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<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-replace-modules">
<title>Replace Modules</title>
<para>
Modules that are imported can also be disabled. The option declarations,
config implementation and the imports of a disabled module will be ignored, allowing another
to take it's place. This can be used to import a set of modules from another
channel while keeping the rest of the system on a stable release.
</para>
<para>
<literal>disabledModules</literal> is a top level attribute like
<literal>imports</literal>, <literal>options</literal> and
<literal>config</literal>. It contains a list of modules that will be
disabled. This can either be the full path to the module or a string with the
filename relative to the modules path (eg. <nixpkgs/nixos/modules> for
nixos).
</para>
<para>
This example will replace the existing postgresql module with the version
defined in the nixos-unstable channel while keeping the rest of the modules
and packages from the original nixos channel. This only overrides the module
definition, this won't use postgresql from nixos-unstable unless explicitly
configured to do so.
</para>
<programlisting>
{ config, lib, pkgs, ... }:
{
disabledModules = [ "services/databases/postgresql.nix" ];
imports =
[ # Use postgresql service from nixos-unstable channel.
# sudo nix-channel --add http://nixos.org/channels/nixos-unstable nixos-unstable
<nixos-unstable/nixos/modules/services/databases/postgresql.nix>
];
services.postgresql.enable = true;
}
</programlisting>
<para>
This example shows how to define a custom module as a replacement for an
existing module. Importing this module will disable the original module
without having to know it's implementation details.
</para>
<programlisting>
{ config, lib, pkgs, ... }:
with lib;
let
cfg = config.programs.man;
in
{
disabledModules = [ "services/programs/man.nix" ];
options = {
programs.man.enable = mkOption {
type = types.bool;
default = true;
description = "Whether to enable manual pages.";
};
};
config = mkIf cfg.enabled {
warnings = [ "disabled manpages for production deployments." ];
};
}
</programlisting>
</section>
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