Книга: Practical Common Lisp

Looking at the Database Contents

Looking at the Database Contents

You can also see the current value of *db* whenever you want by typing *db* at the REPL.

CL-USER> *db*
((:TITLE "Home" :ARTIST "Dixie Chicks" :RATING 9 :RIPPED T)
(:TITLE "Fly" :ARTIST "Dixie Chicks" :RATING 8 :RIPPED T)
(:TITLE "Roses" :ARTIST "Kathy Mattea" :RATING 7 :RIPPED T))

However, that's not a very satisfying way of looking at the output. You can write a dump-db function that dumps out the database in a more human-readable format, like this:

TITLE: Home
ARTIST: Dixie Chicks
RATING: 9
RIPPED: T
TITLE: Fly
ARTIST: Dixie Chicks
RATING: 8
RIPPED: T
TITLE: Roses
ARTIST: Kathy Mattea
RATING: 7
RIPPED: T

The function looks like this:

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

This function works by looping over all the elements of *db* with the DOLIST macro, binding each element to the variable cd in turn. For each value of cd, you use the FORMAT function to print it.

Admittedly, the FORMAT call is a little cryptic. However, FORMAT isn't particularly more complicated than C or Perl's printf function or Python's string-% operator. In Chapter 18 I'll discuss FORMAT in greater detail. For now we can take this call bit by bit. As you saw in Chapter 2, FORMAT takes at least two arguments, the first being the stream where it sends its output; t is shorthand for the stream *standard-output*.

The second argument to FORMAT is a format string that can contain both literal text and directives telling FORMAT things such as how to interpolate the rest of its arguments. Format directives start with ~ (much the way printf's directives start with %). FORMAT understands dozens of directives, each with their own set of options.[26] However, for now I'll just focus on the ones you need to write dump-db.

The ~a directive is the aesthetic directive; it means to consume one argument and output it in a human-readable form. This will render keywords without the leading : and strings without quotation marks. For instance:

CL-USER> (format t "~a" "Dixie Chicks")
Dixie Chicks
NIL

or:

CL-USER> (format t "~a" :title)
TITLE
NIL

The ~t directive is for tabulating. The ~10t tells FORMAT to emit enough spaces to move to the tenth column before processing the next ~a. A ~t doesn't consume any arguments.

CL-USER> (format t "~a:~10t~a" :artist "Dixie Chicks")
ARTIST: Dixie Chicks
NIL

Now things get slightly more complicated. When FORMAT sees ~{ the next argument to be consumed must be a list. FORMAT loops over that list, processing the directives between the ~{ and ~}, consuming as many elements of the list as needed each time through the list. In dump-db, the FORMAT loop will consume one keyword and one value from the list each time through the loop. The ~% directive doesn't consume any arguments but tells FORMAT to emit a newline. Then after the ~} ends the loop, the last ~% tells FORMAT to emit one more newline to put a blank line between each CD.

Technically, you could have also used FORMAT to loop over the database itself, turning our dump-db function into a one-liner.

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

That's either very cool or very scary depending on your point of view.

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