forked from mirrors/nixpkgs
88 lines
3 KiB
XML
88 lines
3 KiB
XML
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<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-freeform-modules">
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<title>Freeform modules</title>
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<para>
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Freeform modules allow you to define values for option paths that
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have not been declared explicitly. This can be used to add
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attribute-specific types to what would otherwise have to be
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<literal>attrsOf</literal> options in order to accept all attribute
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names.
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</para>
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<para>
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This feature can be enabled by using the attribute
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<literal>freeformType</literal> to define a freeform type. By doing
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this, all assignments without an associated option will be merged
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using the freeform type and combined into the resulting
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<literal>config</literal> set. Since this feature nullifies name
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checking for entire option trees, it is only recommended for use in
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submodules.
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</para>
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<anchor xml:id="ex-freeform-module" />
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<para>
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<emphasis role="strong">Example: Freeform submodule</emphasis>
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</para>
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<para>
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The following shows a submodule assigning a freeform type that
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allows arbitrary attributes with <literal>str</literal> values below
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<literal>settings</literal>, but also declares an option for the
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<literal>settings.port</literal> attribute to have it type-checked
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and assign a default value. See
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<link linkend="ex-settings-typed-attrs">Example: Declaring a
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type-checked <literal>settings</literal> attribute</link> for a more
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complete example.
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</para>
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<programlisting language="bash">
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{ lib, config, ... }: {
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options.settings = lib.mkOption {
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type = lib.types.submodule {
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freeformType = with lib.types; attrsOf str;
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# We want this attribute to be checked for the correct type
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options.port = lib.mkOption {
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type = lib.types.port;
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# Declaring the option also allows defining a default value
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default = 8080;
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};
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};
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};
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}
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</programlisting>
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<para>
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And the following shows what such a module then allows
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</para>
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<programlisting language="bash">
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{
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# Not a declared option, but the freeform type allows this
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settings.logLevel = "debug";
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# Not allowed because the the freeform type only allows strings
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# settings.enable = true;
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# Allowed because there is a port option declared
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settings.port = 80;
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# Not allowed because the port option doesn't allow strings
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# settings.port = "443";
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}
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</programlisting>
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<note>
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<para>
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Freeform attributes cannot depend on other attributes of the same
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set without infinite recursion:
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</para>
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<programlisting language="bash">
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{
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# This throws infinite recursion encountered
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settings.logLevel = lib.mkIf (config.settings.port == 80) "debug";
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}
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</programlisting>
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<para>
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To prevent this, declare options for all attributes that need to
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depend on others. For above example this means to declare
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<literal>logLevel</literal> to be an option.
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</para>
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</note>
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</section>
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