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nixos: nixos/doc/manual/development/option-def.xml to CommonMark

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91
nixos/doc/manual/development/option-def.section.md Normal file
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@ -0,0 +1,91 @@
# Option Definitions {#sec-option-definitions}
Option definitions are generally straight-forward bindings of values to
option names, like
```nix
config = {
services.httpd.enable = true;
};
```
However, sometimes you need to wrap an option definition or set of
option definitions in a *property* to achieve certain effects:
## Delaying Conditionals {#sec-option-definitions-delaying-conditionals .unnumbered}
If a set of option definitions is conditional on the value of another
option, you may need to use `mkIf`. Consider, for instance:
```nix
config = if config.services.httpd.enable then {
environment.systemPackages = [ ... ];
...
} else {};
```
This definition will cause Nix to fail with an "infinite recursion"
error. Why? Because the value of `config.services.httpd.enable` depends
on the value being constructed here. After all, you could also write the
clearly circular and contradictory:
```nix
config = if config.services.httpd.enable then {
services.httpd.enable = false;
} else {
services.httpd.enable = true;
};
```
The solution is to write:
```nix
config = mkIf config.services.httpd.enable {
environment.systemPackages = [ ... ];
...
};
```
The special function `mkIf` causes the evaluation of the conditional to
be "pushed down" into the individual definitions, as if you had written:
```nix
config = {
environment.systemPackages = if config.services.httpd.enable then [ ... ] else [];
...
};
```
## Setting Priorities {#sec-option-definitions-setting-priorities .unnumbered}
A module can override the definitions of an option in other modules by
setting a *priority*. 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
`mkOverride`, e.g.
```nix
services.openssh.enable = mkOverride 10 false;
```
This definition causes all other definitions with priorities above 10 to
be discarded. The function `mkForce` is equal to `mkOverride 50`.
## Merging Configurations {#sec-option-definitions-merging .unnumbered}
In conjunction with `mkIf`, 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
`mkMerge`:
```nix
config = mkMerge
[ # Unconditional stuff.
{ environment.systemPackages = [ ... ];
}
# Conditional stuff.
(mkIf config.services.bla.enable {
environment.systemPackages = [ ... ];
})
];
```

99
nixos/doc/manual/development/option-def.xml
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@ -1,99 +0,0 @@
<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-option-definitions">
<title>Option Definitions</title>
<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>
<simplesect xml:id="sec-option-definitions-delaying-conditionals">
<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:
<programlisting>
config = if config.services.httpd.enable then {
services.httpd.enable = false;
} else {
services.httpd.enable = true;
};
</programlisting>
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:
<programlisting>
config = {
environment.systemPackages = if config.services.httpd.enable then [ <replaceable>...</replaceable> ] else [];
<replaceable>...</replaceable>
};
</programlisting>
</para>
</simplesect>
<simplesect xml:id="sec-option-definitions-setting-priorities">
<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>
<simplesect xml:id="sec-option-definitions-merging">
<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.
{ environment.systemPackages = [ <replaceable>...</replaceable> ];
}
# Conditional stuff.
(mkIf config.services.bla.enable {
environment.systemPackages = [ <replaceable>...</replaceable> ];
})
];
</programlisting>
</para>
</simplesect>
</section>

2
nixos/doc/manual/development/writing-modules.xml
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@ -181,7 +181,7 @@ in {
</example>
<xi:include href="../from_md/development/option-declarations.section.xml" />
<xi:include href="../from_md/development/option-types.section.xml" />
<xi:include href="option-def.xml" />
<xi:include href="../from_md/development/option-def.section.xml" />
<xi:include href="../from_md/development/assertions.section.xml" />
<xi:include href="meta-attributes.xml" />
<xi:include href="importing-modules.xml" />

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nixos/doc/manual/from_md/development/option-def.section.xml Normal file
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<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-option-definitions">
<title>Option Definitions</title>
<para>
Option definitions are generally straight-forward bindings of values
to option names, like
</para>
<programlisting language="bash">
config = {
services.httpd.enable = true;
};
</programlisting>
<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>
<section xml:id="sec-option-definitions-delaying-conditionals">
<title>Delaying Conditionals</title>
<para>
If a set of option definitions is conditional on the value of
another option, you may need to use <literal>mkIf</literal>.
Consider, for instance:
</para>
<programlisting language="bash">
config = if config.services.httpd.enable then {
environment.systemPackages = [ ... ];
...
} else {};
</programlisting>
<para>
This definition will cause Nix to fail with an <quote>infinite
recursion</quote> error. Why? Because the value of
<literal>config.services.httpd.enable</literal> depends on the
value being constructed here. After all, you could also write the
clearly circular and contradictory:
</para>
<programlisting language="bash">
config = if config.services.httpd.enable then {
services.httpd.enable = false;
} else {
services.httpd.enable = true;
};
</programlisting>
<para>
The solution is to write:
</para>
<programlisting language="bash">
config = mkIf config.services.httpd.enable {
environment.systemPackages = [ ... ];
...
};
</programlisting>
<para>
The special function <literal>mkIf</literal> causes the evaluation
of the conditional to be <quote>pushed down</quote> into the
individual definitions, as if you had written:
</para>
<programlisting language="bash">
config = {
environment.systemPackages = if config.services.httpd.enable then [ ... ] else [];
...
};
</programlisting>
</section>
<section xml:id="sec-option-definitions-setting-priorities">
<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
<literal>mkOverride</literal>, e.g.
</para>
<programlisting language="bash">
services.openssh.enable = mkOverride 10 false;
</programlisting>
<para>
This definition causes all other definitions with priorities above
10 to be discarded. The function <literal>mkForce</literal> is
equal to <literal>mkOverride 50</literal>.
</para>
</section>
<section xml:id="sec-option-definitions-merging">
<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 <literal>mkMerge</literal>:
</para>
<programlisting language="bash">
config = mkMerge
[ # Unconditional stuff.
{ environment.systemPackages = [ ... ];
}
# Conditional stuff.
(mkIf config.services.bla.enable {
environment.systemPackages = [ ... ];
})
];
</programlisting>
</section>
</section>