Interface for using external diffing tools

[seanfarley]

Tassilo Horn wrote a fantastic blog post about getting difftastic output into magit. After some private discussion with them and others, I was wondering if we could open a discussion about formalizing an interface with magit so that external diff tools could be used as plugins. From the discussion, some design points came up:

• displaying hunks within the familiar diff sections list (to allow jumping around files and hunks)
• not relying (as the blog post does) on internal functions
• probably lots of other things that I forgot

This discussion is just to open up dialog about how welcomed an interface like this would be. Or perhaps this is part of a larger system of having a plugin api in general.

[verhovsky]

difftastic parses files and diffs abstract syntax trees (ASTs), so it can generate nicer diffs than the default of just diffing raw lines/characters.

I wanted to use difftastic system-wide by putting this in my ~/.gitconfig

[diff]
	external = difft

but this causes Magit's diff highlighting to stop working when looking at previous commits or stashes. Magit just displays difftastic's output as-is and doesn't show colors (which difftastic outputs). So it's unusable, I have to pick either Magit or difftastic.

[tarsius]

Use magit-git-global-arguments to disable settings in just Magit.

[verhovsky]

ChatGPT told me you're telling me to do this

(after! magit
  (setq magit-git-global-arguments
    (append magit-git-global-arguments '("--no-ext-diff"))))

but I get a "Corrupt Git configuration" when I open Doom Emacs.

[tarsius]

Not every git command accepts the --no-ext-diff argument -- those that do not deal with diffs at all, have no reason to do so.

As the doc-string points out, you can only use arguments that can be used in between git and <subcommand> and the valid arguments are listed in the git(1) manpage. Looking at the default value, you will notice that the -c argument is used a lot. So what you have to do now, is to figure out what variable=value corresponds to the --no-ext-diff argument. The git-diff(1) and git-config(1) are good places to start, one of them is bound to provide that information.

[tarsius]

I hope that this summer or fall I will finally get around to looking at difftastic more closely.