Exercise 1.2.19

Pretty Printing

You may or may not have noticed already that Lisp has a way to print Lisp code, formatted in such a way that it's more straightforward to read. By default, whenever possible the pretty-printer outputs the code all on one line; if there's no room left in the default margins, then it breaks the code up into multiple lines.

To see the pretty-printer in action, I'll be showing you some pretty ugly code. Normally you would never write Lisp this way---but it shows you how Lisp can take badly formatted code and give you something better.

In this case, type out this example all on one line in the REPL before you hit Return/Enter.

(pprint '(defun monkey (a b c) "a monkey function" (let ((d 4) (e 5) (f 6)) (values (list a b c) (list d e f)))))

What You Should See

* (pprint '(defun monkey (a b c) "a monkey function" (let ((d 4) (e 5) (f 6)) (values (list a b c) (list d e f)))))
(DEFUN MONKEY (A B C)
  "a monkey function"
  (LET ((D 4) (E 5) (F 6))
    (VALUES (LIST A B C) (LIST D E F))))
NIL

Remember how I introduced "quoting" in the Common Lisp Bootcamp chapter? It turns Lisp code into data, so the function definition above is never evaluated by Lisp. It's read, but not eval'd. Otherwise, don't worry too much about the function definition for now. It's just illustrative, to show how the pretty printer splits up a long line into multiple lines.

Now, if you didn't quote the above function definition, it would be evaluated, and then you'd actually be pretty-printing the return value of the function instead of the code. Not very useful in this case---but say you wanted to pretty-print the result of a loop that collects a whole lot of data? It would be useful then.

Lisp's Pretty Printing facilities are pretty complex, and that's to say nothing of pretty-printing with format. Throughout this book, you'll see more ways to take advantage of the Pretty Printer along the way. But that's all for Chapter 1.2!