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 SXEmacs 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 SXEmacs under
a certain name, such as twiddle-files
, do not call it by that name
in your program. Call it mylib-twiddle-files
in your program,
and send mail to ‘sxemacs-devel@sxemacs.org’ suggesting we add
it to SXEmacs. 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 SXEmacs 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, of course, 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 will not work right. See Compiling Macros.
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 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 do not 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 SXEmacs 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 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 SXEmacs primitives. Use the
standard names instead.
- Redefining a SXEmacs 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 SXEmacs, 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 SXEmacs 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.
- Do not use
next-line
or previous-line
in programs; nearly
always, forward-line
is more convenient as well as more
predictable and robust. See Text Lines.
- Do not 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 The Mark.
In particular, do not 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 The Echo Area.
- When you encounter an error condition, call the function
error
(or signal
). The function error
does not return.
See Signaling Errors.
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 Recursive Editing.
- In some other systems there is a convention of choosing variable names
that begin and end with ‘*’. We do not use that convention in
SXEmacs 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.
- Use names starting with a space for temporary buffers (see Buffer Names), or at least call
buffer-disable-undo
on them. Otherwise
they may stay referenced by internal undo variables after getting
killed. If this happens before dumping (see Building SXEmacs), this
may cause fatal error when portable dumper is used.
- Indent each function with C-M-q (
indent-sexp
) using the
default indentation parameters.
- Do not 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 SXEmacs
itself.