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

[zimbatm]

Hi Shea,

did you look at direnv[1] yet? I have a feeling that it would mix with
emacs pretty well and implement all that you need in a more generic way
(which is a pro and con).

Essentially what I do in my projects is use the following .envrc:
```bash
use nix
```
The `use nix` directive calls nix-shell under the hood so then the usual
default.nix and shell.nix are loaded.

In emacs the plugin would need to call `direnv export json` on enter/leave
of a context. This gives you a diff of environment variables to apply.
There is a bit more complexity when implementing the security framework but
that's about it.

I'm just curious if you thought of that approach, keep up the good work!

Cheers,
z

[1]: http://direnv.net/

On Mon, 5 Sep 2016 at 17:11 Shea Levy <shea at shealevy.com> wrote:

> 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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> >>
> >>
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