Книга: Practical Common Lisp

Formatting Lisp Code

Formatting Lisp Code

While code formatting is, strictly speaking, neither a syntactic nor a semantic matter, proper formatting is important to reading and writing code fluently and idiomatically. The key to formatting Lisp code is to indent it properly. The indentation should reflect the structure of the code so that you don't need to count parentheses to see what goes with what. In general, each new level of nesting gets indented a bit more, and, if line breaks are necessary, items at the same level of nesting are lined up. Thus, a function call that needs to be broken up across multiple lines might be written like this:

(some-function arg-with-a-long-name

Macro and special forms that implement control constructs are typically indented a little differently: the "body" elements are indented two spaces relative to the opening parenthesis of the form. Thus:

(defun print-list (list)
(dolist (i list)
(format t "item: ~a~%" i)))

However, you don't need to worry too much about these rules because a proper Lisp environment such as SLIME will take care of it for you. In fact, one of the advantages of Lisp's regular syntax is that it's fairly easy for software such as editors to know how to indent it. Since the indentation is supposed to reflect the structure of the code and the structure is marked by parentheses, it's easy to let the editor indent your code for you.

In SLIME, hitting Tab at the beginning of each line will cause it to be indented appropriately, or you can re-indent a whole expression by positioning the cursor on the opening parenthesis and typing C-M-q. Or you can re-indent the whole body of a function from anywhere within it by typing C-c M-q.

Indeed, experienced Lisp programmers tend to rely on their editor handling indenting automatically, not just to make their code look nice but to detect typos: once you get used to how code is supposed to be indented, a misplaced parenthesis will be instantly recognizable by the weird indentation your editor gives you. For example, suppose you were writing a function that was supposed to look like this:

(defun foo ()
(if (test)

Now suppose you accidentally left off the closing parenthesis after test. Because you don't bother counting parentheses, you quite likely would have added an extra parenthesis at the end of the DEFUN form, giving you this code:

(defun foo ()
(if (test

However, if you had been indenting by hitting Tab at the beginning of each line, you wouldn't have code like that. Instead you'd have this:

(defun foo ()
(if (test

Seeing the then and else clauses indented way out under the condition rather than just indented slightly relative to the IF shows you immediately that something is awry.

Another important formatting rule is that closing parentheses are always put on the same line as the last element of the list they're closing. That is, don't write this:

(defun foo ()
(dotimes (i 10)
(format t "~d. hello~%" i)

but instead write this:

(defun foo ()
(dotimes (i 10)
(format t "~d. hello~%" i)))

The string of )))s at the end may seem forbidding, but as long your code is properly indented the parentheses should fade away—no need to give them undue prominence by spreading them across several lines.

Finally, comments should be prefaced with one to four semicolons depending on the scope of the comment as follows:

;;;; Four semicolons are used for a file header comment.
;;; A comment with three semicolons will usually be a paragraph
;;; comment that applies to a large section of code that follows,
(defun foo (x)
(dotimes (i x)
;; Two semicolons indicate this comment applies to the code
;; that follows. Note that this comment is indented the same
;; as the code that follows.
(another i) ; this comment applies to this line only
(and-another) ; and this is for this line

Now you're ready to start looking in greater detail at the major building blocks of Lisp programs, functions, variables, and macros. Up next: functions.

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