake. matchable is tremendously useful for destructuring. And sql-de-lite is a joy to use.

(define db (open-database "db.sqlite3"))

(define (init-database)
  (if (null? (schema db))
      (exec (sql db "CREATE TABLE burgers(id INTEGER PRIMARY KEY,
                                          date NUMBER,
                                          description TEXT,
                                          url TEXT,
                                          permalink TEXT)"))))

(define (update-database data)
  (match-let* (((date description url permalink) data)
               (latest (query fetch-value (sql db "SELECT date FROM burgers
                                                               WHERE date = ?;")
    (if (not latest)
        (exec (sql db "INSERT INTO burgers(date, description, url,permalink)
                              VALUES(?, ?, ?, ?);")
              date description url permalink))))

The functions presented are used in a straight-forward manner:

(define (main)
  (update-database (burger-metadata (fetch-fragment))))


This concludes the first part. All that’s left is actually serving the data for a feed reader. Besides reading out data from the SQLite database it’s necessary to turn it in an Atom feed with sxml-serializer, atom and spiffy. I use a few more eggs, including rfc3339, format and spiffy-uri-match. First of all, a few helpers:

(define db (open-database "db.sqlite3"))

(define news-items 10)

(define (fetch-latest)
  (query fetch-all
         (sql db "SELECT * FROM burgers ORDER BY date DESC LIMIT ?;")

(define (updated-at)
  (query fetch-value
         (sql db "SELECT date FROM burgers ORDER BY date DESC LIMIT 1;")))

(define (unix->datetime seconds)
  (rfc3339->string (seconds->rfc3339 seconds)))

(define (unix->date seconds)
  (let ((record (seconds->rfc3339 seconds)))
    (format "~4d-~2,'0d-~2,'0d"
            (rfc3339-year record)
            (rfc3339-month record)
            (rfc3339-day record))))

(define (feed-item url description)
     (p (img (@ (src ,url))))
     (p ,description))))

The feed is merely a serialization of the SXML as generated by the atom egg’s API:

(define (feed)
     title: (make-title "Fette Brause")
     id: ""
     updated: (unix->datetime (updated-at))
     authors: (list (make-author name: "Vasilij Schneidermann"))
     links: (list (make-link uri: ""))
     entries: (map (match-lambda
                    ((id date description url permalink)
                      id: permalink
                      title: (make-title (unix->date date))
                      updated: (unix->datetime date)
                      links: (list (make-link uri: permalink))
                      content: (make-content (feed-item url description)
                                             type: 'html))))

All that’s left now is actually serving the content:

(define (main)
   `((".*" .
        `(((/ "")
           (GET ,(lambda (c)
                    body: (feed)
                    headers: '((content-type "application/xml")))))))))))
  (server-bind-address "")
  (server-port 8001)
  (set-buffering-mode! (current-output-port) #:line)
  (access-log (current-output-port))


This makes for two moving parts, one being a binary that has to be run periodically by a scheduler (like, cron or systemd timers or whatever else floats your boat), the other one being a web server I just reverse proxy with nginx:

server {
    listen 80;
    listen [::]:80;

    location / {
        proxy_redirect off;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

Yes, it’s that simple. Full sources are on Github as usual.

Update: I wasn’t reading closely enough when it came to the ID of each atom feed item. It’s important to ensure it is sufficiently unique and never changes so that a feed reader can use it for identifying each news item. In this case the permalink is good enough, but for blogs made with Hyde one needs to make sure a format string with two placeholders for tag and date is used.