#+title: difftastic.el - Wrapper for difftastic #+author: Przemysław Kryger #+language: en #+startup: showeverything #+startup: literallinks #+options: toc:nil num:nil author:nil

#+html: #+html: #+html:

The =difftastic= Emacs package is designed to integrate [[https://github.com/wilfred/difftastic][difftastic]] - a structural diff tool - into your Emacs workflow, enhancing your code review and comparison experience. This package automatically displays =difftastic='s output within Emacs using faces from your user theme, ensuring consistency with your overall coding environment.

** Table of Contents :noexport: :properties: :toc: :include all :end: :contents:

• [[#features][Features]]
• [[#installation][Installation]]
  • [[#installing-from-melpa][Installing from MELPA]]
  • [[#installing-from-github][Installing from GitHub]]
  • [[#manual-installation][Manual Installation]]
• [[#configuration][Configuration]]
  • [[#manual-key-bindings-configuration][Manual Key Bindings Configuration]]
• [[#usage][Usage]]
  • [[#general-usage][General Usage]]
  • [[#specifying-difftastic-arguments][Specifying difftastic Arguments]]
  • [[#difftastic-mode-commands][difftastic-mode Commands]]
• [[#customization][Customization]]
  • [[#face-customization][Face Customization]]
  • [[#window-management][Window Management]]
  • [[#difftastic-mode-behavior][difftastic-mode Behavior]]
  • [[#process-environment][Process Environment]]
• [[#contributing][Contributing]]
  • [[#testing][Testing]]
  • [[#documentation-autoring][Documentation Autoring]] :end:

** Features :properties: :custom_id: features :end:

• Configure faces to your likening. By default =magit-diff-*= faces from your user them are used for consistent visual experience.
• Chunks and file navigation using n / N (or M-n) and p / P (or M-p), as well as C-M-f, C-M-b, and C-M-SPC in generated diffs.
• Hide chunks and files with TAB when on a chunk/file header. Use C-u TAB to hide whole file.
• DWIM workflows from =magit=.
• Use difftastic do compare files and buffers (also directly from =dired=).
• Rerun =difftastic= with g to use current window width to "reflow" content and/or to force language change (when called with prefix).
• Use double prefix argument to specify all =difftastic= arguments.

** Installation :properties: :custom_id: installation :end: *** Installing from MELPA :properties: :custom_id: installing-from-melpa :end: The easiest way to install and keep =difftastic= up-to-date is using Emacs' built-in package manager. =difftastic= is available in the MELPA repository. Refer to https://melpa.org/#/getting-started for how to install a package from MELPA.

Please see [[#configuration][Configuration]] section for example configuration.

You can use any of the package managers that supports installation from MELPA. It can be one of (but not limited to): one of the built-in =package=, =use-package=, or any other package manger that handles autoloads generation, for example (in alphabetical order) [[https://github.com/emacscollective/borg][Borg]], [[https://github.com/progfolio/elpaca][Elpaca]], [[https://github.com/quelpa/quelpa][Quelpa]], or [[https://github.com/radian-software/straight.el][straight.el]].

*** Installing from GitHub :properties: :custom_id: installing-from-github :end: The preferred method is to use built-in =use-package=. Add the following to your Emacs configuration file (usually =/.emacs= or =/.emacs.d/init.el=):

#+begin_src emacs-lisp :results value silent (use-package difftastic :defer t :vc (:url "https://github.com/pkryger/difftastic.el.git" :rev :newest)) #+end_src

Alternatively, you can do a manual checkout and install it from there, for example:

1. Clone this repository to a directory of your choice, for example =~/src/difftastic=.
2. Add the following lines to your Emacs configuration file:

#+begin_src emacs-lisp :results value silent (use-package difftastic :defer t :vc t :load-path "~/src/difftastic") #+end_src

Yet another option is to use any of the package managers that supports installation from GitHub or a an existing checkout. That could be =package-vc=, or any of package managers listed in [[#installing-from-melpa][Installing from MELPA]].

*** Manual Installation :properties: :custom_id: manual-installation :end: Note, that this method does not generate autoloads. As a consequence it will cause the whole package and it's dependencies (including =magit=) to be loaded at startup. If you want to avoid this, ensure autoloads set up on Emacs startup. See [[#installing-from-melpa][Installing from MELPA]] a few package managers that can generate autoloads when package is installed.

1. Clone this repository to a directory of your choice, for example =~/src/difftastic=.
2. Add the following line to your Emacs configuration file:

#+begin_src emacs-lisp :results value silent (add-to-list 'load-path "~/src/difftastic") (require 'difftastic) (require 'difftastic-bindings) #+end_src

** Configuration :properties: :custom_id: configuration :end: This section assumes you have =difftastic='s autoloads set up at Emacs startup. If you have installed =difftastic= using built-in =package= or =use-package= then you should be all set.

To configure =difftastic= commands in relevant =magit= prefixes and keymaps, use the following code snippet in your Emacs configuration:

#+begin_src emacs-lisp :results value silent (difftastic-bindings-mode) #+end_src

Or, if you use =use-package=:

#+begin_src emacs-lisp :results value silent (use-package difftastic-bindings :ensure difftastic ;; or nil if you prefer manual installation :config (difftastic-bindings-mode)) #+end_src

By default this will bind:

• M-d to =difftastic-magit-diff= and M-c to =difftastic-magit-show= in =magit-diff= prefix,
• M-RET to =difftastic-magit-show= in =magit-blame= transient prefix and in =magit-blame-read-only-map= keymap,
• M-d to =difftastic-magit-diff-buffer-file= in =magit-file-dispatch= prefix,
• M-= to =difftastic-dired-diff= in =dired-mode-map=.

Please refer to =difftastic-bindings-alist= documentation to see how to change default bindings. You need to toggle the =difftastic-bindings-mode= off and on again to apply the changes.

The =difftastic-bindings=mode= was designed to have minimal dependencies and be reasonably fast to load, while providing a mechanism to bind =difftastic= commands, such that they are available in relevant contexts.

*** Manual Key Bindings Configuration :properties: :custom_id: manual_key_Bindings_configuration :end:

If you don't want to use mechanism delivered by =difftastic-bindings-mode= you can write your own configuration. As a starting point the following snippets demonstrate how to achieve partial effect similar to the one provided by =difftastic-bindings-mode= in default configuration:

#+begin_src emacs-lisp :results value silent (require 'difftastic) (require 'transient)

(let ((suffix [("D" "Difftastic diff (dwim)" difftastic-magit-diff) ("S" "Difftastic show" difftastic-magit-show)])) (with-eval-after-load 'magit-diff (unless (equal (transient-parse-suffix 'magit-diff suffix) (transient-get-suffix 'magit-diff '(-1 -1))) (transient-append-suffix 'magit-diff '(-1 -1) suffix)))) (let ((suffix '("M-RET" "Difftastic show" difftastic-magit-show))) (with-eval-after-load 'magit-blame (unless (equal (transient-parse-suffix 'magit-blame suffix) (transient-get-suffix 'magit-blame "b")) (transient-append-suffix 'magit-blame "b" suffix)) (keymap-set magit-blame-read-only-mode-map "M-RET" #'difftastic-magit-show))) #+end_src

Or, if you use =use-package=:

#+begin_src emacs-lisp :results value silent (use-package difftastic :defer t :init (use-package transient ; to silence compiler warnings :autoload (transient-get-suffix transient-parse-suffix))

(use-package magit-blame
  :defer t :ensure magit
  :bind
  (:map magit-blame-read-only-mode-map
        ("M-RET" . #'difftastic-magit-show))
  :config
  (let ((suffix '("M-RET" "Difftastic show" difftastic-magit-show)))
    (unless (equal (transient-parse-suffix 'magit-blame suffix)
                   (transient-get-suffix 'magit-blame "b"))
      (transient-append-suffix 'magit-blame "b" suffix)))
(use-package magit-diff
  :defer t :ensure magit
  :config
  (let ((suffix [("M-d" "Difftastic diff (dwim)" difftastic-magit-diff)
                 ("M-c" "Difftastic show" difftastic-magit-show)]))
    (unless (equal (transient-parse-suffix 'magit-diff suffix)
                   (transient-get-suffix 'magit-diff '(-1 -1)))
      (transient-append-suffix 'magit-diff '(-1 -1) suffix)))))

#+end_src

** Usage :properties: :custom_id: usage :end:

*** General Usage :properties: :custom_id: general-usage :end:

The following commands and functions are meant to help invoking =difftastic= depending on context and desired outcome.

• =difftastic-magit-diff= - show the result of =git diff ARGS -- FILES= with =difftastic=. This is the main entry point for DWIM action, so it tries to guess revision or range.
• =difftastic-magit-show= - show the result of =git show ARG= with =difftastic=. It tries to guess =ARG=, and ask for it when can't. When called with prefix argument it will ask for =ARG=.
• =difftastic-magit-diff-buffer-file= - show diff for the blob or file visited in the current buffer with =difftastic=. When the buffer visits a blob, then show the respective commit. When the buffer visits a file, then show the differences between HEAD and the working tree.
• =difftastic-forge-pullreq-show-diff= - show the result of =git diff BASE...HEAD= with =difftastic=. When buffer is a =forge= pull request buffer, of point is at a pull-request, then show diff for that pull request. Otherwise, ask for pull request branches to compare.
• =difftastic-forge-create-pulreq-show-diff= - show diff for a new pull request. This has been designed to be used in =forge-edit-post-hook=.
• =difftastic-files= - show the result of =difft FILE-A FILE-B=. When called with prefix argument it will ask for language to use, instead of relaying on =difftastic='s detection mechanism.
• =difftastic-buffers= - show the result of =difft BUFFER-A BUFFER-B=. Language is guessed based on buffers modes. When called with prefix argument it will ask for language to use.
• =difftastic-file-bufer= - show the result of =difft BUFFER BUFFER-FILE. Language is guessed based on buffer mode. When called with prefix argument it will ask for language to use.
• =difftastic-dired-diff= - same as =dired-diff=, but with =difftastic-files= instead of the built-in =diff=.
• =difftastic-git-diff-range= - transform =ARGS= for difftastic and show the result of =git diff ARGS REV-OR-RANGE -- FILES= with =difftastic=.

*** Specifying =difftastic= Arguments :properties: :custom_id: specifying-difftastic-arguments :end:

All [[#general-usage][above]] commands (and =difftastic-rerun= described [[#difftastic-mode-commands][below]]) support specification of =difft= arguments. When a command is called with a double prefix argument a popup is presented allowing to specify desired arguments. This is in addition to a command specific handling of a single prefix argument.

In order to aid arguments entry and provide similarity to workflows in =magit= and =forge=, a =transient= prefix is used for the popup. For example, some - less commonly used - arguments are not visible in default configuration. Type C-x l in the menu to make them visible. Type C-h C-h for =difftastic= help (=man difft=). Any other =transient= commands should work as well.

When =difftastic-use-transient-arguments= is non-nil, and extra difftastic arguments were saved in =transient= (as described in Info node =(transient)Saving Values=), these values will be used for future =difft= invocations. However, when there's a need to tune these arguments for a specific =difft= call, a difftastic command can be called with double prefix argument to bring a popup allowing to specify arguments. Any arguments changed in the popup (that were not saved) will be used for the following =difft= invocation only.

Note that in some cases arguments will take precedence over standard and computed values, for example =--width= is one such a argument.

*** =difftastic-mode= Commands :properties: :custom_id: difftastic-mode-commands :end: When a buffer shows =difftastic= output the following commands can be used. Commands are followed by their default keybindings (in parenthesis).

• =difftastic-rerun= (g) - rerun difftastic for the current buffer. It runs difftastic again in the current buffer, but respects the window configuration. It uses =difftastic-rerun-requested-window-width-function= which, by default, returns current window width (instead of =difftastic-requested-window-width-function=). It will also reuse current buffer and will not call =difftastic-display-buffer-function=. When called with prefix argument it will ask for language to use.
• =difftastic-next-chunk= (n), =difftastic-next-file= (N or M-n) - move point to a next logical chunk or a next file respectively.
• =difftastic-previous-chunk= (p), =difftastic-previous-file= (P or M-p) - move point to a previous logical chunk or a previous file respectively.
• =difftastic-toggle-chunk= (TAB or C-i) - toggle visibility of a chunk at point. The point has to be in a chunk header. When called with a prefix toggle all file chunks from the header to the end of the file. See also =difftastic-hide-chunk= and =difftastic-show-chunk=.
• =forward-sexp= (C-M-f) - move point to end of current chunk or to an end of next chunk when point is already at the end of the chunk. When called with argument move by that many chunks. Binding is from a default =global-map=.
• =backward-sexp= (C-M-b) - move point to beginning of current chunk or to a beginning of previous chunk when point is already at the beginning of the chunk. When called with argument move by that many chunks. Binding is from a default =global-map=.
• =mark-sexp= (C-M-SPC) - set mark and move point to end of current chunk or to an end of next chunk when point is already at the end of the chunk. When called with argument move by that many chunks. Binding is from a default =global-map=.
• =difftastic-diff-visit-file= (RET), =difftastic-diff-visit-file-other-window=, =difftastic-diff-visit-file-other-frame= - from a diff visit appropriate version of a chunk file. This has been modeled after =magit-diff-visit-file=, but there are some differences, please see documentation for =difftastic-diff-visit-file=.
• =difftastic-diff-visit-worktree-file= (C-RET, C-j), =difftastic-diff-visit-worktree-file-other-window=, =difftastic-diff-visit-worktree-file-other-frame= - from a diff visit appropriate version of a chunk file. This has been modeled after =magit-diff-visit-worktree-file=, but there are some differences, please see documentation for =difftastic-diff-visit-worktree-file=.

** Customization :properties: :custom_id: customization :end: *** Face Customization :properties: :custom_id: face-customization :end: You can customize the appearance of =difftastic= output by adjusting the faces used for highlighting. To customize a faces, use the following code snippet in your configuration:

#+begin_src emacs-lisp :results value silent ;; Customize faces used to display difftastic output. (setq difftastic-normal-colors-vector (vector ;; use black face from ansi-color' (aref ansi-color-normal-colors-vector 0) ;; use face for removed marker from difftastic' (aref difftastic-normal-colors-vector 1) ;; use face for added marker from difftastic' (aref difftastic-normal-colors-vector 2) 'my-section-face 'my-comment-face 'my-string-face 'my-warning-face ;; use white face from ansi-color' (aref ansi-color-normal-colors-vector 7)))

;; Customize highlight faces (setq difftastic-highlight-alist `((,(aref difftastic-normal-colors-vector 2) . my-added-highlight) (,(aref difftastic-normal-colors-vector 1) . my-removed-highlight)))

;; Disable highlight faces (use difftastic's default) (setq difftastic-highlight-alist nil) #+end_src

*** Window Management :properties: :custom_id: window-management :end: The =difftastic= relies on the =difft= command line tool to produce an output that can be displayed in an Emacs buffer window. In short: it runs the =difft=, converts ANSI codes into user defined colors and displays it in window. The =difft= can be instructed with a hint to help it produce a content that can fit into user output, by specifying a requested width. However, the latter is not always respected.

The =difftastic= provides a few variables to let you customize these aspects of interaction with =difft=:

• =difftastic-requested-window-width-function= - this function is called for a first (i.e., not a rerun) call to =difft=. It shall return the requested width of the output. For example this can be a half of a current frame (or a window) if the output is meant to be presented side by side.
• =difftastic-rerun-requested-window-width-function= - this function is called for a rerun (i.e., not a first) call to =difft=. It shall return requested window width of the output. For example this can be a current window width if the output is meant to fill the whole window.
• =difftastic-display-buffer-function= - this function is called after a first call to =difft=. It is meant to select an appropriate Emacs mechanism to display the =difft= output.

*** =difftastic-mode= Behavior :properties: :custom_id: difftastic-mode-behavior :end:

• =difftastic-visibility-indicator= - controls whether and how to show hidden/visible chunk/files.
• =difftastic-buttonize-urls= - controls whether to transform URLs into buttons in difftastic buffers.
• =difftastic-diff-visit-avoid-head-blob= - controls whether to avoid visiting blob of a =HEAD= revision when visiting file form a =difftastic-mode= buffer.

*** Process Environment :properties: :custom_id: process-environment :end: The =difftastic-difft-environment= variable allows users to customize the environment in which =difft= runs. This is particularly useful for temporary configurations via =let=-binding.

This variable is defined as a =defvar= (not =defcustom=) since it's primarily intended for programmatic use rather than permanent customization.

** Contributing :properties: :custom_id: contributing :end: Contributions are welcome! Feel free to submit issues and pull requests on the [[https://github.com/pkryger/difftastic.el][GitHub repository]].

*** Testing :properties: :custom_id: testing :end: When creating a pull request make sure all tests in [[file:test/difftastic.t.el]] are passing. When adding a new functionality, please strive to add tests for it as well.

This repository uses a [[https://github.com/pkryger/el-mock.el][custom fork of =el-mock=]] which has a few more features and some slight semantic differences. Please make sure you're using that fork before you report issues with tests.

To run tests interactively:

• open the [[file:test/difftastic.t.el]]
• type M-x eval-buffer
• open the [[file:test/difftastic-bindings.t.el]]
• type M-x eval-buffer
• type M-x ert t

Alternatively you can use [[https://github.com/cask/cask][Cask]] to run tests in batch mode. There's a convenience [[file:Makefile]] with a =test= target, so you can just type make test.

It seems that byte compilation interferres with [[https://github.com/rejeep/el-mock.el][el-mock]]. In order to get the tests to pass you may need to:

• type M-x eval-buffer in [[file:difftastic.el]] and in [[file:difftastic-bindings.el]] when running test interactively with M-x ert ,
• remove all .elc files in the development directory when running tests in batch mode.

This repository uses [[https://coveralls.io][Coveralls]] to track test coverage. After a PR has been approved for a Gighub Action run, a report will be published [[https://coveralls.io/github/pkryger/difftastic.el][Coveralls difftastic repo]]. Please check it out if there's no outstanding relevant lines.

You can run all checks performed by Github Actions, by typing: make bytecompile lint relint checkdoc commentary test.

*** Documentation Autoring :properties: :custom_id: documentation-authoring :end: This package uses the following packages to help with documentation authoring:

• [[https://github.com/pkryger/org-commentary.el][org-commentary.el]] (which is different from the one available on MELPA!) to generate and validate commentary section in =difftastic.el=. Please see the package documentation for usage instructions.

• [[https://github.com/alphapapa/org-make-toc][org-make-toc]] to generate and validate table of contents in the =README.org= file.

Appropriate functions from both of these packages are added to =after-save-hook= and =before-save-hook= respectively, when packages are available in user's Emacs (see =dir-locals.el=).

** Acknowledgments :noexport: :properties: :custom_id: acknowledgments :end: This package was inspired by the need for an integration of =difftastic= within Emacs, enhancing the code review process for developers.

This work is based on Tassilo Horn's [[https://tsdh.org/posts/2022-08-01-difftastic-diffing-with-magit.html][blog entry]].

=magit-diff= keybindings and a concept of updating faces comes from a Shiv Jha-Mathur's [[https://shivjm.blog/better-magit-diffs/][blog entry]].

This all has been strongly influenced by - a class in itself - [[https://github.com/magit/magit][Magit]] and [[https://github.com/magit/transient][Transient]] Emacs packages by Jonas Bernoulli.

** Similar Packages :noexport: :properties: :custom_id: similar-packages :end: *** Diff ANSI :properties: :custom_id: diff-ansi :end: There's a [[https://codeberg.org/ideasman42/emacs-diff-ansi][diff-ansi]] package available. I haven't spent much time on it, but at a first glance it doesn't seem that it supports =difftastic= out of box. Perhaps it is possible to configure it to support =difftastic= as a custom tool.

** License :noexport: :properties: :custom_id: license :end: This package is licensed under the [[https://www.gnu.org/licenses/gpl-3.0.en.html][GPLv3 License]].

---

Happy coding! If you encounter any issues or have suggestions for improvements, please don't hesitate to reach out on the [[https://github.com/pkryger/difftastic.el][GitHub repository]]. Your feedback is highly appreciated.

LocalWords: MELPA DWIM