···

       
30
       
30
       
 
       


     

       
31
       
31
       
 
       
Sets can be nested, and in fact dots in option names are shorthand for

     

       
32
       
32
       
 
       
defining a set containing another set. For instance,

     

       
33
       
33
       
-
       
[`services.httpd.enable`](options.html#opt-services.httpd.enable) defines a set named

     

       
33
       
33
       
+
       
[](#opt-services.httpd.enable) defines a set named

     

       
34
       
34
       
 
       
`services` that contains a set named `httpd`, which in turn contains an

     

       
35
       
35
       
 
       
option definition named `enable` with value `true`. This means that the

     

       
36
       
36
       
 
       
example above can also be written as:

     

···

       
34
       
34
       
 
       
```

     

       
35
       
35
       
 
       


     

       
36
       
36
       
 
       
Note that both `configuration.nix` and `kde.nix` define the option

     

       
37
       
37
       
-
       
[`environment.systemPackages`](options.html#opt-environment.systemPackages). When multiple modules define an

     

       
37
       
37
       
+
       
[](#opt-environment.systemPackages). When multiple modules define an

     

       
38
       
38
       
 
       
option, NixOS will try to *merge* the definitions. In the case of

     

       
39
       
39
       
-
       
[`environment.systemPackages`](options.html#opt-environment.systemPackages), that's easy: the lists of

     

       
39
       
39
       
+
       
[](#opt-environment.systemPackages), that's easy: the lists of

     

       
40
       
40
       
 
       
packages can simply be concatenated. The value in `configuration.nix` is

     

       
41
       
41
       
 
       
merged last, so for list-type options, it will appear at the end of the

     

       
42
       
42
       
 
       
merged list. If you want it to appear first, you can use `mkBefore`:

     
···

       
49
       
49
       
 
       
kernel modules.

     

       
50
       
50
       
 
       


     

       
51
       
51
       
 
       
For other types of options, a merge may not be possible. For instance,

     

       
52
       
52
       
-
       
if two modules define [`services.httpd.adminAddr`](options.html#opt-services.httpd.adminAddr),

     

       
52
       
52
       
+
       
if two modules define [](#opt-services.httpd.adminAddr),

     

       
53
       
53
       
 
       
`nixos-rebuild` will give an error:

     

       
54
       
54
       
 
       


     

       
55
       
55
       
 
       
```plain

     
···

       
68
       
68
       
 
       
for: it contains the complete, merged system configuration. That is,

     

       
69
       
69
       
 
       
`config` is the result of combining the configurations returned by every

     

       
70
       
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module [^1] . For example, here is a module that adds some packages to

     

       
71
       
71
       
-
       
[`environment.systemPackages`](options.html#opt-environment.systemPackages) only if

     

       
72
       
72
       
-
       
[`services.xserver.enable`](options.html#opt-services.xserver.enable) is set to `true` somewhere else:

     

       
71
       
71
       
+
       
[](#opt-environment.systemPackages) only if

     

       
72
       
72
       
+
       
[](#opt-services.xserver.enable) is set to `true` somewhere else:

     

       
73
       
73
       
 
       


     

       
74
       
74
       
 
       
```nix

     

       
75
       
75
       
 
       
{ config, pkgs, ... }:

     

···

       
37
       
37
       
 
       
  <para>

     

       
38
       
38
       
 
       
    Sets can be nested, and in fact dots in option names are shorthand

     

       
39
       
39
       
 
       
    for defining a set containing another set. For instance,

     

       
40
       
40
       
-
       
    <link xlink:href="options.html#opt-services.httpd.enable"><literal>services.httpd.enable</literal></link>

     

       
41
       
41
       
-
       
    defines a set named <literal>services</literal> that contains a set

     

       
42
       
42
       
-
       
    named <literal>httpd</literal>, which in turn contains an option

     

       
40
       
40
       
+
       
    <xref linkend="opt-services.httpd.enable" /> defines a set named

     

       
41
       
41
       
+
       
    <literal>services</literal> that contains a set named

     

       
42
       
42
       
+
       
    <literal>httpd</literal>, which in turn contains an option

     

       
43
       
43
       
 
       
    definition named <literal>enable</literal> with value

     

       
44
       
44
       
 
       
    <literal>true</literal>. This means that the example above can also

     

       
45
       
45
       
 
       
    be written as:

     

···

       
40
       
40
       
 
       
  <para>

     

       
41
       
41
       
 
       
    Note that both <literal>configuration.nix</literal> and

     

       
42
       
42
       
 
       
    <literal>kde.nix</literal> define the option

     

       
43
       
43
       
-
       
    <link xlink:href="options.html#opt-environment.systemPackages"><literal>environment.systemPackages</literal></link>.

     

       
44
       
44
       
-
       
    When multiple modules define an option, NixOS will try to

     

       
43
       
43
       
+
       
    <xref linkend="opt-environment.systemPackages" />. When multiple

     

       
44
       
44
       
+
       
    modules define an option, NixOS will try to

     

       
45
       
45
       
 
       
    <emphasis>merge</emphasis> the definitions. In the case of

     

       
46
       
46
       
-
       
    <link xlink:href="options.html#opt-environment.systemPackages"><literal>environment.systemPackages</literal></link>,

     

       
47
       
47
       
-
       
    that’s easy: the lists of packages can simply be concatenated. The

     

       
48
       
48
       
-
       
    value in <literal>configuration.nix</literal> is merged last, so for

     

       
46
       
46
       
+
       
    <xref linkend="opt-environment.systemPackages" />, that’s easy: the

     

       
47
       
47
       
+
       
    lists of packages can simply be concatenated. The value in

     

       
48
       
48
       
+
       
    <literal>configuration.nix</literal> is merged last, so for

     

       
49
       
49
       
 
       
    list-type options, it will appear at the end of the merged list. If

     

       
50
       
50
       
 
       
    you want it to appear first, you can use

     

       
51
       
51
       
 
       
    <literal>mkBefore</literal>:

     
···

       
60
       
60
       
 
       
  <para>

     

       
61
       
61
       
 
       
    For other types of options, a merge may not be possible. For

     

       
62
       
62
       
 
       
    instance, if two modules define

     

       
63
       
63
       
-
       
    <link xlink:href="options.html#opt-services.httpd.adminAddr"><literal>services.httpd.adminAddr</literal></link>,

     

       
63
       
63
       
+
       
    <xref linkend="opt-services.httpd.adminAddr" />,

     

       
64
       
64
       
 
       
    <literal>nixos-rebuild</literal> will give an error:

     

       
65
       
65
       
 
       
  </para>

     

       
66
       
66
       
 
       
  <programlisting>

     
···

       
89
       
89
       
 
       
        configuration value depends on itself.

     

       
90
       
90
       
 
       
      </para>

     

       
91
       
91
       
 
       
    </footnote> . For example, here is a module that adds some packages

     

       
92
       
92
       
-
       
    to

     

       
93
       
93
       
-
       
    <link xlink:href="options.html#opt-environment.systemPackages"><literal>environment.systemPackages</literal></link>

     

       
94
       
94
       
-
       
    only if

     

       
95
       
95
       
-
       
    <link xlink:href="options.html#opt-services.xserver.enable"><literal>services.xserver.enable</literal></link>

     

       
96
       
96
       
-
       
    is set to <literal>true</literal> somewhere else:

     

       
92
       
92
       
+
       
    to <xref linkend="opt-environment.systemPackages" /> only if

     

       
93
       
93
       
+
       
    <xref linkend="opt-services.xserver.enable" /> is set to

     

       
94
       
94
       
+
       
    <literal>true</literal> somewhere else:

     

       
97
       
95
       
 
       
  </para>

     

       
98
       
96
       
 
       
  <programlisting language="bash">

     

       
99
       
97
       
 
       
{ config, pkgs, ... }: