Clack

Revenge of Lisp in Web

Chapter 6: Using Clack.App.Directory

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Clack.App.Directory is a real example of a Clack application that uses <component>. This application returns directory information as an HTML page.

Using this application is pretty easy:

;; mount /path/to/your/home/ to http://localhost:5000/
(clack:clackup
 (make-instance 'clack.app.directory:<clack-app-directory>
    :root #p"/path/to/your/home")
 :port 5000)

Open your web browser and go to http://localhost:5000/. You will see a list of files in your home directory.

Can you see why this application was implemented not just as a function but by using <component>? When you create the instance, you set :root, which changes the behavior of the application. It is difficult to achieve this functionality by using functions.

I know functions returning a closure can do similar things. But that would be complicated. Moreover, there is another advantage of using classes: inheritance. To tell the truth, <clack-app-directory> doesn't inherit <component> directly. It inehrits <clack-app-file>, whose functionality is to simply return a file (not directory information). What <clack-app-directory> per se has implemented is only the feature to return a list of files in a directory.

If you are curious about these stuffs, please take a look at directory.lisp and file.lisp in the core/apps/ directory. These will tell you the details of how components are used in Clack applications.