A. Tips and Standards

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

A.1 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 XEmacs 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 XEmacs 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 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.

• 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.
• 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 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 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.
• 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.
• 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 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 XEmacs, 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 XEmacs 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 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.
• 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 XEmacs 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.

A.2 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 XEmacs 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 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.

A.3 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 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 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 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.

• 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.

A.4 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.

This and following examples are taken from the Emacs sources.

(setq base-version-list ; there was a base (assoc (subseq 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 modeline (redraw-modeline)))

Every function that has no documentation string (because it 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.

‘;;;’

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 XEmacs ;;; 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))

‘;;;;’

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 XEmacs, 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 (xemacs)Comments section ‘Manipulating Comments’ in The XEmacs User’s Manual.

This document was generated by Aidan Kehoe on December 27, 2016 using texi2html 1.82.