In addition to the obligatory request parameter, playlist takes a number of query parameters. The most important in some ways is playlist-id, which identifies which playlist object the page should display and manipulate. For this parameter, you can take advantage of define-url-function‘s “sticky parameter” feature. Normally, the playlist-id won’t be supplied explicitly, defaulting to the value returned by the playlist-id function, namely, the IP address of the client machine on which the browser is running. However, users can also manipulate their playlists from different machines than the ones running their MP3 clients by allowing this value to be explicitly specified. And if it’s specified once, define-url-function will arrange for it to “stick” by setting a cookie in the browser. Later you’ll define a URL function that generates a list of all existing playlists, which users can use to pick a playlist other than the one for the machines they’re browsing from.

The file, what, and values parameters are used with the :add-songs action. By declaring values to be of type base-64-list, the define-url-function infrastructure will take care of decoding the value submitted by the “Add all” form. The other parameters are used with other actions as noted in the comments.

Since will create a new playlist if necessary, this will always return a playlist object. Then you take care of any necessary queue manipulation, dispatching on the value of the action parameter in order to call one of the playlist functions.

The function playlist-toolbar generates a toolbar containing links to playlist to perform the various :action manipulations. And delete-songs-link generates a link to playlist with the parameter set to :delete-songs and the appropriate arguments to delete an individual file, or all files on an album, by a particular artist or in a specific genre.