[Nix-dev] Anyone against giving the nixpkgs attrset a reference to itself?

[Nicolas Pierron]

Hi Mathijs,

On Mon, Jul 16, 2012 at 12:32 AM, Mathijs Kwik <mathijs at bluescreen303.nl> wrote:
> I would like to add the final nixpkgs attrset (defaults + overrides)
> to itself (named finalPkgs).
> As this is a somewhat strange thing to do, I thought it would be best
> to ask first :)

strange ?  No, this is the base principle of NixOS.  In fact I would
almost recommend you to use a similar idea because I think we should
get rid of __override.

Ideally, all Nixpkgs files should look like [2].

> Extra bonuspoints:
> Currently, uncommenting the "with" statement on line 3 of [2] leads to
> "infinite recursion detected".
> Can anyone find out why? I was under the impression that "with" just
> brought stuff into scope, so as long as I don't use anything that
> would recurse to bottom, laziness should save me.

The way names are resolve by the with statement implies that we know
every attribute of the with statement.  Unfortunately, this is only
possible when the update operator (//) is finished, which implies
knowing all attributes of the left-hand-side and the right-hand-side.

So when you write:

let
  set1 = pkgs: { a = ...; b = ...; };
  set2 = pkgs: with pkgs; { c = ...; d = ...; };
in
  (import <nixpkgs/pkgs/lib>).fix (pkgs: (set1 pkgs // set2 pkgs))

You will cause an infinite recursion because you the with statement in
set2 need the results of the update (//) to finish.

    with { attr = <LAZY> }

By the way, I will recommend to fold with  recursiveUpdateUntil  with
a predicate which throw if one of the operand is a derivation.

with (import <nixpkgs/pkgs/lib>);

let
  noConflictUpdate = path: l: r:
    if isDerivation l || isDerivation r then
      throw "Cannot merge a derivation with anything else."
    else
       false;

  pkgsMerge = groupOfPackages:
    fix (pkgs: fold (recursiveUpdateUntil noConflictUpdate) {} groupOfPackages);
in
  pkgsMerge [
    (import ./emacs.nix)
    (import ./kde.nix)
  ]

Then we can extend this list with various index files either search at
default locations, given as arguments or listed in an environment
variable.

[1] https://github.com/bluescreen303/nix-config/blob/master/bluepkgs/default.nix
[2] https://github.com/bluescreen303/nix-config/blob/master/bluepkgs/extra.nix

-- 
Nicolas Pierron
http://www.linkedin.com/in/nicolasbpierron - http://nbp.name/