nixos/doc: Add freeform modules documentation

Co-Authored-By: Robert Hensing <robert@roberthensing.nl>
2020-08-14 22:49:35 +02:00
parent 42cf8130d7
commit 25d75155f3
3 changed files with 108 additions and 2 deletions
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+<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-freeform-modules">
+ <title>Freeform modules</title>
+ <para>
+  Freeform modules allow you to define values for option paths that have not been declared explicitly. This can be used to add attribute-specific types to what would otherwise have to be <literal>attrsOf</literal> options in order to accept all attribute names.
+ </para>
+ <para>
+  This feature can be enabled by using the attribute <literal>freeformType</literal> to define a freeform type. By doing this, all assignments without an associated option will be merged using the freeform type and combined into the resulting <literal>config</literal> set. Since this feature nullifies name checking for entire option trees, it is only recommended for use in submodules.
+ </para>
+ <example xml:id="ex-freeform-module">
+  <title>Freeform submodule</title>
+  <para>
+   The following shows a submodule assigning a freeform type that allows arbitrary attributes with <literal>str</literal> values below <literal>settings</literal>, but also declares an option for the <literal>settings.port</literal> attribute to have it type-checked and assign a default value. See <xref linkend="ex-settings-typed-attrs"/> for a more complete example.
+  </para>
+ <programlisting>
+{ lib, config, ... }: {
+
+  options.settings = lib.mkOption {
+    type = lib.types.submodule {
+
+      freeformType = with lib.types; attrsOf str;
+
+      # We want this attribute to be checked for the correct type
+      options.port = lib.mkOption {
+        type = lib.types.port;
+        # Declaring the option also allows defining a default value
+        default = 8080;
+      };
+
+    };
+  };
+}
+ </programlisting>
+ <para>
+  And the following shows what such a module then allows
+ </para>
+ <programlisting>
+{
+  # Not a declared option, but the freeform type allows this
+  settings.logLevel = "debug";
+
+  # Not allowed because the the freeform type only allows strings
+  # settings.enable = true;
+
+  # Allowed because there is a port option declared
+  settings.port = 80;
+
+  # Not allowed because the port option doesn't allow strings
+  # settings.port = "443";
+}
+ </programlisting>
+ </example>
+ <note>
+  <para>
+   Freeform attributes cannot depend on other attributes of the same set without infinite recursion:
+<programlisting>
+{
+  # This throws infinite recursion encountered
+  settings.logLevel = lib.mkIf (config.settings.port == 80) "debug";
+}
+</programlisting>
+   To prevent this, declare options for all attributes that need to depend on others. For above example this means to declare <literal>logLevel</literal> to be an option.
+  </para>
+ </note>
+</section>