1 files changed, 79 insertions, 0 deletions
diff --git a/nixos/doc/manual/development/freeform-modules.section.md b/nixos/doc/manual/development/freeform-modules.section.md
new file mode 100644
index 00000000000..10e876b96d5
--- /dev/null
+++ b/nixos/doc/manual/development/freeform-modules.section.md
@@ -0,0 +1,79 @@
+# Freeform modules {#sec-freeform-modules}
+
+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 `attrsOf` options in order to
+accept all attribute names.
+
+This feature can be enabled by using the attribute `freeformType` 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 `config` set. Since this feature nullifies name
+checking for entire option trees, it is only recommended for use in
+submodules.
+
+::: {#ex-freeform-module .example}
+::: {.title}
+**Example: Freeform submodule**
+:::
+The following shows a submodule assigning a freeform type that allows
+arbitrary attributes with `str` values below `settings`, but also
+declares an option for the `settings.port` attribute to have it
+type-checked and assign a default value. See
+[Example: Declaring a type-checked `settings` attribute](#ex-settings-typed-attrs)
+for a more complete example.
+
+```nix
+{ 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;
+      };
+
+    };
+  };
+}
+```
+
+And the following shows what such a module then allows
+
+```nix
+{
+  # 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";
+}
+```
+:::
+
+::: {.note}
+Freeform attributes cannot depend on other attributes of the same set
+without infinite recursion:
+
+```nix
+{
+  # This throws infinite recursion encountered
+  settings.logLevel = lib.mkIf (config.settings.port == 80) "debug";
+}
+```
+
+To prevent this, declare options for all attributes that need to depend
+on others. For above example this means to declare `logLevel` to be an
+option.
+:::