emacs-lisp-style-guide

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• The Emacs Lisp Style Guide :PROPERTIES: :CUSTOM_ID: the-emacs-lisp-style-guide :END:

  #+BEGIN_QUOTE Role models are important.

  -- Officer Alex J. Murphy / RoboCop #+END_QUOTE

  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][Source Code Layout & Organization]]
• [[#syntax][Syntax]]
• [[#naming][Naming]]
• [[#macros][Macros]]
• [[#functions][Functions]]
  • [[#macro-declarations][Macro Declarations]]
  • [[#loading-and-autoloading][Loading and Autoloading]]
• [[#lists][Lists]]
• [[#comments][Comments]]
  • [[#comment-annotations][Comment Annotations]]
  • [[#docstrings][Docstrings]]
• [[#tools][Tools]]
• [[#existential][Existential]]
• [[#contributing][Contributing]]
• [[#license][License]]
• [[#spread-the-word][Spread the Word]]

** Source Code Layout & Organization :PROPERTIES: :CUSTOM_ID: source-code-layout-organization :END:

#+BEGIN_QUOTE 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) #+END_QUOTE

#+BEGIN_NOTE 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 ==. #+END_NOTE

• Use spaces for indentation. No hard tabs.

In practical terms this means you should add the following to your Emacs config:

#+BEGIN_SRC emacs-lisp emacs-lisp (setq-default indent-tabs-mode nil) #+END_SRC

An even better idea would be to force the use of spaces using =.dir-locals.el= in each of your Emacs Lisp projects.

#+BEGIN_SRC emacs-lisp ((emacs-lisp-mode (indent-tabs-mode nil))) #+END_SRC

• For regular functions, vertically align function arguments.

#+BEGIN_SRC emacs-lisp ;; good (format "%s %d" something something-else)

;; bad (format "%s %d" something something-else) #+END_SRC

• If the first argument is on a new line, align it with the function's name.

#+BEGIN_SRC emacs-lisp ;; good (format "%s %d" something something-else)

;; bad (format "%s %d" something something-else) #+END_SRC

• 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.

  #+BEGIN_SRC emacs-lisp ;; 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)) #+END_SRC

• Note that the "then" clause of an =if= form is a special argument, indent it by 4 spaces.

  #+BEGIN_SRC emacs-lisp ;; good (if something then-clause else-clause)

  ;; bad (if something then-clause else-clause) #+END_SRC

• Vertically align =let= bindings.

  #+BEGIN_SRC emacs-lisp ;; good (let ((thing1 "some stuff") (thing2 "other stuff")) ...)

  ;; bad (let ((thing1 "some stuff") (thing2 "other stuff")) ...) #+END_SRC

• 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:

  #+BEGIN_SRC conf bash$ git config --global core.autocrlf true #+END_SRC

• 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.

  #+BEGIN_SRC emacs-lisp ;; good (foo (bar baz) quux)

  ;; bad (foo(bar baz)quux) (foo ( bar baz ) quux) #+END_SRC

• Place all trailing parentheses on a single line instead of distinct lines.

  #+BEGIN_SRC emacs-lisp ;; good; single line (when something (something-else))

  ;; bad; distinct lines (when something (something-else) ) #+END_SRC

• Use empty lines between top-level forms.

  #+BEGIN_SRC emacs-lisp ;; good (defvar x ...)

  (defun foo ...)

  ;; bad (defvar x ...) (defun foo ...) #+END_SRC

  An exception to the rule is the grouping of related =def=s together.

  #+BEGIN_SRC emacs-lisp ;; good (defconst min-rows 10) (defconst max-rows 20) (defconst min-cols 15) (defconst max-cols 30) #+END_SRC

• 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.

  #+BEGIN_SRC emacs-lisp ;;; -- lexical-binding: t; -- #+END_SRC

** Syntax :PROPERTIES: :CUSTOM_ID: syntax :END:

• Don't wrap the else clause of an =if= in a =progn= (it's wrapped in =progn= implicitly).

#+BEGIN_SRC emacs-lisp ;; good (if something if-clause (something) (something-else))

 ;; bad
 (if something
     if-clause
   (progn
     (something)
     (something-else)))

#+END_SRC

• Use =when= instead of =(if ... (progn ...)=.

#+BEGIN_SRC emacs-lisp ;; good (when pred (foo) (bar))

 ;; bad
 (if pred
   (progn
     (foo)
     (bar)))

#+END_SRC

• Use =unless= instead of =(when (not ...) ...)=.

#+BEGIN_SRC emacs-lisp ;; good (unless pred (foo) (bar))

;; bad (when (not pred) (foo) (bar)) #+END_SRC

• 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.

#+BEGIN_SRC emacs-lisp ;; good (if (null lst) ...)

 (if (or (not foo) something) ...)

 ;; bad
 (if (not lst))

 (if (and (null foo) bar) ...)

#+END_SRC

• When doing comparisons, keep in mind that the functions =<=, =>=, etc. accept a variable number of arguments as of Emacs 24.4.

#+BEGIN_SRC emacs-lisp ;; Preferred (< 5 x 10)

;; Old (and (> x 5) (< x 10)) #+END_SRC

• Use =t= as the catch-all test expression in =cond=.

#+BEGIN_SRC emacs-lisp ;; good (cond ((< n 0) "negative") ((> n 0) "positive") (t "zero"))

;; bad (cond ((< n 0) "negative") ((> n 0) "positive") (:else "zero")) #+END_SRC

• Use =(1+ x)= & =(1- x)= instead of =(+ x 1)= and =(- x 1)=.

• Use =with-eval-after-load= instead of =eval-after-load=.

#+BEGIN_SRC emacs-lisp ;; good (with-eval-after-load "foo" (bar) (baz))

;; bad (eval-after-load "foo" '(progn (bar) (baz))) #+END_SRC

** Naming :PROPERTIES: :CUSTOM_ID: naming :END:

#+BEGIN_QUOTE The only real difficulties in programming are cache invalidation and naming things.

-- Phil Karlton #+END_QUOTE

• Use =lisp-case= for function and variable names.

#+BEGIN_SRC emacs-lisp ;; good (defvar some-var ...) (defun some-fun ...)

;; bad (defvar someVar ...) (defun somefun ...) (defvar some_fun ...) #+END_SRC

• Prefix top-level names with the name of the library they belong to in order to avoid name clashes.

#+BEGIN_SRC emacs-lisp ;; good (defun projectile-project-root ...)

;; bad (defun project-root ...) #+END_SRC

• Prefix unused local (lexically scoped) variables with =_=.

#+BEGIN_SRC emacs-lisp ;; good (lambda (x _y) x)

;; bad (lambda (x y) x) #+END_SRC

• 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=).

#+BEGIN_SRC emacs-lisp ;; good (defun palindromep ...) (defun only-one-p ...)

;; bad (defun palindrome? ...) ; Scheme style (defun is-palindrome ...) ; Java style #+END_SRC

• Face names [[https://www.gnu.org/software/emacs/manual/html_node/elisp/Defining-Faces.html][should not]] end in =-face=.

#+BEGIN_SRC emacs-lisp ;; good (defface widget-inactive ...)

;; bad (defface widget-inactive-face ...) #+END_SRC

** 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.

#+BEGIN_SRC emacs-lisp ;;; 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))) #+END_SRC

• Never hard quote a lambda, it impedes byte-compilation.

#+BEGIN_SRC emacs-lisp ;;; Good (lambda (x) (car x))

;;; Ok, but redundant. #'(lambda (x) (car x))

;;; Bad '(lambda (x) (car x)) #+END_SRC

• Don't wrap functions in anonymous functions when you don't need to.

#+BEGIN_SRC emacs-lisp ;; good (cl-remove-if-not #'evenp numbers)

;; bad (cl-remove-if-not (lambda (x) (evenp x)) numbers) #+END_SRC

• 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=).

#+BEGIN_SRC emacs-lisp ;; 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)) #+END_SRC

*** 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:

#+BEGIN_SRC emacs-lisp ;;; 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) #+END_SRC

• Use =seq-do= or =dolist= instead of =mapcar= if you don't intend to concatenate the result.

#+BEGIN_SRC emacs-lisp ;;; 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) #+END_SRC

• Use =dolist= instead of calling =seq-do= over a lambda. Reserve =seq-do= for single function calls.

#+BEGIN_SRC emacs-lisp ;;; 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)) #+END_SRC

** Comments :PROPERTIES: :CUSTOM_ID: comments :END:

#+BEGIN_QUOTE 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 #+END_QUOTE

• 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.

#+BEGIN_SRC emacs-lisp ;;; 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)) #+END_SRC

• Comments longer than a word begin with a capital letter and use punctuation. Separate sentences with two spaces.

• Avoid superfluous comments.

#+BEGIN_SRC emacs-lisp ;; bad (1+ counter) ; increments counter by one #+END_SRC

• Keep existing comments up-to-date. An outdated comment is worse than no comment at all.

#+BEGIN_QUOTE Good code is like a good joke - it needs no explanation. -- Russ Olsen #+END_QUOTE

• Avoid writing comments to explain bad code. Refactor the code to make it self-explanatory.

#+BEGIN_QUOTE Do, or do not. There is no try. -- Yoda #+END_QUOTE

*** 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..."
  ...)

#+END_SRC

** Tools :PROPERTIES: :CUSTOM_ID: tools :END:

• Use =checkdoc= to check for docstring style issues.

  • Many people in the Emacs community use =checkdoc= with [[http://flycheck.readthedocs.org/en/latest/][Flycheck]].

• Use [[https://github.com/purcell/package-lint][package-lint]] to check packages before submission to repositories such as [[https://melpa.org/][MELPA]].

  • See the =package-lint= README about integration with [[http://www.flycheck.org/en/latest/][Flycheck]].

** Existential :PROPERTIES: :CUSTOM_ID: existential :END:

• Be consistent. In an ideal world, be consistent with these guidelines.
• Use common sense.

• 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]]