Peter Siebel - Practical Common Lisp

In ID3v2.4, UCS-2 is replaced by the virtually identical UTF-16, and UTF-16BE and UTF-8 are added as additional encodings.

The 2.4 version of the ID3 format also supports placing a footer at the end of a tag, which makes it easier to find a tag appended to the end of a file.

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.

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.

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-classto 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.

Ensuring that kind of interfield consistency would be a fine application for :aftermethods on the accessor generic functions. For instance, you could define this :aftermethod to keep sizein sync with the informationstring:

(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))))

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/.

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.

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.

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.

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

http://lisplets.sourceforge.net/

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.

Loading PortableAllegroServe will create some other packages for the compatibility libraries, but the packages you'll care about are those three.

The ~@followed by a newline tells FORMAT to ignore whitespace after the newline, which allows you to indent your code nicely without adding a bunch of whitespace to the HTML. Since white-space is typically not significant in HTML, this doesn't matter to the browser, but it makes the generated HTML source look a bit nicer to humans.

FOO is a recursive tautological acronym for FOO Outputs Output .

For information about the meaning of the other parameters, see the AllegroServe documentation and RFC 2109, which describes the cookie mechanism.

You need to use LET* rather than a LET to allow the default value forms for parameters to refer to parameters that appear earlier in the parameter list. For example, you could write this:

(define-url-function (request (x integer 10) (y integer (* 2 x))) ...)

and the value of y, if not explicitly supplied, would be twice the value of x.

The general theory behind interning objects is that if you're going to compare a particular value many times, it's worth it to pay the cost of interning it. The value-normalizerruns once when you insert a value into the table and, as you'll see, once at the beginning of each query. Since a query can involve invoking the equality-predicateonce per row in the table, the amortized cost of interning the values will quickly approach zero.

As always, the first causality of concise exposition in programming books is proper error handling; in production code you'd probably want to define your own error type, such as the following, and signal it instead:

(error 'illegal-column-value :value value :column column)

Then you'd want to think about where you can add restarts that might be able to recover from this condition. And, finally, in any given application you could establish condition handlers that would choose from among those restarts.

If any MP3 files have malformed data in the track and year frames, PARSE-INTEGER could signal an error. One way to deal with that is to pass PARSE-INTEGER the :junk-allowedargument of T , which will cause it to ignore any non-numeric junk following the number and to return NIL if no number can be found in the string. Or, if you want practice at using the condition system, you could define an error and signal it from these functions when the data is malformed and also establish a few restarts to allow these functions to recover.

This query will also return all the songs performed by the Dixie Chicks. If you want to limit it to songs by artists other than the Dixie Chicks, you need a more complex :wherefunction. Since the :whereargument can be any function, it's certainly possible; you could remove the Dixie Chicks' own songs with this query: