Good code is automatically formatted by tools like Black or Prettier so that you and your team spend less time on formatting and more time on building features. It's best if your editor can run code formatters each time you save a file, so that you don't have to look at badly formatted code or get surprised when things change just before you commit. However, running a code formatter on save suffers from the following two problems:
- It takes some time (e.g. around 200ms for Black on an empty file), which makes the editor feel less responsive.
- It invariably moves your cursor (point) somewhere unexpected if the changes made by the code formatter are too close to point's position.
The approach is as follows:
- Run code formatters on
after-save-hook, rather than
before-save-hook, and do so asynchronously. Once the formatter has finished running, check if the buffer has been modified since it started; only apply the changes if not.
- After running the code formatter, generate an RCS patch showing the changes and then apply it to the buffer. This prevents changes elsewhere in the buffer from moving point. If a patch region happens to include point, then use a dynamic programming algorithm for string alignment to determine where point should be moved so that it remains in the same place relative to its surroundings. Finally, if the vertical position of point relative to the window has changed, adjust the scroll position to maintain maximum visual continuity. (This includes iterating through all windows displaying the buffer, if there are more than one.) The dynamic programming algorithm runs in quadratic time, which is why it is only applied if necessary and to a single patch region.
To get started with Apheleia, install it with
straight.el as follows:
(straight-use-package '(apheleia :host github :repo "raxod502/apheleia"))
To your init-file, add the following form:
The autoloading has been configured so that this will not cause Apheleia to be loaded until you save a file.
apheleia-formatters: Alist mapping names of formatters (symbols like
prettier) to commands used to run those formatters (such as
(npx "prettier" input)). See the docstring for more information.
You can manipulate this alist using standard Emacs functions. For example, to add some command-line options to Black, you could use:
(setf (alist-get 'black apheleia-formatters) '("black" "--option" "..." "-"))
apheleia-mode-alist: Alist mapping major modes and filename regexps to names of formatters to use in those modes and files. See the docstring for more information.
apheleia-formatter: Optional buffer-local variable specifying the formatter to use in this buffer. Overrides
You can run
M-x apheleia-mode to toggle automatic formatting on save
in a single buffer, or
M-x apheleia-global-mode to toggle the
default setting for all buffers. Also, even if
apheleia-mode is not
enabled, you can run
M-x apheleia-format-buffer to manually invoke
the configured formatter for the current buffer. Running with a prefix
argument will cause the command to prompt you for which formatter to
Apheleia does not currently support TRAMP, and is therefore automatically disabled for remote files.
The following user options are also available:
apheleia-post-format-hook: Normal hook run after Apheleia formats a buffer. Run if the formatting is successful, even when no changes are made to the buffer.
apheleia-max-alignment-size: The maximum number of characters that a diff region can have to be processed using Apheleia's dynamic programming algorithm for point alignment. This cannot be too big or Emacs will hang noticeably on large reformatting operations, since the DP algorithm is quadratic-time.
Please see the contributor guide for my projects.
I got the idea for using RCS patches to avoid moving point too much from prettier-js, although that package does not implement the dynamic programming algorithm which Apheleia uses to guarantee stability of point even within a formatted region.
Note that despite this inspiration, Apheleia is a clean-room implementation which is free of the copyright terms of prettier-js.