How to access url query parameters in Clack, Lucerne or Caveman

edit: I found Snooze (by Sly and Yasnippet’s author) easier and cleaner in this regard. It also has built-in settings to choose where to catch errors: with Slime’s debugger, with a full stacktrace in the browser or displaying a custom error page.

If you’re using Lucerne don’t search more like I did, its with-params macro works with url query parameters (as well as POST parameters).

If you’re accessing the url hello?search=kw, this works:

@route app "/hello"
(defview index (name)
  (with-params (search)
    (render-template (+index+)
                     :search search)))

An illustration with a POST parameter from the “utweet” example:

@route app (:post "/tweet")
(defview tweet ()
  (if (lucerne-auth:logged-in-p)
      (let ((user (current-user)))
        (with-params (tweet)
          (utweet.models:tweet user tweet))
        (redirect "/"))
      (render-template (+index+)
                       :error "You are not logged in.")))

The macro is implemented like this:

;; https://github.com/eudoxia0/lucerne/blob/master/src/http.lisp
(defmacro with-params (params &body body)
  "Extract the parameters in @cl:param(param) from the @c(*request*), and bind
them for use in @cl:param(body)."
  `(let ,(loop for param in params collecting
               `(,param (let ((str (parameter *request*
                                              ,(intern (string-downcase
                                                        (symbol-name param))
                                                       :keyword))))
                          (if (equal str "")
                              nil
                              str))))
     ,@body))

For Caveman it is possible but a bit awkward and inconsistent.

There’s an example for Ningle on the related StackOverflow question.

And in Clack generally ?

It is only scarcely documented on Clack’s api documentation.

We can access the parameters with (clack.request:query-parameter lucerne:*request*). So to get the value of a given param:

(assoc "a-param" (clack.request:query-parameter lucerne:*request*) :test 'string=)

and this returns the key and the value, so we need another cdr to get the value…

(defun query-param (param)
  (cdr (assoc param (clack.request:query-parameter lucerne:*request*) :test #'string=)))