287
Loading PortableAllegroServe will create some other packages for the compatibility libraries, but the packages you'll care about are those three.
288
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.
289
FOO is a recursive tautological acronym for
290
For information about the meaning of the other parameters, see the AllegroServe documentation and RFC 2109, which describes the cookie mechanism.
291
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.
292
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-normalizer runs 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-predicate once per row in the table, the amortized cost of interning the values will quickly approach zero.
293
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.
294
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-allowed argument 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.
295
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 :where function. Since the :where argument can be any function, it's certainly possible; you could remove the Dixie Chicks' own songs with this query:
(let* ((dixie-chicks (matching *mp3s* :artist 'Dixie Chicks'))
(same-song (in :song (select :columns :song :from *mp3s* :where dixie-chicks)))
(query #'(lambda (row) (and (not (funcall dixie-chicks row)) (funcall same-song row)))))
(select :columns '(:artist :song) :from *mp3s* :where query))
This obviously isn't quite as convenient. If you were going to write an application that needed to do lots of complex queries, you might want to consider coming up with a more expressive query language.
296
The version of LOOP implemented at M.I.T. before Common Lisp was standardized included a mechanism for extending the LOOP grammar to support iteration over new data structures. Some Common Lisp implementations that inherited their LOOP implementation from that code base may still support that facility, which would make do-rows and map-rows less necessary.
