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!
• 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.
• 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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