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Graham Christensen
2018-05-01 19:57:09 -04:00
parent 77161de454
commit eca5c99bf8
91 changed files with 9705 additions and 7852 deletions
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nixos/doc/manual/development/option-def.xml
+46 -59
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@@ -3,39 +3,36 @@
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
xml:id="sec-option-definitions">
<title>Option Definitions</title>
<title>Option Definitions</title>
<para>Option definitions are generally straight-forward bindings of values to option names, like
<para>
Option definitions are generally straight-forward bindings of values to
option names, like
<programlisting>
config = {
services.httpd.enable = true;
};
</programlisting>
However, sometimes you need to wrap an option definition or set of option
definitions in a <emphasis>property</emphasis> to achieve certain effects:
</para>
However, sometimes you need to wrap an option definition or set of
option definitions in a <emphasis>property</emphasis> to achieve
certain effects:</para>
<simplesect><title>Delaying Conditionals</title>
<para>If a set of option definitions is conditional on the value of
another option, you may need to use <varname>mkIf</varname>.
Consider, for instance:
<simplesect>
<title>Delaying Conditionals</title>
<para>
If a set of option definitions is conditional on the value of another
option, you may need to use <varname>mkIf</varname>. Consider, for instance:
<programlisting>
config = if config.services.httpd.enable then {
environment.systemPackages = [ <replaceable>...</replaceable> ];
<replaceable>...</replaceable>
} else {};
</programlisting>
This definition will cause Nix to fail with an “infinite recursion”
error. Why? Because the value of
<option>config.services.httpd.enable</option> depends on the value
being constructed here. After all, you could also write the clearly
circular and contradictory:
This definition will cause Nix to fail with an “infinite recursion”
error. Why? Because the value of
<option>config.services.httpd.enable</option> depends on the value being
constructed here. After all, you could also write the clearly circular and
contradictory:
<programlisting>
config = if config.services.httpd.enable then {
services.httpd.enable = false;
@@ -43,56 +40,49 @@ config = if config.services.httpd.enable then {
services.httpd.enable = true;
};
</programlisting>
The solution is to write:
The solution is to write:
<programlisting>
config = mkIf config.services.httpd.enable {
environment.systemPackages = [ <replaceable>...</replaceable> ];
<replaceable>...</replaceable>
};
</programlisting>
The special function <varname>mkIf</varname> causes the evaluation of
the conditional to be “pushed down” into the individual definitions,
as if you had written:
The special function <varname>mkIf</varname> causes the evaluation of the
conditional to be “pushed down” into the individual definitions, as if
you had written:
<programlisting>
config = {
environment.systemPackages = if config.services.httpd.enable then [ <replaceable>...</replaceable> ] else [];
<replaceable>...</replaceable>
};
</programlisting>
</para>
</simplesect>
</para>
</simplesect>
<simplesect><title>Setting Priorities</title>
<para>A module can override the definitions of an option in other
modules by setting a <emphasis>priority</emphasis>. All option
definitions that do not have the lowest priority value are discarded.
By default, option definitions have priority 1000. You can specify an
explicit priority by using <varname>mkOverride</varname>, e.g.
<simplesect>
<title>Setting Priorities</title>
<para>
A module can override the definitions of an option in other modules by
setting a <emphasis>priority</emphasis>. All option definitions that do not
have the lowest priority value are discarded. By default, option definitions
have priority 1000. You can specify an explicit priority by using
<varname>mkOverride</varname>, e.g.
<programlisting>
services.openssh.enable = mkOverride 10 false;
</programlisting>
This definition causes all other definitions with priorities above 10 to be
discarded. The function <varname>mkForce</varname> is equal to
<varname>mkOverride 50</varname>.
</para>
</simplesect>
This definition causes all other definitions with priorities above 10
to be discarded. The function <varname>mkForce</varname> is
equal to <varname>mkOverride 50</varname>.</para>
</simplesect>
<simplesect><title>Merging Configurations</title>
<para>In conjunction with <literal>mkIf</literal>, it is sometimes
useful for a module to return multiple sets of option definitions, to
be merged together as if they were declared in separate modules. This
can be done using <varname>mkMerge</varname>:
<simplesect>
<title>Merging Configurations</title>
<para>
In conjunction with <literal>mkIf</literal>, it is sometimes useful for a
module to return multiple sets of option definitions, to be merged together
as if they were declared in separate modules. This can be done using
<varname>mkMerge</varname>:
<programlisting>
config = mkMerge
[ # Unconditional stuff.
@@ -104,9 +94,6 @@ config = mkMerge
})
];
</programlisting>
</para>
</simplesect>
</section>
</para>
</simplesect>
</section>