Practical Common Lisp

276

Character streams support two functions, PEEK-CHAR and UNREAD-CHAR, either of which would be a perfect solution to this problem, but binary streams support no equivalent functions.

277

If a tag had an extended header, you could use this value to determine where the frame data should end. However, if the extended header isn't used, you'd have to use the old algorithm anyway, so it's not worth adding code to do it another way.

278

These flags, in addition to controlling whether the optional fields are included, can affect the parsing of the rest of the tag. In particular, if the seventh bit of the flags is set, then the actual frame data is compressed using the zlib algorithm, and if the sixth bit is set, the data is encrypted. In practice these options are rarely, if ever, used, so you can get away with ignoring them for now. But that would be an area you'd have to address to make this a production-quality ID3 library. One simple half solution would be to change find-frame-class to accept a second argument and pass it the flags; if the frame is compressed or encrypted, you could instantiate a generic frame to hold the data.

279

Ensuring that kind of interfield consistency would be a fine application for :after methods on the accessor generic functions. For instance, you could define this :after method to keep size in sync with the information string:

(defmethod (setf information) :after (value (frame text-info-frame))
  (declare (ignore value))
  (with-slots (encoding size information) frame
    (setf size (encoded-string-length information encoding nil))))

280

Readers new to Web programming will probably need to supplement this introduction with a more in-depth tutorial or two. You can find a good set of online tutorials at http://www.jmarshall.com/easy/.

281

Loading a single Web page may actually involve multiple requests—to render the HTML of a page containing inline images, the browser must request each image individually and then insert each into the appropriate place in the rendered HTML.

282

Much of the complexity around Web programming is a result of trying to work around this fundamental limitation in order to provide a user experience that's more like the interactivity provided by desktop applications.

283

Unfortunately, dynamic is somewhat overloaded in the Web world. The phrase Dynamic HTML refers to HTML containing embedded code, usually in the language JavaScript, that can be executed in the browser without further communication with the Web server. Used with some discretion, Dynamic HTML can improve the usability of a Web-based application since, even with high-speed Internet connections, making a request to a Web server, receiving the response, and rendering the new page can take a noticeable amount of time. To further confuse things, dynamically generated pages (in other words, generated on the server) could also contain Dynamic HTML (code to be run on the client.) For the purposes of this book, you'll stick to dynamically generating plain old nondynamic HTML.

284

http://www.fractalconcept.com/asp/html/mod_lisp.html

285

http://lisplets.sourceforge.net/

286

AllegroServe also provides a framework called Webactions that's analogous to JSPs in the Java world—instead of writing code that generates HTML, with Webactions you write pages that are essentially HTML with a bit of magic foo that turns into code to be run when the page is served. I won't cover Webactions in this book.