The Emacs Lisp Style Guide

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 career as a professional software engineer, feedback and suggestions from members of the Emacs Lisp community, and various highly regarded Emacs Lisp programming resources, such as "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 coding conventions and tips too.

You can generate a PDF or an HTML copy of this guide using Transmuter.

Table of Contents

• Source Code Layout & Organization
• Syntax
• Naming
• Macros
• Functions
  • Macro Declarations
  • Loading and Autoloading
• Comments
  • Comment Annotations
  • Docstrings
• Tools
• Existential
• Contributing
• License
• Spread the Word

Source Code Layout & Organization

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 <tab>.

• Use spaces for indentation. No hard tabs.

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

  ;; good
  (if something
      if-clause
    else-clause)
  
  ;; bad
  (if something
    if-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 defs 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

• 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))

• 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).

Naming

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 should not end in -face.

  ;; good
  (defface widget-inactive ...)
  
  ;; bad
  (defface widget-inactive-face ...)

Macros

• 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

• Use lambdas 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.
  #'(lambda (x) (car x))
  
  ;;; 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

• Always declare the 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 indent specification if the macro arguments should not be aligned like a function (think of defun or with-current-buffer).

  (defmacro define-widget (name &rest forms)
    "Description"
    (declare (debug (sexp body))
             (indent defun))
    ...)

Loading and Autoloading

• Always end each library file with a provide statement and an appropriate comment (the provide statement will allow dependent libraries to use require).

  (provide 'foo)
  
  ;;; foo.el ends here

• 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.

  ;;; 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)

• 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.

  ;;; good
  ;;;###autoload
  (add-to-list 'auto-mode-alist '("\\.foo\\'" . foo-mode))
  
  ;;; bad
  ;;;###autoload
  (foo-setup)

Comments

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

• 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.

  (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))

• 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.

  (defun bar ()
    (sleep 100)) ; OPTIMIZE

• 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

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.

  ;; 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

• Use checkdoc to check for style issues
  • Many in the Emacs community use checkdoc with Flycheck.
• Use package-lint to check packages before submission to repositories such as MELPA.
  • See the package-lint README about integration with flycheck.

Existential

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

Contributing

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!

You can also support the style guide with financial contributions via gittip.

License

This work is licensed under a Creative Commons Attribution 3.0 Unported License

Spread the Word

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,
Bozhidar