[Nix-dev] Announcing nix-buffer, nix-shell for emacs

[Shea Levy]

Hi all,

I've just pushed the initial version of nix-buffer[1], and opened a PR
for adding it to melpa[2]. The description of the 'nix-buffer-enter'
command this library defines:

> Set up the buffer according to the directory-local nix expression.
>
> Looks for dir-locals.nix upward from the current directory.  If found,
> builds the derivation defined there with the 'root' arg set to the
> current buffer file name or directory and evaluates the resulting
> elisp if safe to do so.
>
> Because in practice dir-locals.nix will always want to do things that
> are unsafe in dir-locals.el (e.g. append to 'exec-path'), we don't
> reuse that mechanism and instead just load the file as elisp.  Because
> this allows arbitrary code execution, the first time we're asked to
> load a particular store path we query the user to verify if it's safe
> to load beforehand.
>
> The Lisp code generated by dir-locals.nix should limit itself to
> modifying buffer-local variables, but there is no actual enforcement
> of this.  'setq-local' is your friend.

It may be useful to advise the 'normal-mode' function to call
'nix-buffer-enter' beforehand for local files, so that all visited files
have their environments set up before modes are loaded.

Note that this is my first elisp package, so feedback definitely
welcome!

In addition to the elisp work mentioned in the TODO[3], it would be
useful to add some functions to nixpkgs to build elisp files,
especially:

* A function taking a derivation and building an elisp file that sets up
  the buffer environment analogous to the build environment specified by
  that derivation
* A function taking a list of packages and building an elisp file that
  sets up the buffer environment with all of those packages available
  (in PATH, in the emacs load path, etc.).

Happy hacking!

~Shea

[1]: https://github.com/shlevy/nix-buffer/tree/v1.0
[2]: https://github.com/melpa/melpa/pull/4204
[3]: https://github.com/shlevy/nix-buffer/blob/v1.0/TODO.md
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