From 1441806c27f3775db580360e072ead2afbe00ae9 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Thu, 7 Dec 2017 20:30:45 +0100
Subject: [PATCH] nixpkgs: add nixos function

---
 nixos/doc/manual/release-notes/rl-1809.xml | 14 +++++++
 pkgs/top-level/all-packages.nix            | 46 ++++++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/nixos/doc/manual/release-notes/rl-1809.xml b/nixos/doc/manual/release-notes/rl-1809.xml
index 61f9ec8ba995..29abea1afd6a 100644
--- a/nixos/doc/manual/release-notes/rl-1809.xml
+++ b/nixos/doc/manual/release-notes/rl-1809.xml
@@ -77,6 +77,20 @@ following incompatible changes:</para>
 <itemizedlist>
   <listitem>
     <para>
+      The <literal>pkgs</literal> argument to NixOS modules can now be set directly using <literal>nixpkgs.pkgs</literal>. Previously, only the <literal>system</literal>, <literal>config</literal> and <literal>overlays</literal> arguments could be used to influence <literal>pkgs</literal>.
+    </para>
+  </listitem>
+  <listitem>
+    <para>
+      A NixOS system can now be constructed more easily based on a preexisting invocation of Nixpkgs. For example:
+      <programlisting>
+inherit (pkgs.nixos {
+  boot.loader.grub.enable = false;
+  fileSystems."/".device = "/dev/xvda1";
+}) toplevel kernel initialRamdisk manual;
+      </programlisting>
+
+      This benefits evaluation performance, lets you write Nixpkgs packages that depend on NixOS images and is consistent with a deployment architecture that would be centered around Nixpkgs overlays.
     </para>
   </listitem>
 </itemizedlist>
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index 86500ece4610..1a4fa144708e 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -20690,6 +20690,52 @@ with pkgs;
 
   nixops-dns = callPackage ../tools/package-management/nixops/nixops-dns.nix { };
 
+  /*
+   * Evaluate a NixOS configuration using this evaluation of Nixpkgs.
+   *
+   * With this function you can write, for example, a package that
+   * depends on a custom virtual machine image.
+   *
+   * Parameter: A module, path or list of those that represent the
+   *            configuration of the NixOS system to be constructed.
+   *
+   * Result:    An attribute set containing packages produced by this
+   *            evaluation of NixOS, such as toplevel, kernel and
+   *            initialRamdisk.
+   *            The result can be extended in the modules by defining
+   *            extra options in system.build.
+   *
+   * Unlike in plain NixOS, the nixpkgs.config, nixpkgs.overlays and
+   * nixpkgs.system options will be ignored by default. Instead,
+   * nixpkgs.pkgs will have the default value of pkgs as it was
+   * constructed right after invoking the nixpkgs function (e.g. the
+   * value of import <nixpkgs> { overlays = [./my-overlay.nix]; }
+   * but not the value of (import <nixpkgs> {} // { extra = ...; }).
+   *
+   * If you do want to use the config.nixpkgs options, you are
+   * probably better off by calling nixos/lib/eval-config.nix
+   * directly, even though it is possible to set config.nixpkgs.pkgs.
+   *
+   * For more information about writing NixOS modules, see
+   * https://nixos.org/nixos/manual/index.html#sec-writing-modules
+   *
+   * Note that you will need to have called Nixpkgs with the system
+   * parameter set to the right value for your deployment target.
+   */
+  nixos = configuration:
+    (import (self.path + "/nixos/lib/eval-config.nix") {
+      inherit (pkgs) system;
+      modules = [(
+                  { lib, ... }: {
+                    config.nixpkgs.pkgs = lib.mkDefault pkgs;
+                  }
+                )] ++ (
+                  if builtins.isList configuration
+                  then configuration
+                  else [configuration]
+                );
+    }).config.system.build;
+
   nixui = callPackage ../tools/package-management/nixui { node_webkit = nwjs_0_12; };
 
   nix-bundle = callPackage ../tools/package-management/nix-bundle { };