Support for alejandra and nixpkgs-format

[TLATER]

Support for alejandra and nixpkgs-format

[fbergroth]

I'm using reformatter.el which is convenient: (reformatter-define nix-alejandra :program "alejandra") and then either (add-hook 'nix-mode-hook 'nix-alejandra-on-save-mode) or simply M-x nix-alejandra. Perhaps nix-format.el could use reformatter.

[TLATER]

Hm, that does indeed cover everything I'd like to do. It's even already in my config through elm-mode, so I guess I might just switch to that for now. I'd be very in favor of switching to reformat.el as a backend, it's cool that there is a small standardization effort here, but that's a bit off-topic - separate issue?

I'd still like to see support for more that nix-fmt, though. nixpkgs-fmt and alejandra are big improvements over nix-fmt, so it would be nice to expose new users to them through this package. I don't think we'll see too many new ones created either.

On the other hand, I realize it'd feel a bit unclean to have multiple formatting functions lying around - especially because it'll be hard to remove formatters that don't see much maintenance in the future. It might also be confusingly hard to tell which to use. The maintenance burden would be low here, though, especially if it's backed by reformat.el.

We could also wait for the RFC that is choosing one of them as the default for nixpkgs to resolve, and switch to whichever wins?

[mkaito]

The "official" Alejandra integration suggests https://github.com/lassik/emacs-format-all-the-code, which also supports nixpkgs-format.