The Emacs Lisp Style Guide :PROPERTIES: :CUSTOM_ID: the-emacs-lisp-style-guide :END:
Role models are important.
-- Officer Alex J. Murphy / RoboCop
This Emacs Lisp style guide recommends best practices so that real-world Emacs Lisp programmers can write code that can be maintained by other real-world Emacs Lisp programmers. A style guide that reflects real-world usage gets used, and a style guide that holds to an ideal that has been rejected by the people it is supposed to help risks not getting used at all --- no matter how good it is.
The guide is separated into several sections of related rules. I've tried to add the rationale behind the rules (if it's omitted, I've assumed that it's pretty obvious).
I didn't come up with all the rules out of nowhere; they are mostly based on my extensive experience of using Emacs and creating/maintaining Emacs packages, feedback and suggestions from members of the Emacs Lisp community, and various highly regarded Emacs Lisp programming resources, such as [[https://www.gnu.org/software/emacs/manual/elisp.html]["GNU Emacs Lisp Reference Manual"]].
The guide is still a work in progress; some sections are missing, others are incomplete, some rules are lacking examples, some rules don't have examples that illustrate them clearly enough. In due time these issues will be addressed --- just keep them in mind for now.
Please note, that the Emacs developers maintain a list of [[http://www.gnu.org/software/emacs/manual/html_node/elisp/Tips.html#Tips][coding conventions and tips]] too.
You can generate a PDF or an HTML copy of this guide using [[https://pandoc.org/][Pandoc]].
** Table of Contents :PROPERTIES: :CUSTOM_ID: table-of-contents :END:
** Source Code Layout & Organization :PROPERTIES: :CUSTOM_ID: source-code-layout-organization :END:
Nearly everybody is convinced that every style but their own is ugly and unreadable. Leave out the "but their own" and they're probably right...
-- Jerry Coffin (on indentation)
The indentation guidelines specified here can be taken for granted if
you're writing in Emacs. It will always do the right thing when you hit
=
Use spaces for indentation. No hard tabs.
In practical terms this means you should add the following to your Emacs config:
(setq-default indent-tabs-mode nil)
An even better idea would be to force the use of spaces using =.dir-locals.el= in each of your Emacs Lisp projects.
((emacs-lisp-mode (indent-tabs-mode nil)))
For regular functions, vertically align function arguments.
;; good (format "%s %d" something something-else)
;; bad (format "%s %d" something something-else)
If the first argument is on a new line, align it with the function's name.
;; good (format "%s %d" something something-else)
;; bad (format "%s %d" something something-else)
Some forms are special, they take 1 or more /"special"/ arguments followed by a /"body"/ (an arbitrary number of arguments where only the final return value matters), e.g. =if=, =let=, =with-current-buffer=, etc. The special arguments should either be on the same line as the form's name or be indented by 4 spaces. The body arguments should be indented by 2 spaces.
;; good (when something (something-else))
;; bad - four spaces on the body (when something (something-else))
;; bad - aligned like a regular function (when something (something-else))
Note that the "then" clause of an =if= form is a special argument, indent it by 4 spaces.
;; good (if something then-clause else-clause)
;; bad (if something then-clause else-clause)
Vertically align =let= bindings.
;; good (let ((thing1 "some stuff") (thing2 "other stuff")) ...)
;; bad (let ((thing1 "some stuff") (thing2 "other stuff")) ...)
Use Unix-style line endings. (*BSD/Solaris/Linux/OSX users are covered by default, Windows users have to be extra careful.)
If you're using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:
bash$ git config --global core.autocrlf true
If any text precedes an opening bracket(=(=, ={= and =[=) or follows a closing bracket(=)=, =}= and =]=), separate that text from that bracket with a space. Conversely, leave no space after an opening bracket and before following text, or after preceding text and before a closing bracket.
;; good (foo (bar baz) quux)
;; bad (foo(bar baz)quux) (foo ( bar baz ) quux)
Place all trailing parentheses on a single line instead of distinct lines.
;; good; single line (when something (something-else))
;; bad; distinct lines (when something (something-else) )
Use empty lines between top-level forms.
;; good (defvar x ...)
(defun foo ...)
;; bad (defvar x ...) (defun foo ...)
An exception to the rule is the grouping of related =def=s together.
;; good (defconst min-rows 10) (defconst max-rows 20) (defconst min-cols 15) (defconst max-cols 30)
Do not place blank lines in the middle of a function or macro definition. An exception can be made to indicate grouping of pairwise constructs as found in e.g. =let= and =cond=.
Where feasible, avoid making lines longer than 80 characters.
Avoid trailing whitespace.
Avoid parameter lists with more than three or four positional parameters.
Always enable lexical scoping. This must be done on the first line as a file local variable.
;;; -- lexical-binding: t; --
** Syntax :PROPERTIES: :CUSTOM_ID: syntax :END:
Don't wrap the else clause of an =if= in a =progn= (it's wrapped in =progn= implicitly).
;; good (if something if-clause (something) (something-else))
;; bad (if something if-clause (progn (something) (something-else)))
Use =when= instead of =(if ... (progn ...)=.
;; good (when pred (foo) (bar))
;; bad (if pred (progn (foo) (bar)))
Use =unless= instead of =(when (not ...) ...)=.
;; good (unless pred (foo) (bar))
;; bad (when (not pred) (foo) (bar))
Use =not= instead of =null=, unless your checking whether something is =nil= (empty list). Despite its name (=null= instead of =nullp=), the function =null= is mostly meant to be used as a predicate.
;; good (if (null lst) ...)
(if (or (not foo) something) ...)
;; bad (if (not lst))
(if (and (null foo) bar) ...)
When doing comparisons, keep in mind that the functions =<=, =>=, etc. accept a variable number of arguments as of Emacs 24.4.
;; Preferred (< 5 x 10)
;; Old (and (> x 5) (< x 10))
Use =t= as the catch-all test expression in =cond=.
;; good (cond ((< n 0) "negative") ((> n 0) "positive") (t "zero"))
;; bad (cond ((< n 0) "negative") ((> n 0) "positive") (:else "zero"))
Use =(1+ x)= & =(1- x)= instead of =(+ x 1)= and =(- x 1)=.
Use =with-eval-after-load= instead of =eval-after-load=.
;; good (with-eval-after-load "foo" (bar) (baz))
;; bad (eval-after-load "foo" '(progn (bar) (baz)))
** Naming :PROPERTIES: :CUSTOM_ID: naming :END:
The only real difficulties in programming are cache invalidation and naming things.
-- Phil Karlton
Use =lisp-case= for function and variable names.
;; good (defvar some-var ...) (defun some-fun ...)
;; bad (defvar someVar ...) (defun somefun ...) (defvar some_fun ...)
Prefix top-level names with the name of the library they belong to in order to avoid name clashes.
;; good (defun projectile-project-root ...)
;; bad (defun project-root ...)
Prefix unused local (lexically scoped) variables with =_=.
;; good (lambda (x _y) x)
;; bad (lambda (x y) x)
Use =--= to denote private top-level definitions (e.g. =projectile--private-fun=).
The names of predicate methods (methods that return a boolean value) should end in a =p= if it's a single-word name and a =-p= if it's a multi-word name (e.g., =evenp= and =buffer-live-p=).
;; good (defun palindromep ...) (defun only-one-p ...)
;; bad (defun palindrome? ...) ; Scheme style (defun is-palindrome ...) ; Java style
Face names [[https://www.gnu.org/software/emacs/manual/html_node/elisp/Defining-Faces.html][should not]] end in =-face=.
;; good (defface widget-inactive ...)
;; bad (defface widget-inactive-face ...)
** Macros :PROPERTIES: :CUSTOM_ID: macros :END:
Don't write a macro if a function will do.
Create an example of a macro usage first and the macro afterwards.
Break complicated macros into smaller functions whenever possible.
A macro should usually just provide syntactic sugar and the core of the macro should be a plain function. Doing so will improve composability.
Prefer syntax-quoted forms over building lists manually.
** Functions :PROPERTIES: :CUSTOM_ID: functions :END:
Use =lambda=s for local bindings and function calls, not for hooks or global variables. Define named functions for the latter, they aid readability and customizability.
;;; Good (mapcar (lambda (x) (or (car x) "")) some-list) (let ((predicate (lambda (x) (and (numberp x) (evenp x))))) (funcall predicate 1000))
;;; Bad - Define real functions for these. (defcustom my-predicate (lambda (x) (and (numberp x) (evenp x))) ...) (define-key my-keymap (kbd "C-f") (lambda () (interactive) (forward-char 1))) (add-hook 'my-hook (lambda () (save-some-buffers)))
Never hard quote a lambda, it impedes byte-compilation.
;;; Good (lambda (x) (car x))
;;; Ok, but redundant.
;;; Bad '(lambda (x) (car x))
Don't wrap functions in anonymous functions when you don't need to.
;; good (cl-remove-if-not #'evenp numbers)
;; bad (cl-remove-if-not (lambda (x) (evenp x)) numbers)
Use a sharp quote (=#'=) when quoting function names. It's a good hint for the byte-compiler, which will warn you if the function is undefined. Some macros can also behave differently otherwise (like =cl-labels=).
;; good (cl-remove-if-not #'evenp numbers) (global-set-key (kbd "C-l C-l") #'redraw-display) (cl-labels ((butterfly () (message "42"))) (funcall #'butterfly))
;; bad (cl-remove-if-not 'evenp numbers) (global-set-key (kbd "C-l C-l") 'redraw-display) (cl-labels ((butterfly () (message "42"))) (funcall 'butterfly))
*** Macro Declarations :PROPERTIES: :CUSTOM_ID: macro-declarations :END:
- Always declare the [[http://www.gnu.org/software/emacs/manual/html_node/elisp/Specification-List.html#Specification-List][debug-specification]], this tells edebug which arguments
are meant for evaluation. If all arguments are evaluated, a simple
=(declare (debug t))= is enough.
- Declare the [[https://www.gnu.org/software/emacs/manual/html_node/elisp/Indenting-Macros.html#Indenting-Macros][indent specification]] if the macro arguments should not be
aligned like a function (think of =defun= or =with-current-buffer=).
#+BEGIN_SRC emacs-lisp
(defmacro define-widget (name &rest forms)
"Description"
(declare (debug (sexp body))
(indent defun))
...)
#+END_SRC*** Loading and Autoloading :PROPERTIES: :CUSTOM_ID: loading-and-autoloading :END:
- Always end each library file with a =provide= statement and an
appropriate comment (the =provide= statement will allow dependent
libraries to use =require=).
#+BEGIN_SRC emacs-lisp
(provide 'foo)
;;; foo.el ends here
#+END_SRC
- Always load library dependencies with =require=, rather than =load= or
=load-library= (the former is idempotent, while the others can result
in multiple evaluations).
- Include =autoload= cookies for mode definitions and commonly-used
user-facing functions and commands (i.e. setup functions and commands
that could be bound to a key). Conversely, *do not* provide autoload
cookies for global variables or internal functions.
#+BEGIN_SRC emacs-lisp
;;; good
;;;###autoload
(define-derived-mode foo-mode ...)
;;;###autoload
(define-minor-mode foo-minor-mode ...)
;;;###autoload
(defun foo-setup () ...)
;;; bad
;;;###autoload
(defun foo--internal () ...)
;;;###autoload
(defvar foo-option)
#+END_SRC
- *Do not* provide =autoload= cookies for non-definition top-level forms
(autoloading a library should never alter the behavior of a user's
configuration). The single exception: =auto-mode-alist= can be altered
for new major modes.
#+BEGIN_SRC emacs-lisp
;;; good
;;;###autoload
(add-to-list 'auto-mode-alist '("\\.foo\\'" . foo-mode))
;;; bad
;;;###autoload
(foo-setup)
#+END_SRC** Lists :PROPERTIES: :CUSTOM_ID: lists :END:
Use =dolist= instead of calling the same s-exps over different variables:
;;; good (dolist (hook '(prog-mode-hook text-mode-hook)) (add-hook hook 'turn-on-column-number-mode) (add-hook hook 'turn-off-line-number-mode) (add-hook hook 'linum-mode))
;;; bad (add-hook 'prog-mode-hook 'turn-on-column-number-mode) (add-hook 'prog-mode-hook 'turn-off-line-number-mode) (add-hook 'prog-mode-hook 'linum-mode) (add-hook 'text-mode-hook 'turn-on-column-number-mode) (add-hook 'text-mode-hook 'turn-off-line-number-mode) (add-hook 'text-mode-hook 'linum-mode)
Use =seq-do= or =dolist= instead of =mapcar= if you don't intend to concatenate the result.
;;; good (font-lock-add-keywords nil (mapcar 'downcase list-of-crazy-cased-words)) (seq-do 'load list-of-files-to-load)
;;; bad (mapcar 'load list-of-files-to-load)
Use =dolist= instead of calling =seq-do= over a lambda. Reserve =seq-do= for single function calls.
;;; good (dolist (map (list c-mode-map c++-mode-map)) (define-key map "\C-c\C-c" 'compile))
;;; bad (mapc (lambda (map) (define-key map "\C-c\C-c" 'compile)) (list c-mode-map c++-mode-map))
** Comments :PROPERTIES: :CUSTOM_ID: comments :END:
Good code is its own best documentation. As you're about to add a comment, ask yourself, "How can I improve the code so that this comment isn't needed?" Improve the code and then document it to make it even clearer. -- Steve McConnell
Endeavor to make your code as self-documenting as possible.
Write heading comments with at least three semicolons.
Write top-level comments with three semicolons if it represents a heading, otherwise use two semicolons.
Write comments on a particular fragment of code before that fragment and aligned with it, using two semicolons.
Write margin comments with one semicolon.
Always have at least one space between the semicolon and the text that follows it.
;;; Frob Grovel ;; This is where Frob grovels and where Grovel frobs.
;; This section of code has some important implications: ;; 1. Foo. ;; 2. Bar. ;; 3. Baz.
(defun fnord (zarquon) ;; If zob, then veeblefitz. (quux zot mumble ; Zibblefrotz. frotz))
Comments longer than a word begin with a capital letter and use punctuation. Separate sentences with two spaces.
Avoid superfluous comments.
;; bad (1+ counter) ; increments counter by one
Keep existing comments up-to-date. An outdated comment is worse than no comment at all.
Good code is like a good joke - it needs no explanation. -- Russ Olsen
Avoid writing comments to explain bad code. Refactor the code to make it self-explanatory.
Do, or do not. There is no try. -- Yoda
*** Comment Annotations :PROPERTIES: :CUSTOM_ID: comment-annotations :END:
- Annotations should usually be written on the line immediately above
the relevant code.
- The annotation keyword is followed by a colon and a space, then a note
describing the problem.
- If multiple lines are required to describe the problem, subsequent
lines should be indented as much as the first one.
- Tag the annotation with your initials and a date so its relevance can
be easily verified.
#+BEGIN_SRC emacs-lisp
(defun some-fun ()
;; FIXME: This has crashed occasionally since v1.2.3. It may
;; be related to the BarBazUtil upgrade. (xz 13-1-31)
(baz))
#+END_SRC
- In cases where the problem is so obvious that any documentation would
be redundant, annotations may be left at the end of the offending line
with no note. This usage should be the exception and not the rule.
#+BEGIN_SRC emacs-lisp
(defun bar ()
(sleep 100)) ; OPTIMIZE
#+END_SRC
- Use =TODO= to note missing features or functionality that should be
added at a later date.
- Use =FIXME= to note broken code that needs to be fixed.
- Use =OPTIMIZE= to note slow or inefficient code that may cause
performance problems.
- Use =HACK= to note "code smells" where questionable coding practices
were used and should be refactored away.
- Use =REVIEW= to note anything that should be looked at to confirm it
is working as intended. For example:
=REVIEW: Are we sure this is how the client does X currently?=
- Use other custom annotation keywords if it feels appropriate, but be
sure to document them in your project's =README= or similar.*** Docstrings :PROPERTIES: :CUSTOM_ID: docstrings :END:
Emacs is famous for the breadth, depth, and ubiquity of its
documentation. By taking the time to write docstrings in your package,
you are helping to continue that tradition!
- Begin with a terse, complete sentence. Use imperative language. For
example, prefer "Verify" over "Verifies", and "Check" over "Checks".
- When a function takes arguments, mention what the arguments do,
whether they are required, and so on. Describe the arguments in
UPCASE, and order them as they are used.
- Always capitalize "Emacs".
- Do not indent subsequent lines of a documentation string. This looks
nice in the source code, but looks bizarre when users view the
documentation.
#+BEGIN_SRC emacs-lisp
;; good
(defun goto-line (line &optional buffer)
"Go to LINE, counting from line 1 at beginning of buffer.
If called interactively, a numeric prefix argument specifies
LINE; without a numeric prefix argument, read LINE from the
minibuffer..."
...)
;; bad
(defun goto-line (line &optional buffer)
"Go to LINE, counting from line 1 at beginning of buffer.
If called interactively, a numeric prefix argument specifies
LINE; without a numeric prefix argument, read LINE from the
minibuffer..."
...)
;; also bad
(defun goto-line (line &optional buffer)
"Go to LINE, counting from line 1 at beginning of buffer.
If called interactively, a numeric prefix argument specifies
LINE; without a numeric prefix argument, read LINE from the
minibuffer..."
...)** Tools :PROPERTIES: :CUSTOM_ID: tools :END:
Use =checkdoc= to check for docstring style issues.
Use [[https://github.com/purcell/package-lint][package-lint]] to check packages before submission to repositories such as [[https://melpa.org/][MELPA]].
** Existential :PROPERTIES: :CUSTOM_ID: existential :END:
Contributing :PROPERTIES: :CUSTOM_ID: contributing :END:
Nothing written in this guide is set in stone. It's my desire to work together with everyone interested in Emacs Lisp coding style, so that we could ultimately create a resource that will be beneficial to the entire Emacs community.
Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!
License :PROPERTIES: :CUSTOM_ID: license :END:
[[http://i.creativecommons.org/l/by/3.0/88x31.png]] This work is licensed under a [[http://creativecommons.org/licenses/by/3.0/deed.en_US][Creative Commons Attribution 3.0 Unported License]]
Spread the Word :PROPERTIES: :CUSTOM_ID: spread-the-word :END:
A community-driven style guide is of little use to a community that doesn't know about its existence. Tweet about the guide, share it with your friends and colleagues. Every comment, suggestion or opinion we get makes the guide just a little bit better. And we want to have the best possible guide, don't we?
Cheers,\ [[https://twitter.com/bbatsov][Bozhidar]]