nixpkgs/nixos/doc/manual/development/option-types.xml
Nikolay Amiantov ee4f8c2dc9 nixos doc: document coercedTo type
Also mention about docs in types.nix and fix a small error in related
documentation.

Fixes #26055.
2017-05-24 15:52:26 +03:00

457 lines
17 KiB
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-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><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.int</varname></term>
<listitem><para>An integer.</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>
</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>
</variablelist>
</section>
<section><title>Value Types</title>
<para>Value types are type 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.submodule</varname> <replaceable>o</replaceable></term>
<listitem><para>A set of sub options <replaceable>o</replaceable>.
<replaceable>o</replaceable> can be an attribute set or a function
returning an attribute set. Submodules are used in composed types to
create modular options. Submodule are detailed in <xref
linkend='section-option-types-submodule' />.</para></listitem>
</varlistentry>
</variablelist>
</section>
<section><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.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>types.loaOf</varname> <replaceable>t</replaceable></term>
<listitem><para>An attribute set or a list of <replaceable>t</replaceable>
type. Multiple definitions are merged according to the
value.</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.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>Submodule is a very powerful type that defines a set of sub-options that
are handled like a separate module.
It is especially interesting when used with composed types like
<literal>attrsOf</literal> or <literal>listOf</literal>.</para>
<para>The submodule type take 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.
The option set can be defined directly (<xref linkend='ex-submodule-direct'
/>) or as reference (<xref linkend='ex-submodule-reference' />).</para>
<para>Submodule option definitions are type-checked accordingly to the options
declarations. It is possible to declare submodule options inside a submodule
sub-options for even higher modularity.</para>
<example xml:id='ex-submodule-direct'><title>Directly defined submodule</title>
<screen>
options.mod = mkOption {
name = "mod";
description = "submodule example";
type = with types; listOf (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; listOf (submodule modOptions);
};</screen></example>
<section><title>Composed with <literal>listOf</literal></title>
<para>When composed with <literal>listOf</literal>, submodule allows multiple
definitions of the submodule option set.</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>
</section>
<section><title>Composed with <literal>attrsOf</literal></title>
<para>When composed with <literal>attrsOf</literal>, submodule allows multiple
named definitions of the submodule option set.</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>
<section><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 = 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><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 usally 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>