nixpkgs/nixos/doc/manual/development/option-declarations.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-option-declarations">

<title>Option Declarations</title>

<para>An option declaration specifies the name, type and description
of a NixOS configuration option.  It is invalid to define an option
that hasn’t been declared in any module.  An option declaration
generally looks like this:

<programlisting>
options = {
  <replaceable>name</replaceable> = mkOption {
    type = <replaceable>type specification</replaceable>;
    default = <replaceable>default value</replaceable>;
    example = <replaceable>example value</replaceable>;
    description = "<replaceable>Description for use in the NixOS manual.</replaceable>";
  };
};
</programlisting>

</para>

<para>The function <varname>mkOption</varname> accepts the following arguments.

<variablelist>

  <varlistentry>
    <term><varname>type</varname></term>
    <listitem>
      <para>The type of the option (see <xref linkend='sec-option-types' />).
      It may be omitted, but that’s not advisable since it may lead to errors
      that are hard to diagnose.</para>
    </listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>default</varname></term>
    <listitem>
      <para>The default value used if no value is defined by any
      module.  A default is not required; in that case, if the option
      value is never used, an error will be thrown.</para>
    </listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>example</varname></term>
    <listitem>
      <para>An example value that will be shown in the NixOS manual.</para>
    </listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>description</varname></term>
    <listitem>
      <para>A textual description of the option, in DocBook format,
      that will be included in the NixOS manual.</para>
    </listitem>
  </varlistentry>

</variablelist>

</para>

</section>