GNU Info
(elisp)Documentation Tips
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--documentation strings now take up
very little space in a running Emacs.
* 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 that line's
contents don't stand on their own, the result looks bad. In
particular, start the first line with a capital letter and end
with a period.
The documentation string is not limited to one line; use as many
lines as you need to explain the details of how to use the
function or variable. Please use complete sentences in the
additional lines.
* For consistency, phrase the verb in the first sentence of a
function's documentation string as an imperative-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
each sentence has a proper subject.
* 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."
* When a command is meaningful only in a certain mode or situation,
do mention that in the documentation string. For example, the
documentation of `dired-find-file' is:
In Dired, visit the file or directory named on this line.
* 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 should not be wider than
67 characters or it will look bad in the output of `apropos'.
You can fill the text if that looks good. However, rather than
blindly filling the entire documentation string, you can often
make it much more readable by choosing certain 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 `*'. Note: Defining 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 `eval' refers to its second argument as
`FORM', because the actual argument name is `form':
Evaluate FORM and return its value.
Also write metasyntactic variables in capital letters, such as
when you show the decomposition of a list or vector into subunits,
some of which may vary. `KEY' and `VALUE' in the following example
illustrate this practice:
The argument TABLE should be an alist whose elements
have the form (KEY . VALUE). Here, KEY is ...
* If a line in a documentation string begins with an
open-parenthesis, write a backslash before the open-parenthesis,
like this:
The argument FOO can be either a number
\(a buffer position) or a string (a file name).
This prevents the open-parenthesis from being treated as the start
of a defun (Note: Defuns.).
* 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. (In this
manual, we use a different convention, with single-quotes for all
symbols.)
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'.
To make a hyperlink to Info documentation, write the name of the
Info node in single quotes, preceded by `info node' or `Info
node'. The Info file name defaults to `emacs'. For example,
See Info node `Font Lock' and Info node `(elisp)Font Lock Basics'.
* 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.) Note: Keys 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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