nixpkgs/nixos/doc/manual/configuration/modularity.xml

<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="sec-modularity">
 <title>Modularity</title>

 <para>
  The NixOS configuration mechanism is modular. If your
  <filename>configuration.nix</filename> becomes too big, you can split it into
  multiple files. Likewise, if you have multiple NixOS configurations (e.g. for
  different computers) with some commonality, you can move the common
  configuration into a shared file.
 </para>

 <para>
  Modules have exactly the same syntax as
  <filename>configuration.nix</filename>. In fact,
  <filename>configuration.nix</filename> is itself a module. You can use other
  modules by including them from <filename>configuration.nix</filename>, e.g.:
<programlisting>
{ config, pkgs, ... }:

{ imports = [ ./vpn.nix ./kde.nix ];
  <xref linkend="opt-services.httpd.enable"/> = true;
  <xref linkend="opt-environment.systemPackages"/> = [ pkgs.emacs ];
  <replaceable>...</replaceable>
}
</programlisting>
  Here, we include two modules from the same directory,
  <filename>vpn.nix</filename> and <filename>kde.nix</filename>. The latter
  might look like this:
<programlisting>
{ config, pkgs, ... }:

{ <xref linkend="opt-services.xserver.enable"/> = true;
  <xref linkend="opt-services.xserver.displayManager.sddm.enable"/> = true;
  <xref linkend="opt-services.xserver.desktopManager.plasma5.enable"/> = true;
}
</programlisting>
  Note that both <filename>configuration.nix</filename> and
  <filename>kde.nix</filename> define the option
  <xref linkend="opt-environment.systemPackages"/>. When multiple modules
  define an option, NixOS will try to <emphasis>merge</emphasis> the
  definitions. In the case of <xref linkend="opt-environment.systemPackages"/>,
  that’s easy: the lists of packages can simply be concatenated. The value in
  <filename>configuration.nix</filename> is merged last, so for list-type
  options, it will appear at the end of the merged list. If you want it to
  appear first, you can use <varname>mkBefore</varname>:
<programlisting>
<xref linkend="opt-boot.kernelModules"/> = mkBefore [ "kvm-intel" ];
</programlisting>
  This causes the <literal>kvm-intel</literal> kernel module to be loaded
  before any other kernel modules.
 </para>

 <para>
  For other types of options, a merge may not be possible. For instance, if two
  modules define <xref linkend="opt-services.httpd.adminAddr"/>,
  <command>nixos-rebuild</command> will give an error:
<screen>
The unique option `services.httpd.adminAddr' is defined multiple times, in `/etc/nixos/httpd.nix' and `/etc/nixos/configuration.nix'.
</screen>
  When that happens, it’s possible to force one definition take precedence
  over the others:
<programlisting>
<xref linkend="opt-services.httpd.adminAddr"/> = pkgs.lib.mkForce "bob@example.org";
</programlisting>
 </para>

 <para>
  When using multiple modules, you may need to access configuration values
  defined in other modules. This is what the <varname>config</varname> function
  argument is for: it contains the complete, merged system configuration. That
  is, <varname>config</varname> is the result of combining the configurations
  returned by every module
  <footnote xml:id="footnote-nix-is-lazy">
   <para>
    If you’re wondering how it’s possible that the (indirect)
    <emphasis>result</emphasis> of a function is passed as an
    <emphasis>input</emphasis> to that same function: that’s because Nix is a
    “lazy” language — it only computes values when they are needed. This
    works as long as no individual configuration value depends on itself.
   </para>
  </footnote>
  . For example, here is a module that adds some packages to
  <xref linkend="opt-environment.systemPackages"/> only if
  <xref linkend="opt-services.xserver.enable"/> is set to
  <literal>true</literal> somewhere else:
<programlisting>
{ config, pkgs, ... }:

{ <xref linkend="opt-environment.systemPackages"/> =
    if config.<xref linkend="opt-services.xserver.enable"/> then
      [ pkgs.firefox
        pkgs.thunderbird
      ]
    else
      [ ];
}
</programlisting>
 </para>

 <para>
  With multiple modules, it may not be obvious what the final value of a
  configuration option is. The command <option>nixos-option</option> allows you
  to find out:
<screen>
$ nixos-option <xref linkend="opt-services.xserver.enable"/>
true

$ nixos-option <xref linkend="opt-boot.kernelModules"/>
[ "tun" "ipv6" "loop" <replaceable>...</replaceable> ]
</screen>
  Interactive exploration of the configuration is possible using
  <command
xlink:href="https://github.com/edolstra/nix-repl">nix-repl</command>,
  a read-eval-print loop for Nix expressions. It’s not installed by default;
  run <literal>nix-env -i nix-repl</literal> to get it. A typical use:
<screen>
$ nix-repl '&lt;nixpkgs/nixos>'

nix-repl> config.<xref linkend="opt-networking.hostName"/>
"mandark"

nix-repl> map (x: x.hostName) config.<xref linkend="opt-services.httpd.virtualHosts"/>
[ "example.org" "example.gov" ]
</screen>
 </para>
</section>