42. Tips and Conventions

• Emacs Lisp Coding Conventions
• Tips for Making Compiled Code Fast
• Tips for Documentation Strings
• Tips on Writing Comments
• Conventional Headers for Emacs Libraries

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, and describes conventions Emacs Lisp programmers should follow.

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42.1. Emacs Lisp Coding Conventions

Here are conventions that you should follow when writing Emacs 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 copy-list. Believe it or not, there is more than one plausible way to define copy-list. Play it safe; append your name prefix to produce a name like foo-copy-list or mylib-copy-list 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@gnu.org' 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 a file requires certain other library programs to be loaded beforehand, then the comments at the beginning of the file should say so. Also, use require to make sure they are loaded.
• If one file foo uses a macro defined in another file bar, foo should contain this expression before the first use of the macro:

  (eval-when-compile (require 'bar))
  

  (And the library 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. Using eval-when-compile avoids loading bar when the compiled version of foo is used.
• When defining a major mode, please follow the major mode conventions. See section Major Mode Conventions.
• When defining a minor mode, please follow the minor mode conventions. See section Conventions for Writing Minor Modes.
• If the purpose of a function is to tell you whether a certain condition is true or false, give the function a name that ends in `p'. If the name is one word, add just `p'; if the name is multiple words, add `-p'. Examples are framep and frame-live-p.
• If a user option variable records a true-or-false condition, give it a name that ends in `-flag'.
• 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 do not block them. Instead, define sequences consisting of C-c followed by a control character, a digit, or certain punctuation characters. These sequences are reserved for major modes. Changing all the Emacs major modes to follow this convention was a lot of work. Abandoning this convention would make that work go to waste, and inconvenience users.
• Sequences consisting of C-c followed by {, }, <, >, : or ; are also reserved for major modes.
• Sequences consisting of C-c followed by any other punctuation character are allocated for minor modes. Using them in a major mode is not absolutely prohibited, but if you do that, the major mode binding may be shadowed from time to time by minor modes.
• Function keys F5 through F9 without modifier keys are reserved for users to define.
• Do 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.
• Do 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.
• Anything which acts like a temporary mode or state which the user can enter and leave should define ESC ESC of ESC ESC ESC as a way to escape. For a state which accepts ordinary Emacs commands, or more generally any kind of state in which ESC followed by a function key or arrow key is potentially meaningful, then you must not define ESC ESC, since that would preclude recognizing an escape sequence after ESC. In these states, you should define ESC ESC ESC as the way to escape. Otherwise, define ESC ESC instead.
• 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.
• Special major modes used for read-only text should usually redefine mouse-2 and RET to trace some sort of reference in the text. Modes such as Dired, Info, Compilation, and Occur redefine it in this way.
• When a package provides a modification of ordinary Emacs behavior, it is good to include a command to enable and disable the feature, Provide a command named whatever-mode which turns the feature on or off, and make it autoload (see section Autoload). Design the package so that simply loading it has no visible effect--that should not enable the feature. Users will request the feature by invoking the command.
• It is a bad idea to define aliases for the Emacs primitives. Use the standard names instead.
• Redefining (or advising) an Emacs primitive is discouraged. 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.
• 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 call functions that set the mark, unless setting the mark is one of the intended features of your program. 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 any of 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.
• Use lists rather than vectors, except when there is a particular reason to use a vector. Lisp has more facilities for manipulating lists than for vectors, and working with lists is usually more convenient. Vectors are advantageous for tables that are substantial in size and are accessed in random order (not searched front to back), provided there is no need to insert or delete elements (only lists allow that).
• 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.
• An error message should start with a capital letter but should not end with a period.
• Many commands that take a long time to execute display a message that says `Operating...' when they start, and change it to `Operating...done' when they finish. Please keep the style of these messages uniform: no space around the ellipsis, and no period at the end.
• 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 special-purpose buffers.) The users will find Emacs more coherent if all libraries use the same conventions.
• Try to avoid compiler warnings about undefined free variables, by adding defvar definitions for these variables. If you bind a variable in one function, and use it or set it in another function, the compiler warns about the latter function unless the variable has a definition. But often these variables have short names, and it is not clean for Lisp packages to define such variable names. Therefore, you should rename the variable to start with the name prefix used for the other functions and variables in your package.
• 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 the sequence in one or two significant places.
• Please put a copyright notice on the file if you give copies to anyone. Use a message like this one:

  ;; Copyright (C) year name
  
  ;; This program is free software; you can redistribute it and/or
  ;; modify it under the terms of the GNU General Public License as
  ;; published by the Free Software Foundation; either version 2 of
  ;; the License, or (at your option) any later version.
  
  ;; This program is distributed in the hope that it will be
  ;; useful, but WITHOUT ANY WARRANTY; without even the implied
  ;; warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
  ;; PURPOSE.  See the GNU General Public License for more details.
  
  ;; You should have received a copy of the GNU General Public
  ;; License along with this program; if not, write to the Free
  ;; Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
  ;; MA 02111-1307 USA
  

  If you have signed papers to assign the copyright to the Foundation, then use `Free Software Foundation, Inc.' as name. Otherwise, use your name.

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42.2. Tips for Making Compiled Code Fast

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

• Profile your program with the `profile' library or the `elp' library. See the files `profile.el' and `elp.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, member, assq, or assoc is even faster than explicit iteration. It can 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):

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

• 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.

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42.3. Tips for Documentation Strings

Here are some tips and conventions for the writing of documentation strings. You can check many of these conventions by running the command M-x checkdoc-minor-mode.

• Every command, function, or variable intended for users to know about should have a documentation string.
• An internal variable or subroutine of a Lisp program might as well have a documentation string. In earlier Emacs versions, you could save space by using a comment instead of a documentation string, but that is no longer the case.
• The first line of the documentation string should consist of one or two complete sentences that 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 that 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 function's 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!
• When the user tries to use a disabled command, Emacs displays just the first paragraph of its documentation string--everything through the first blank line. If you wish, you can choose which information to include before the first blank line so as to make this display useful.
• 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 be set only 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. Help mode automatically creates a hyperlink when a documentation string uses a symbol name inside single quotes, if the symbol has either a function or a variable definition. You do not need to do anything special to make use of this feature. However, when a symbol has both a function definition and a variable definition, and you want to refer to just one of them, you can specify which one by writing one of the words `variable', `option', `function', or `command', immediately before the symbol name. (Case makes no difference in recognizing these indicator words.) For example, if you write

  This function sets the variable `buffer-file-name'.
  

  then the hyperlink will refer only to the variable documentation of buffer-file-name, and not to its function documentation. If a symbol has a function definition and/or a variable definition, but those are irrelevant to the use of the symbol that you are documenting, you can write the word `symbol' before the symbol name to prevent making any hyperlink. For example,

  If the argument KIND-OF-RESULT is the symbol `list',
  this function returns a list of all the objects
  that satisfy the criterion.
  

  does not make a hyperlink to the documentation, irrelevant here, of the function list.
• 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 the construct `\\[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.

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42.4. Tips on Writing Comments

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

^top`;'

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. This and 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

^top`;;'

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 (presumably one that is used 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 values.

^top`;;;'

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 lines 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))

^top`;;;;'

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 number of semicolons. See section `Manipulating Comments' in The GNU Emacs Manual.

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42.5. Conventional Headers for Emacs Libraries

Emacs 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.
...
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.

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:

^top`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>

^top`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.

^top`Created'

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

^top`Version'

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

^top`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).

^top`Keywords'

This line lists keywords for the finder-by-keyword help command. Please use that command to see a list of the meaningful keywords. This field is important; it's how people will find your package when they're looking for things by topic area. To separate the keywords, you can use spaces, commas, or both.

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:

^top`;;; Commentary:'

This begins introductory comments that explain how the library works. It should come right after the copying permissions, terminated by a `Change Log', `History' or `Code' comment line. This text is used by the Finder package, so it should make sense in that context.

^top`;;; Documentation'

This has been used in some files in place of `;;; Commentary:', but `;;; Commentary:' is preferred.

^top`;;; 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.

^top`;;; Code:'

This begins the actual code of the program.

^top`;;; 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.