what is cd in (cd *db*)?

[mohsen]

In Practical Common Lisp there's a code that helps display a list in a much cleaner form

Code: Select all

(defun dump-db ()
  (dolist (cd *db*)
    (format t "~{~a:~10t~a~%~}~%" cd)))

The syntax for dolist is described as
dolist (var list-form [result-form]) declaration* {tag | statement}*
but I i can't quite figure out what cd is in (cd *db*)

could someone help me crack this one?

That would be great!!

Thanks,
-m

[ramarren]

Code: Select all

(dolist (cd *db*)
dolist (var list-form [result-form])

In the syntax descriptions in CLHS top level parentheses are omitted and [] denotes optional component. This is modified Backus-Naur Form. Knowing this you should be able to figure out what "cd" corresponds to.

[JamesF]

Hint: that's not the only place you see that symbol. Look at how it's used elsewhere in the function, and it should start to make sense.
The documentation should also help.

[Warren Wilkinson]

Code: Select all

(dolist (a '(1 2 3 4))
   --body--code--)

Could be rewritten as

Code: Select all

(destructuring-bind (a . rest) '(1 2 3 4)
   (tagbody :start 
      --body--code--
      (setf a (car rest) rest (cdr rest))
      (when rest (goto :start))))

If that helps.

[Vivitron]

dolist (var list-form [result-form]) declaration* {tag | statement}*

Cd is the var and *db* is the list-form, and the optional result-form is not used. There can be any number of declaration; there are none. There can be any number of tag or statement; there are no tags and the only statement is (format t ...).

I second Rmarren's suggestion to take a look at BNF.

Roughly: "For every album in the database, print it like this."

Historical note: when PCL was published CDs were a common physical medium for distributing albums