Add overlays mechanism to Nixpkgs.

This patch add a new argument to Nixpkgs default expression named "overlays".

By default, the value of the argument is either taken from the environment variable `NIXPKGS_OVERLAYS`,
or from the directory `~/.nixpkgs/overlays/`.  If the environment variable does not name a valid directory
then this mechanism would fallback on the home directory.  If the home directory does not exists it will
fallback on an empty list of overlays.

The overlays directory should contain the list of extra Nixpkgs stages which would be used to extend the
content of Nixpkgs, with additional set of packages.  The overlays, i-e directory, files, symbolic links
are used in alphabetical order.

The simplest overlay which extends Nixpkgs with nothing looks like:

```nix
self: super: {
}
```

More refined overlays can use `super` as the basis for building new packages, and `self` as a way to query
the final result of the fix-point.

An example of overlay which extends Nixpkgs with a small set of packages can be found at:
  https://github.com/nbp/nixpkgs-mozilla/blob/nixpkgs-overlay/moz-overlay.nix

To use this file, checkout the repository and add a symbolic link to
the `moz-overlay.nix` file in `~/.nixpkgs/overlays` directory.

1 files changed, 2 insertions, 62 deletions

diff --git a/doc/functions.xml b/doc/functions.xml
index f6a0a4352f63..6374c15ddf2b 100644
--- a/doc/functions.xml
+++ b/doc/functions.xml
@@ -17,66 +17,6 @@
     derivations or even the whole package set.
   </para>
 
-  <section xml:id="sec-pkgs-overridePackages">
-    <title>pkgs.overridePackages</title>
-
-    <para>
-      This function inside the nixpkgs expression (<varname>pkgs</varname>)
-      can be used to override the set of packages itself.
-    </para>
-    <para>
-      Warning: this function is expensive and must not be used from within
-      the nixpkgs repository.
-    </para>
-    <para>
-      Example usage:
-
-      <programlisting>let
-    pkgs = import &lt;nixpkgs&gt; {};
-    newpkgs = pkgs.overridePackages (self: super: {
-      foo = super.foo.override { ... };
-    });
-  in ...</programlisting>
-    </para>
-
-    <para>
-      The resulting <varname>newpkgs</varname> will have the new <varname>foo</varname>
-      expression, and all other expressions depending on <varname>foo</varname> will also
-      use the new <varname>foo</varname> expression.
-    </para>
-
-    <para>
-      The behavior of this function is similar to <link 
-      linkend="sec-modify-via-packageOverrides">config.packageOverrides</link>.
-    </para>
-
-    <para>
-      The <varname>self</varname> parameter refers to the final package set with the
-      applied overrides. Using this parameter may lead to infinite recursion if not
-      used consciously.
-    </para>
-
-    <para>
-      The <varname>super</varname> parameter refers to the old package set.
-      It's equivalent to <varname>pkgs</varname> in the above example.
-    </para>
-
-    <para>
-      Note that in previous versions of nixpkgs, this method replaced any changes from <link 
-      linkend="sec-modify-via-packageOverrides">config.packageOverrides</link>,
-      along with that from previous calls if this function was called repeatedly.
-      Now those previous changes will be preserved so this function can be "chained" meaningfully.
-      To recover the old behavior, make sure <varname>config.packageOverrides</varname> is unset,
-      and call this only once off a "freshly" imported nixpkgs:
-
-      <programlisting>let
-    pkgs = import &lt;nixpkgs&gt; { config: {}; };
-    newpkgs = pkgs.overridePackages ...;
-  in ...</programlisting>
-    </para>
-
-  </section>
-
   <section xml:id="sec-pkg-override">
     <title>&lt;pkg&gt;.override</title>
 
@@ -91,9 +31,9 @@
       Example usages:
 
       <programlisting>pkgs.foo.override { arg1 = val1; arg2 = val2; ... }</programlisting>
-      <programlisting>pkgs.overridePackages (self: super: {
+      <programlisting>import pkgs.path { overlays = [ (self: super: {
     foo = super.foo.override { barSupport = true ; };
-  })</programlisting>
+  })]};</programlisting>
       <programlisting>mypkg = pkgs.callPackage ./mypkg.nix {
     mydep = pkgs.mydep.override { ... };
   }</programlisting>