Getting Clack

The easiest way to install Clack is to use Quicklisp. After installing Quicklisp, just use:

(ql:quickload :clack)

on your Common Lisp implementation. This command downloads and installs Clack and all dependencies automatically.

To check if everything is installed correctly, run a web server with Clack on your REPL:

  (lambda (env)
    (declare (ignore env))
    '(200 (:content-type "text/plain") ("Hello, Clack!"))))

;-> Hunchentoot server is started.
    Listening on localhost:5000.
    #<CLACK.HANDLER:<HANDLER> #x30200A102B3D>

Clack starts a web server on localhost port 5000 by default. Open your web browser and go to You should get “Hello, Clack!” message. You may not understand the details of the above code. Don’t worry. I’ll explain it in the next chapter.

To stop the server, use:

(clack:stop *)