Re: &key and how it's used

[Sam S. <sd...@gn...>]

> * Yves S. Garret <lbh...@tz...> [2012-06-17 14:20:13 -0400]:
>>
>  1) how *db* is accessed, that's where all the data is held and

(defun select (selector-fn)
  (remove-if-not selector-fn *db*))
 
(defun where (&key title artist rating (ripped nil ripped-p))
  #'(lambda (cd)
    (and
      (if title    (equal (getf cd :title)  title)  t) 
      (if artist   (equal (getf cd :artist) artist) t)
      (if rating   (equal (getf cd :rating) rating) t)
      (if ripped-p (equal (getf cd :ripped) ripped) t))))

SELECT returns elements of *DB* which satisfy SELECTOR-FN
WHERE returns a function which __APPARENTLY__ can be used as an argument
to SELECT.
e.g.,
(select (where :title "foo"))
will return the objects with the given title.


> 2) what the heck is &key?

a lambda list keyword.
http://www.lispworks.com/documentation/HyperSpec/Body/03_da.htm

it is a way to pass optional arguments to functions in a position
independent way.
e.g.:

(defun foo (&key (a 1) (b 2) (c 3))
  (list a b c))

(foo :b 10) => (1 10 3)
(foo :a 4 :c 5) => (4 2 5)
(foo :c 5 :a 4) => (4 2 5)

> If I'm missing something absurdly obvious, please feel free to point it out.

You might get a more detailed and informative reply to such a general
Lisp question on the comp.lang.lisp newsgroup.

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