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

[Shea Levy]

Sure! In my .emacs, I have:

> (defun nix-buffer-find-file-hook ()
>   (when (not (file-remote-p (buffer-file-name)))
>     (nix-buffer)))
> (add-hook 'find-file-hook 'nix-buffer-find-file-hook)

I have coq 8.5pl1 installed in my configuration.nix, but I have a coq
project that requires 8.4pl6, so I have in that project's root directory
dir-locals.nix:

> let pkgs = import <nixpkgs> {}; in
>   pkgs.nixBufferBuilders.withPackages [ pkgs.coq ]

(note that this relies on a recent nixpkgs commit[1]). Then whenever I
open a .v file in the project, ProofGeneral uses coq 8.4 rather than
8.5.

If I wanted, my dir-locals.nix could take a 'root' argument, which would
point to the file I opened.

~Shea

[1]: https://github.com/NixOS/nixpkgs/commit/05c132486d8cfae600bbfe8c9ac5d799b298afed

Bas van Dijk <v.dijk.bas at gmail.com> writes:

> Hi Shea,
>
> As a Nix and Emacs user this looks very interesting. I didn't know about
> Buffer-Local Variables before.
>
> Can you share some applications of nix-buffer / how you are using it?
>
> Thanks!
>
> Bas
>
> On 5 September 2016 at 08:06, Shea Levy <shea at shealevy.com> wrote:
>
>> 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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>> nix-dev at lists.science.uu.nl
>> http://lists.science.uu.nl/mailman/listinfo/nix-dev
>>
>>
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