# Apheleia Good code is automatically formatted by tools like [Black](https://github.com/python/black) or [Prettier](https://prettier.io/) 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: 1. It takes some time (e.g. around 200ms for Black on an empty file), which makes the editor feel less responsive. 2. It invariably moves your cursor (point) somewhere unexpected if the changes made by the code formatter are too close to point's position. Apheleia is an [Emacs](https://www.gnu.org/software/emacs/) package which solves both of these problems comprehensively for all languages, allowing you to say goodbye to language-specific packages such as [Blacken](https://github.com/proofit404/blacken) and [prettier-js](https://github.com/prettier/prettier-emacs). The approach is as follows: 1. 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. 2. After running the code formatter, generate an [RCS patch](https://tools.ietf.org/doc/tcllib/html/rcs.html#section4) 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](https://en.wikipedia.org/wiki/Needleman%E2%80%93Wunsch_algorithm) 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. ## User guide To get started with Apheleia, install it with [`straight.el`](https://github.com/raxod502/straight.el) as follows: (straight-use-package '(apheleia :host github :repo "raxod502/apheleia")) Alternatively, you can use another source-based package manager such as [Quelpa](https://framagit.org/steckerhalter/quelpa), [El-Get](https://github.com/dimitri/el-get), or [Borg](https://github.com/emacscollective/borg). Apheleia is not currently listed on [MELPA](https://melpa.org/) or [GNU ELPA](https://elpa.gnu.org/). To your init-file, add the following form: (apheleia-global-mode +1) The autoloading has been configured so that this will *not* cause Apheleia to be loaded until you save a file. By default, Apheleia is configured to format with [Black](https://github.com/python/black), [Prettier](https://prettier.io/), and [Gofmt](https://golang.org/cmd/gofmt/) on save in all relevant major modes. To configure this, you can adjust the values of the following variables: * `apheleia-formatters`: Alist mapping names of formatters (symbols like `black` and `prettier`) to commands used to run those formatters (such as `("black" "-")` and `(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: ```elisp (setf (alist-get 'black apheleia-formatters) '("black" "--option" "..." "-")) ``` * There are a list of symbols that are interpreted by apheleia specially when formatting a command (example: `npx`). Any non-string entries in a formatter that doesn't equal one of these symbols is evaluated and replaced in place. This can be used to pass certain flags to the formatter process depending on the state of the current buffer. For example: ```elisp (push '(shfmt . ("beautysh" "-filename" filepath (when-let ((indent (bound-and-true-p sh-basic-offset))) (list "--indent-size" (number-to-string indent))) (when indent-tabs-mode "--tab") "-")) apheleia-formatters) ``` This adds an entry to `apheleia-formatters` for the `beautysh` formatter. The evaluated entries makes it so that the `--tab` flag is only passed to `beautysh` when the value of `indent-tabs-mode` is true. Similarly the indent-size flag is passed the exact value of the `sh-basic-offset` variable only when it is bound. Observe that one of these evaluations returns a list of flags whereas the other returns a single string. These are substituted into the command as you'd expect. * `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. * You can use this variable to configure multiple formatters for the same buffer by setting the `cdr` of an entry to a list of formatters to run instead of a single formatter. For example you may want to run `isort` and `black` one after the other. ```elisp (setf (alist-get 'isort apheleia-formatters) '("isort" "--stdout" "-")) (setf (alist-get 'python-mode apheleia-mode-alist) '(isort black)) ``` This will make apheleia run `isort` on the current buffer and then `black` on the result of `isort` and then use the final output to format the current buffer. **Warning**: At the moment there's no smart or configurable error handling in place. This means if one of the configured formatters fail (for example if `isort` isn't installed) then apheleia just doesn't format the buffer at all, even if `black` is installed. **Warning:** If a formatter uses `file` (rather than `filepath` or `input` or none of these keywords), it can't be chained after another formatter, because `file` implies that the formatter must read from the *original* file, not an intermediate temporary file. For this reason it's suggested to avoid the use of `file` in general. * `apheleia-formatter`: Optional buffer-local variable specifying the formatter to use in this buffer. Overrides `apheleia-mode-alist`. 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 run. 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. ## Contributing Please see [the contributor guide for my projects](https://github.com/raxod502/contributor-guide). ## Acknowledgements I got the idea for using RCS patches to avoid moving point too much from [prettier-js](https://github.com/prettier/prettier-emacs), 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.