Tips and Standards

This chapter describes no additional features of Emacs Lisp. Instead it gives advice on making effective use of the features described in the previous chapters.

• Style Tips: Writing clean and robust programs.
• Compilation Tips: Making compiled code run fast.
• Documentation Tips: Writing readable documentation strings.
• Comment Tips: Conventions for writing comments.
• Library Headers: Standard headers for library packages.

Writing Clean Lisp Programs

Here are some tips for avoiding common errors in writing Lisp code intended for widespread use:

• Since all global variables share the same name space, and all functions share another name space, you should choose a short word to distinguish your program from other Lisp programs. Then take care to begin the names of all global variables, constants, and functions with the chosen prefix. This helps avoid name conflicts. This recommendation applies even to names for traditional Lisp primitives that are not primitives in Emacs Lisp--even to cadr. Believe it or not, there is more than one plausible way to define cadr. Play it safe; append your name prefix to produce a name like foo-cadr or mylib-cadr instead. If you write a function that you think ought to be added to Emacs under a certain name, such as twiddle-files, don't call it by that name in your program. Call it mylib-twiddle-files in your program, and send mail to `bug-gnu-emacs@prep.ai.mit.edu' suggesting we add it to Emacs. If and when we do, we can change the name easily enough. If one prefix is insufficient, your package may use two or three alternative common prefixes, so long as they make sense. Separate the prefix from the rest of the symbol name with a hyphen, `-'. This will be consistent with Emacs itself and with most Emacs Lisp programs.
• It is often useful to put a call to provide in each separate library program, at least if there is more than one entry point to the program.
• If one file foo uses a macro defined in another file bar, foo should contain (require 'bar) before the first use of the macro. (And bar should contain (provide 'bar), to make the require work.) This will cause bar to be loaded when you byte-compile foo. Otherwise, you risk compiling foo without the necessary macro loaded, and that would produce compiled code that won't work right. See section Macros and Byte Compilation.
• If you define a major mode, make sure to run a hook variable using run-hooks, just as the existing major modes do. See section Hooks.
• Please do not define C-c letter as a key in your major modes. These sequences are reserved for users; they are the only sequences reserved for users, so we cannot do without them. Instead, define sequences consisting of C-c followed by a non-letter. These sequences are reserved for major modes. Changing all the major modes in Emacs 18 so they would follow this convention was a lot of work. Abandoning this convention would waste that work and inconvenience the users.
• You should not bind C-h following any prefix character (including C-c). If you don't bind C-h, it is automatically available as a help character for listing the subcommands of the prefix character.
• You should not bind a key sequence ending in ESC except following another ESC. (That is, it is ok to bind a sequence ending in ESC ESC.) The reason for this rule is that a non-prefix binding for ESC in any context prevents recognition of escape sequences as function keys in that context.
• Applications should not bind mouse events based on button 1 with the shift key held down. These events include S-mouse-1, M-S-mouse-1, C-S-mouse-1, and so on. They are reserved for users.
• Modes should redefine mouse-2 as a command to follow some sort of reference in the text of a buffer, if users usually would not want to alter the text in that buffer by hand. Modes such as Dired, Info, Compilation, and Occur redefine it in this way.
• It is a bad idea to define aliases for the Emacs primitives. Use the standard names instead.
• Redefining an Emacs primitive is an even worse idea. It may do the right thing for a particular program, but there is no telling what other programs might break as a result.
• If a file does replace any of the functions or library programs of standard Emacs, prominent comments at the beginning of the file should say which functions are replaced, and how the behavior of the replacements differs from that of the originals.
• If a file requires certain standard library programs to be loaded beforehand, then the comments at the beginning of the file should say so.
• Please keep the names of your Emacs Lisp source files to 13 characters or less. This way, if the files are compiled, the compiled files' names will be 14 characters or less, which is short enough to fit on all kinds of Unix systems.
• Don't use next-line or previous-line in programs; nearly always, forward-line is more convenient as well as more predictable and robust. See section Motion by Text Lines.
• Don't use functions that set the mark in your Lisp code (unless you are writing a command to set the mark). The mark is a user-level feature, so it is incorrect to change the mark except to supply a value for the user's benefit. See section The Mark. In particular, don't use these functions:
  • beginning-of-buffer, end-of-buffer
  • replace-string, replace-regexp
  If you just want to move point, or replace a certain string, without any of the other features intended for interactive users, you can replace these functions with one or two lines of simple Lisp code.
• The recommended way to print a message in the echo area is with the message function, not princ. See section The Echo Area.
• When you encounter an error condition, call the function error (or signal). The function error does not return. See section How to Signal an Error. Do not use message, throw, sleep-for, or beep to report errors.
• Try to avoid using recursive edits. Instead, do what the Rmail e command does: use a new local keymap that contains one command defined to switch back to the old local keymap. Or do what the edit-options command does: switch to another buffer and let the user switch back at will. See section Recursive Editing.
• In some other systems there is a convention of choosing variable names that begin and end with `*'. We don't use that convention in Emacs Lisp, so please don't use it in your programs. (Emacs uses such names only for program-generated buffers.) The users will find Emacs more coherent if all libraries use the same conventions.
• Indent each function with C-M-q (indent-sexp) using the default indentation parameters.
• Don't make a habit of putting close-parentheses on lines by themselves; Lisp programmers find this disconcerting. Once in a while, when there is a sequence of many consecutive close-parentheses, it may make sense to split them in one or two significant places.
• Please put a copyright notice on the file if you give copies to anyone. Use the same lines that appear at the top of the Lisp files in Emacs itself. If you have not signed papers to assign the copyright to the Foundation, then place your name in the copyright notice in place of the Foundation's name.

Tips for Making Compiled Code Fast

Here are ways of improving the execution speed of byte-compiled Lisp programs.

• Use the `profile' library to profile your program. See the file `profile.el' for instructions.
• Use iteration rather than recursion whenever possible. Function calls are slow in Emacs Lisp even when a compiled function is calling another compiled function.
• Using the primitive list-searching functions memq, assq or assoc is even faster than explicit iteration. It may be worth rearranging a data structure so that one of these primitive search functions can be used.
• Certain built-in functions are handled specially in byte-compiled code, avoiding the need for an ordinary function call. It is a good idea to use these functions rather than alternatives. To see whether a function is handled specially by the compiler, examine its byte-compile property. If the property is non-nil, then the function is handled specially. For example, the following input will show you that aref is compiled specially (see section Functions that Operate on Arrays) while elt is not (see section Sequences):

  (get 'aref 'byte-compile)
       => byte-compile-two-args
  
  (get 'elt 'byte-compile)
       => nil
  

• If calling a small function accounts for a substantial part of your program's running time, make the function inline. This eliminates the function call overhead. Since making a function inline reduces the flexibility of changing the program, don't do it unless it gives a noticeable speedup in something slow enough that users care about the speed. See section Inline Functions.

Tips for Documentation Strings

Here are some tips for the writing of documentation strings.

• Every command, function or variable intended for users to know about should have a documentation string.
• An internal subroutine of a Lisp program need not have a documentation string, and you can save space by using a comment instead.
• The first line of the documentation string should consist of one or two complete sentences which stand on their own as a summary. M-x apropos displays just the first line, and if it doesn't stand on its own, the result looks bad. In particular, start the first line with a capital letter and end with a period. The documentation string can have additional lines which expand on the details of how to use the function or variable. The additional lines should be made up of complete sentences also, but they may be filled if that looks good.
• For consistency, phrase the verb in the first sentence of a documentation string as an infinitive with "to" omitted. For instance, use "Return the cons of A and B." in preference to "Returns the cons of A and B." Usually it looks good to do likewise for the rest of the first paragraph. Subsequent paragraphs usually look better if they have proper subjects.
• Write documentation strings in the active voice, not the passive, and in the present tense, not the future. For instance, use "Return a list containing A and B." instead of "A list containing A and B will be returned."
• Avoid using the word "cause" (or its equivalents) unnecessarily. Instead of, "Cause Emacs to display text in boldface," write just "Display text in boldface."
• Do not start or end a documentation string with whitespace.
• Format the documentation string so that it fits in an Emacs window on an 80 column screen. It is a good idea for most lines to be no wider than 60 characters. The first line can be wider if necessary to fit the information that ought to be there. However, rather than simply filling the entire documentation string, you can make it much more readable by choosing line breaks with care. Use blank lines between topics if the documentation string is long.
• Do not indent subsequent lines of a documentation string so that the text is lined up in the source code with the text of the first line. This looks nice in the source code, but looks bizarre when users view the documentation. Remember that the indentation before the starting double-quote is not part of the string!
• A variable's documentation string should start with `*' if the variable is one that users would often want to set interactively. If the value is a long list, or a function, or if the variable would only be set in init files, then don't start the documentation string with `*'. See section Defining Global Variables.
• The documentation string for a variable that is a yes-or-no flag should start with words such as "Non-nil means...", to make it clear that all non-nil values are equivalent and indicate explicitly what nil and non-nil mean.
• When a function's documentation string mentions the value of an argument of the function, use the argument name in capital letters as if it were a name for that value. Thus, the documentation string of the function / refers to its second argument as `DIVISOR', because the actual argument name is divisor. Also use all caps for meta-syntactic variables, such as when you show the decomposition of a list or vector into subunits, some of which may vary.
• When a documentation string refers to a Lisp symbol, write it as it would be printed (which usually means in lower case), with single-quotes around it. For example: ``lambda''. There are two exceptions: write t and nil without single-quotes.
• Don't write key sequences directly in documentation strings. Instead, use the `\\[...]' construct to stand for them. For example, instead of writing `C-f', write `\\[forward-char]'. When Emacs displays the documentation string, it substitutes whatever key is currently bound to forward-char. (This is normally `C-f', but it may be some other character if the user has moved key bindings.) See section Substituting Key Bindings in Documentation.
• In documentation strings for a major mode, you will want to refer to the key bindings of that mode's local map, rather than global ones. Therefore, use the construct `\\<...>' once in the documentation string to specify which key map to use. Do this before the first use of `\\[...]'. The text inside the `\\<...>' should be the name of the variable containing the local keymap for the major mode. It is not practical to use `\\[...]' very many times, because display of the documentation string will become slow. So use this to describe the most important commands in your major mode, and then use `\\{...}' to display the rest of the mode's keymap.
• Don't use the term "Elisp", since that is or was a trademark. Use the term "Emacs Lisp".

Tips on Writing Comments

We recommend these conventions for where to put comments and how to indent them:

`;'

Comments that start with a single semicolon, `;', should all be aligned to the same column on the right of the source code. Such comments usually explain how the code on the same line does its job. In Lisp mode and related modes, the M-; (indent-for-comment) command automatically inserts such a `;' in the right place, or aligns such a comment if it is already present. (The following examples are taken from the Emacs sources.)

(setq base-version-list                 ; there was a base
      (assoc (substring fn 0 start-vn)  ; version to which
             file-version-assoc-list))  ; this looks like
                                        ; a subversion

`;;'

Comments that start with two semicolons, `;;', should be aligned to the same level of indentation as the code. Such comments usually describe the purpose of the following lines or the state of the program at that point. For example:

(prog1 (setq auto-fill-function
             ...
             ...
  ;; update mode line
  (force-mode-line-update)))

Every function that has no documentation string (because it is use only internally within the package it belongs to), should have instead a two-semicolon comment right before the function, explaining what the function does and how to call it properly. Explain precisely what each argument means and how the function interprets its possible value.

`;;;'

Comments that start with three semicolons, `;;;', should start at the left margin. Such comments are used outside function definitions to make general statements explaining the design principles of the program. For example:

;;; This Lisp code is run in Emacs
;;; when it is to operate as a server
;;; for other processes.

Another use for triple-semicolon comments is for commenting out line within a function. We use triple-semicolons for this precisely so that they remain at the left margin.

(defun foo (a)
;;; This is no longer necessary.
;;;  (force-mode-line-update)
  (message "Finished with %s" a))

`;;;;'

Comments that start with four semicolons, `;;;;', should be aligned to the left margin and are used for headings of major sections of a program. For example:

;;;; The kill ring

The indentation commands of the Lisp modes in Emacs, such as M-; (indent-for-comment) and TAB (lisp-indent-line) automatically indent comments according to these conventions, depending on the the number of semicolons. See section `Manipulating Comments' in The GNU Emacs Manual.

Conventional Headers for Emacs Libraries

Emacs 19 has conventions for using special comments in Lisp libraries to divide them into sections and give information such as who wrote them. This section explains these conventions. First, an example:

;;; lisp-mnt.el -- minor mode for Emacs Lisp maintainers

;; Copyright (C) 1992 Free Software Foundation, Inc.

;; Author: Eric S. Raymond <esr@snark.thyrsus.com>
;; Maintainer: Eric S. Raymond <esr@snark.thyrsus.com>
;; Created: 14 Jul 1992
;; Version: 1.2
;; Keywords: docs

;; This file is part of GNU Emacs.
copying conditions...

The very first line should have this format:

;;; filename -- description

The description should be complete in one line.

After the copyright notice come several header comment lines, each beginning with `;; header-name:'. Here is a table of the conventional possibilities for header-name:

`Author'

This line states the name and net address of at least the principal author of the library. If there are multiple authors, you can list them on continuation lines led by ;; and a tab character, like this:

;; Author: Ashwin Ram <Ram-Ashwin@cs.yale.edu>
;;      Dave Sill <de5@ornl.gov>
;;      Dave Brennan <brennan@hal.com>
;;      Eric Raymond <esr@snark.thyrsus.com>

`Maintainer'

This line should contain a single name/address as in the Author line, or an address only, or the string `FSF'. If there is no maintainer line, the person(s) in the Author field are presumed to be the maintainers. The example above is mildly bogus because the maintainer line is redundant. The idea behind the `Author' and `Maintainer' lines is to make possible a Lisp function to "send mail to the maintainer" without having to mine the name out by hand. Be sure to surround the network address with `<...>' if you include the person's full name as well as the network address.

`Created'

This optional line gives the original creation date of the file. For historical interest only.

`Version'

If you wish to record version numbers for the individual Lisp program, put them in this line.

`Adapted-By'

In this header line, place the name of the person who adapted the library for installation (to make it fit the style conventions, for example).

`Keywords'

This line lists keywords for the finder-by-keyword help command. This field is important; it's how people will find your package when they're looking for things by topic area.

Just about every Lisp library ought to have the `Author' and `Keywords' header comment lines. Use the others if they are appropriate. You can also put in header lines with other header names--they have no standard meanings, so they can't do any harm.

We use additional stylized comments to subdivide the contents of the library file. Here is a table of them:

`;;; Commentary:'

This begins introductory comments that explain how the library works. It should come right after the copying permissions.

`;;; Change log:'

This begins change log information stored in the library file (if you store the change history there). For most of the Lisp files distributed with Emacs, the change history is kept in the file `ChangeLog' and not in the source file at all; these files do not have a `;;; Change log:' line.

`;;; Code:'

This begins the actual code of the program.

`;;; filename ends here'

This is the footer line; it appears at the very end of the file. Its purpose is to enable people to detect truncated versions of the file from the lack of a footer line.