<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-types">
 <title>Options Types</title>

 <para>
  Option types are a way to put constraints on the values a module option can
  take. Types are also responsible of how values are merged in case of multiple
  value definitions.
 </para>

 <section xml:id="sec-option-types-basic">
  <title>Basic Types</title>

  <para>
   Basic types are the simplest available types in the module system. Basic
   types include multiple string types that mainly differ in how definition
   merging is handled.
  </para>

  <variablelist>
   <varlistentry>
    <term>
     <varname>types.bool</varname>
    </term>
    <listitem>
     <para>
      A boolean, its values can be <literal>true</literal> or
      <literal>false</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.path</varname>
    </term>
    <listitem>
     <para>
      A filesystem path, defined as anything that when coerced to a string
      starts with a slash. Even if derivations can be considered as path, the
      more specific <literal>types.package</literal> should be preferred.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.package</varname>
    </term>
    <listitem>
     <para>
      A derivation or a store path.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.anything</varname>
    </term>
    <listitem>
     <para>
      A type that accepts any value and recursively merges attribute sets together.
      This type is recommended when the option type is unknown.
      <example xml:id="ex-types-anything">
       <title><literal>types.anything</literal> Example</title>
       <para>
        Two definitions of this type like
<programlisting>
{
  str = lib.mkDefault "foo";
  pkg.hello = pkgs.hello;
  fun.fun = x: x + 1;
}
</programlisting>
<programlisting>
{
  str = lib.mkIf true "bar";
  pkg.gcc = pkgs.gcc;
  fun.fun = lib.mkForce (x: x + 2);
}
</programlisting>
        will get merged to
<programlisting>
{
  str = "bar";
  pkg.gcc = pkgs.gcc;
  pkg.hello = pkgs.hello;
  fun.fun = x: x + 2;
}
</programlisting>
       </para>
      </example>
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.attrs</varname>
    </term>
    <listitem>
     <para>
      A free-form attribute set.
      <warning><para>
       This type will be deprecated in the future because it doesn't recurse
       into attribute sets, silently drops earlier attribute definitions, and
       doesn't discharge <literal>lib.mkDefault</literal>, <literal>lib.mkIf
       </literal> and co. For allowing arbitrary attribute sets, prefer
       <literal>types.attrsOf types.anything</literal> instead which doesn't
       have these problems.
      </para></warning>
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   Integer-related types:
  </para>

  <variablelist>
   <varlistentry>
    <term>
     <varname>types.int</varname>
    </term>
    <listitem>
     <para>
      A signed integer.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.ints.{s8, s16, s32}</varname>
    </term>
    <listitem>
     <para>
      Signed integers with a fixed length (8, 16 or 32 bits). They go from
      <inlineequation><mathphrase>−2<superscript>n</superscript>/2</mathphrase>
      </inlineequation> to <inlineequation>
      <mathphrase>2<superscript>n</superscript>/2−1</mathphrase>
      </inlineequation> respectively (e.g. <literal>−128</literal> to
      <literal>127</literal> for 8 bits).
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.ints.unsigned</varname>
    </term>
    <listitem>
     <para>
      An unsigned integer (that is >= 0).
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id='types.ints.ux'>
    <term>
     <varname>types.ints.{u8, u16, u32}</varname>
    </term>
    <listitem>
     <para>
      Unsigned integers with a fixed length (8, 16 or 32 bits). They go from
      <inlineequation><mathphrase>0</mathphrase></inlineequation> to
      <inlineequation>
      <mathphrase>2<superscript>n</superscript>−1</mathphrase>
      </inlineequation> respectively (e.g. <literal>0</literal> to
      <literal>255</literal> for 8 bits).
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.ints.positive</varname>
    </term>
    <listitem>
     <para>
      A positive integer (that is > 0).
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.port</varname>
    </term>
    <listitem>
     <para>
      A port number. This type is an alias to
      <link linkend='types.ints.ux'><varname>types.ints.u16</varname></link>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   String-related types:
  </para>

  <variablelist>
   <varlistentry>
    <term>
     <varname>types.str</varname>
    </term>
    <listitem>
     <para>
      A string. Multiple definitions cannot be merged.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.lines</varname>
    </term>
    <listitem>
     <para>
      A string. Multiple definitions are concatenated with a new line
      <literal>"\n"</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.commas</varname>
    </term>
    <listitem>
     <para>
      A string. Multiple definitions are concatenated with a comma
      <literal>","</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.envVar</varname>
    </term>
    <listitem>
     <para>
      A string. Multiple definitions are concatenated with a collon
      <literal>":"</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.strMatching</varname>
    </term>
    <listitem>
     <para>
      A string matching a specific regular expression. Multiple definitions
      cannot be merged. The regular expression is processed using
      <literal>builtins.match</literal>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </section>

 <section xml:id="sec-option-types-value">
  <title>Value Types</title>

  <para>
   Value types are types that take a value parameter.
  </para>

  <variablelist>
   <varlistentry>
    <term>
     <varname>types.enum</varname> <replaceable>l</replaceable>
    </term>
    <listitem>
     <para>
      One element of the list <replaceable>l</replaceable>, e.g.
      <literal>types.enum [ "left" "right" ]</literal>. Multiple definitions
      cannot be merged.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.separatedString</varname> <replaceable>sep</replaceable>
    </term>
    <listitem>
     <para>
      A string with a custom separator <replaceable>sep</replaceable>, e.g.
      <literal>types.separatedString "|"</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.ints.between</varname> <replaceable>lowest</replaceable> <replaceable>highest</replaceable>
    </term>
    <listitem>
     <para>
      An integer between <replaceable>lowest</replaceable> and
      <replaceable>highest</replaceable> (both inclusive). Useful for creating
      types like <literal>types.port</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.submodule</varname> <replaceable>o</replaceable>
    </term>
    <listitem>
     <para>
      A set of sub options <replaceable>o</replaceable>.
      <replaceable>o</replaceable> can be an attribute set, a function
      returning an attribute set, or a path to a file containing such a value. Submodules are used in
      composed types to create modular options. This is equivalent to
      <literal>types.submoduleWith { modules = toList o; shorthandOnlyDefinesConfig = true; }</literal>.
      Submodules are detailed in
      <xref
          linkend='section-option-types-submodule' />.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
     <term>
       <varname>types.submoduleWith</varname> {
        <replaceable>modules</replaceable>,
        <replaceable>specialArgs</replaceable> ? {},
        <replaceable>shorthandOnlyDefinesConfig</replaceable> ? false }
     </term>
     <listitem>
       <para>
         Like <varname>types.submodule</varname>, but more flexible and with better defaults.
         It has parameters
         <itemizedlist>
           <listitem><para>
             <replaceable>modules</replaceable>
             A list of modules to use by default for this submodule type. This gets combined
             with all option definitions to build the final list of modules that will be included.
             <note><para>
               Only options defined with this argument are included in rendered documentation.
             </para></note>
           </para></listitem>
           <listitem><para>
             <replaceable>specialArgs</replaceable>
             An attribute set of extra arguments to be passed to the module functions.
             The option <literal>_module.args</literal> should be used instead
             for most arguments since it allows overriding. <replaceable>specialArgs</replaceable> should only be
             used for arguments that can&apos;t go through the module fixed-point, because of
             infinite recursion or other problems. An example is overriding the
             <varname>lib</varname> argument, because <varname>lib</varname> itself is used
             to define <literal>_module.args</literal>, which makes using
             <literal>_module.args</literal> to define it impossible.
           </para></listitem>
           <listitem><para>
             <replaceable>shorthandOnlyDefinesConfig</replaceable>
             Whether definitions of this type should default to the <literal>config</literal>
             section of a module (see <xref linkend='ex-module-syntax'/>) if it is an attribute
             set. Enabling this only has a benefit when the submodule defines an option named
             <literal>config</literal> or <literal>options</literal>. In such a case it would
             allow the option to be set with <literal>the-submodule.config = "value"</literal>
             instead of requiring <literal>the-submodule.config.config = "value"</literal>.
             This is because only when modules <emphasis>don&apos;t</emphasis> set the
             <literal>config</literal> or <literal>options</literal> keys, all keys are interpreted
             as option definitions in the <literal>config</literal> section. Enabling this option
             implicitly puts all attributes in the <literal>config</literal> section.
           </para>
           <para>
             With this option enabled, defining a non-<literal>config</literal> section requires
             using a function: <literal>the-submodule = { ... }: { options = { ... }; }</literal>.
           </para></listitem>
         </itemizedlist>
       </para>
     </listitem>
   </varlistentry>
  </variablelist>
 </section>

 <section xml:id="sec-option-types-composed">
  <title>Composed Types</title>

  <para>
   Composed types are types that take a type as parameter. <literal>listOf
   int</literal> and <literal>either int str</literal> are examples of composed
   types.
  </para>

  <variablelist>
   <varlistentry>
    <term>
     <varname>types.listOf</varname> <replaceable>t</replaceable>
    </term>
    <listitem>
     <para>
      A list of <replaceable>t</replaceable> type, e.g. <literal>types.listOf
      int</literal>. Multiple definitions are merged with list concatenation.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.attrsOf</varname> <replaceable>t</replaceable>
    </term>
    <listitem>
     <para>
      An attribute set of where all the values are of
      <replaceable>t</replaceable> type. Multiple definitions result in the
      joined attribute set.
      <note><para>
       This type is <emphasis>strict</emphasis> in its values, which in turn
       means attributes cannot depend on other attributes. See <varname>
       types.lazyAttrsOf</varname> for a lazy version.
      </para></note>
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.lazyAttrsOf</varname> <replaceable>t</replaceable>
    </term>
    <listitem>
     <para>
      An attribute set of where all the values are of
      <replaceable>t</replaceable> type. Multiple definitions result in the
      joined attribute set. This is the lazy version of <varname>types.attrsOf
      </varname>, allowing attributes to depend on each other.
      <warning><para>
       This version does not fully support conditional definitions! With an
       option <varname>foo</varname> of this type and a definition
       <literal>foo.attr = lib.mkIf false 10</literal>, evaluating
       <literal>foo ? attr</literal> will return <literal>true</literal>
       even though it should be false. Accessing the value will then throw
       an error. For types <replaceable>t</replaceable> that have an
       <literal>emptyValue</literal> defined, that value will be returned
       instead of throwing an error. So if the type of <literal>foo.attr</literal>
       was <literal>lazyAttrsOf (nullOr int)</literal>, <literal>null</literal>
       would be returned instead for the same <literal>mkIf false</literal> definition.
      </para></warning>
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.nullOr</varname> <replaceable>t</replaceable>
    </term>
    <listitem>
     <para>
      <literal>null</literal> or type <replaceable>t</replaceable>. Multiple
      definitions are merged according to type <replaceable>t</replaceable>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.uniq</varname> <replaceable>t</replaceable>
    </term>
    <listitem>
     <para>
      Ensures that type <replaceable>t</replaceable> cannot be merged. It is
      used to ensure option definitions are declared only once.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.either</varname> <replaceable>t1</replaceable> <replaceable>t2</replaceable>
    </term>
    <listitem>
     <para>
      Type <replaceable>t1</replaceable> or type <replaceable>t2</replaceable>,
      e.g. <literal>with types; either int str</literal>. Multiple definitions
      cannot be merged.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.oneOf</varname> [ <replaceable>t1</replaceable> <replaceable>t2</replaceable> ... ]
    </term>
    <listitem>
     <para>
      Type <replaceable>t1</replaceable> or type <replaceable>t2</replaceable> and so forth,
      e.g. <literal>with types; oneOf [ int str bool ]</literal>. Multiple definitions
      cannot be merged.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>types.coercedTo</varname> <replaceable>from</replaceable> <replaceable>f</replaceable> <replaceable>to</replaceable>
    </term>
    <listitem>
     <para>
      Type <replaceable>to</replaceable> or type
      <replaceable>from</replaceable> which will be coerced to type
      <replaceable>to</replaceable> using function <replaceable>f</replaceable>
      which takes an argument of type <replaceable>from</replaceable> and
      return a value of type <replaceable>to</replaceable>. Can be used to
      preserve backwards compatibility of an option if its type was changed.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </section>

 <section xml:id='section-option-types-submodule'>
  <title>Submodule</title>

  <para>
   <literal>submodule</literal> is a very powerful type that defines a set of
   sub-options that are handled like a separate module.
  </para>

  <para>
   It takes a parameter <replaceable>o</replaceable>, that should be a set, or
   a function returning a set with an <literal>options</literal> key defining
   the sub-options. Submodule option definitions are type-checked accordingly
   to the <literal>options</literal> declarations. Of course, you can nest
   submodule option definitons for even higher modularity.
  </para>

  <para>
   The option set can be defined directly
   (<xref linkend='ex-submodule-direct' />) or as reference
   (<xref linkend='ex-submodule-reference' />).
  </para>

  <example xml:id='ex-submodule-direct'>
   <title>Directly defined submodule</title>
<screen>
options.mod = mkOption {
  description = "submodule example";
  type = with types; submodule {
    options = {
      foo = mkOption {
        type = int;
      };
      bar = mkOption {
        type = str;
      };
    };
  };
};</screen>
  </example>

  <example xml:id='ex-submodule-reference'>
   <title>Submodule defined as a reference</title>
<screen>
let
  modOptions = {
    options = {
      foo = mkOption {
        type = int;
      };
      bar = mkOption {
        type = int;
      };
    };
  };
in
options.mod = mkOption {
  description = "submodule example";
  type = with types; submodule modOptions;
};</screen>
  </example>

  <para>
   The <literal>submodule</literal> type is especially interesting when used
   with composed types like <literal>attrsOf</literal> or
   <literal>listOf</literal>. When composed with <literal>listOf</literal>
   (<xref linkend='ex-submodule-listof-declaration' />),
   <literal>submodule</literal> allows multiple definitions of the submodule
   option set (<xref linkend='ex-submodule-listof-definition' />).
  </para>

  <example xml:id='ex-submodule-listof-declaration'>
   <title>Declaration of a list of submodules</title>
<screen>
options.mod = mkOption {
  description = "submodule example";
  type = with types; listOf (submodule {
    options = {
      foo = mkOption {
        type = int;
      };
      bar = mkOption {
        type = str;
      };
    };
  });
};</screen>
  </example>

  <example xml:id='ex-submodule-listof-definition'>
   <title>Definition of a list of submodules</title>
<screen>
config.mod = [
  { foo = 1; bar = "one"; }
  { foo = 2; bar = "two"; }
];</screen>
  </example>

  <para>
   When composed with <literal>attrsOf</literal>
   (<xref linkend='ex-submodule-attrsof-declaration' />),
   <literal>submodule</literal> allows multiple named definitions of the
   submodule option set (<xref linkend='ex-submodule-attrsof-definition' />).
  </para>

  <example xml:id='ex-submodule-attrsof-declaration'>
   <title>Declaration of attribute sets of submodules</title>
<screen>
options.mod = mkOption {
  description = "submodule example";
  type = with types; attrsOf (submodule {
    options = {
      foo = mkOption {
        type = int;
      };
      bar = mkOption {
        type = str;
      };
    };
  });
};</screen>
  </example>

  <example xml:id='ex-submodule-attrsof-definition'>
   <title>Declaration of attribute sets of submodules</title>
<screen>
config.mod.one = { foo = 1; bar = "one"; };
config.mod.two = { foo = 2; bar = "two"; };</screen>
  </example>
 </section>

 <section xml:id="sec-option-types-extending">
  <title>Extending types</title>

  <para>
   Types are mainly characterized by their <literal>check</literal> and
   <literal>merge</literal> functions.
  </para>

  <variablelist>
   <varlistentry>
    <term>
     <varname>check</varname>
    </term>
    <listitem>
     <para>
      The function to type check the value. Takes a value as parameter and
      return a boolean. It is possible to extend a type check with the
      <literal>addCheck</literal> function
      (<xref
          linkend='ex-extending-type-check-1' />), or to fully
      override the check function
      (<xref linkend='ex-extending-type-check-2' />).
     </para>
     <example xml:id='ex-extending-type-check-1'>
      <title>Adding a type check</title>
<screen>
byte = mkOption {
  description = "An integer between 0 and 255.";
  type = types.addCheck types.int (x: x &gt;= 0 &amp;&amp; x &lt;= 255);
};</screen>
     </example>
     <example xml:id='ex-extending-type-check-2'>
      <title>Overriding a type check</title>
<screen>
nixThings = mkOption {
  description = "words that start with 'nix'";
  type = types.str // {
    check = (x: lib.hasPrefix "nix" x)
  };
};</screen>
     </example>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>merge</varname>
    </term>
    <listitem>
     <para>
      Function to merge the options values when multiple values are set. The
      function takes two parameters, <literal>loc</literal> the option path as
      a list of strings, and <literal>defs</literal> the list of defined values
      as a list. It is possible to override a type merge function for custom
      needs.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </section>

 <section xml:id="sec-option-types-custom">
  <title>Custom Types</title>

  <para>
   Custom types can be created with the <literal>mkOptionType</literal>
   function. As type creation includes some more complex topics such as
   submodule handling, it is recommended to get familiar with
   <filename
  xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/types.nix">types.nix</filename>
   code before creating a new type.
  </para>

  <para>
   The only required parameter is <literal>name</literal>.
  </para>

  <variablelist>
   <varlistentry>
    <term>
     <varname>name</varname>
    </term>
    <listitem>
     <para>
      A string representation of the type function name.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>definition</varname>
    </term>
    <listitem>
     <para>
      Description of the type used in documentation. Give information of the
      type and any of its arguments.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>check</varname>
    </term>
    <listitem>
     <para>
      A function to type check the definition value. Takes the definition value
      as a parameter and returns a boolean indicating the type check result,
      <literal>true</literal> for success and <literal>false</literal> for
      failure.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>merge</varname>
    </term>
    <listitem>
     <para>
      A function to merge multiple definitions values. Takes two parameters:
     </para>
     <variablelist>
      <varlistentry>
       <term>
        <replaceable>loc</replaceable>
       </term>
       <listitem>
        <para>
         The option path as a list of strings, e.g. <literal>["boot" "loader
         "grub" "enable"]</literal>.
        </para>
       </listitem>
      </varlistentry>
      <varlistentry>
       <term>
        <replaceable>defs</replaceable>
       </term>
       <listitem>
        <para>
         The list of sets of defined <literal>value</literal> and
         <literal>file</literal> where the value was defined, e.g. <literal>[ {
         file = "/foo.nix"; value = 1; } { file = "/bar.nix"; value = 2 }
         ]</literal>. The <literal>merge</literal> function should return the
         merged value or throw an error in case the values are impossible or
         not meant to be merged.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>getSubOptions</varname>
    </term>
    <listitem>
     <para>
      For composed types that can take a submodule as type parameter, this
      function generate sub-options documentation. It takes the current option
      prefix as a list and return the set of sub-options. Usually defined in a
      recursive manner by adding a term to the prefix, e.g. <literal>prefix:
      elemType.getSubOptions (prefix ++
      [<replaceable>"prefix"</replaceable>])</literal> where
      <replaceable>"prefix"</replaceable> is the newly added prefix.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>getSubModules</varname>
    </term>
    <listitem>
     <para>
      For composed types that can take a submodule as type parameter, this
      function should return the type parameters submodules. If the type
      parameter is called <literal>elemType</literal>, the function should just
      recursively look into submodules by returning
      <literal>elemType.getSubModules;</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>substSubModules</varname>
    </term>
    <listitem>
     <para>
      For composed types that can take a submodule as type parameter, this
      function can be used to substitute the parameter of a submodule type. It
      takes a module as parameter and return the type with the submodule
      options substituted. It is usually defined as a type function call with a
      recursive call to <literal>substSubModules</literal>, e.g for a type
      <literal>composedType</literal> that take an <literal>elemtype</literal>
      type parameter, this function should be defined as <literal>m:
      composedType (elemType.substSubModules m)</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>typeMerge</varname>
    </term>
    <listitem>
     <para>
      A function to merge multiple type declarations. Takes the type to merge
      <literal>functor</literal> as parameter. A <literal>null</literal> return
      value means that type cannot be merged.
     </para>
     <variablelist>
      <varlistentry>
       <term>
        <replaceable>f</replaceable>
       </term>
       <listitem>
        <para>
         The type to merge <literal>functor</literal>.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
     <para>
      Note: There is a generic <literal>defaultTypeMerge</literal> that work
      with most of value and composed types.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>functor</varname>
    </term>
    <listitem>
     <para>
      An attribute set representing the type. It is used for type operations
      and has the following keys:
     </para>
     <variablelist>
      <varlistentry>
       <term>
        <varname>type</varname>
       </term>
       <listitem>
        <para>
         The type function.
        </para>
       </listitem>
      </varlistentry>
      <varlistentry>
       <term>
        <varname>wrapped</varname>
       </term>
       <listitem>
        <para>
         Holds the type parameter for composed types.
        </para>
       </listitem>
      </varlistentry>
      <varlistentry>
       <term>
        <varname>payload</varname>
       </term>
       <listitem>
        <para>
         Holds the value parameter for value types. The types that have a
         <literal>payload</literal> are the <literal>enum</literal>,
         <literal>separatedString</literal> and <literal>submodule</literal>
         types.
        </para>
       </listitem>
      </varlistentry>
      <varlistentry>
       <term>
        <varname>binOp</varname>
       </term>
       <listitem>
        <para>
         A binary operation that can merge the payloads of two same types.
         Defined as a function that take two payloads as parameters and return
         the payloads merged.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </listitem>
   </varlistentry>
  </variablelist>
 </section>
</section>
