lib.types: Add deferredModule

lib/modules.nix

@@ -462,6 +462,7 @@ rec {
           config = addFreeformType (addMeta (m.config or {}));
         }
     else
+      lib.throwIfNot (isAttrs m) "module ${file} (${key}) does not look like a module."
       { _file = toString m._file or file;
         key = toString m.key or key;
         disabledModules = m.disabledModules or [];

lib/tests/modules.sh

@@ -194,6 +194,13 @@ checkConfigOutput '^"submodule"$' options.submodule.type.description ./declare-s
 ## Paths should be allowed as values and work as expected
 checkConfigOutput '^true$' config.submodule.enable ./declare-submoduleWith-path.nix
 
+## deferredModule
+# default module is merged into nodes.foo
+checkConfigOutput '"beta"' config.nodes.foo.settingsDict.c ./deferred-module.nix
+# errors from the default module are reported with accurate location
+checkConfigError 'In `the-file-that-contains-the-bad-config.nix, via option default'\'': "bogus"' config.nodes.foo.bottom ./deferred-module.nix
+checkConfigError '.*lib/tests/modules/deferred-module-error.nix, via option deferred [(]:anon-1:anon-1:anon-1[)] does not look like a module.' config.result ./deferred-module-error.nix
+
 # Check the file location information is propagated into submodules
 checkConfigOutput the-file.nix config.submodule.internalFiles.0 ./submoduleFiles.nix
 

lib/tests/modules/deferred-module-error.nix

@@ -0,0 +1,20 @@
+{ config, lib, ... }:
+let
+  inherit (lib) types mkOption setDefaultModuleLocation evalModules;
+  inherit (types) deferredModule lazyAttrsOf submodule str raw enum;
+in
+{
+  options = {
+    deferred = mkOption {
+      type = deferredModule;
+    };
+    result = mkOption {
+      default = (evalModules { modules = [ config.deferred ]; }).config.result;
+    };
+  };
+  config = {
+    deferred = { ... }:
+      # this should be an attrset, so this fails
+      true;
+  };
+}

lib/tests/modules/deferred-module.nix

@@ -0,0 +1,58 @@
+{ lib, ... }:
+let
+  inherit (lib) types mkOption setDefaultModuleLocation;
+  inherit (types) deferredModule lazyAttrsOf submodule str raw enum;
+in
+{
+  imports = [
+    # generic module, declaring submodules:
+    #   - nodes.<name>
+    #   - default
+    # where all nodes include the default
+    ({ config, ... }: {
+      _file = "generic.nix";
+      options.nodes = mkOption {
+        type = lazyAttrsOf (submodule { imports = [ config.default ]; });
+        default = {};
+      };
+      options.default = mkOption {
+        type = deferredModule;
+        default = { };
+        description = ''
+          Module that is included in all nodes.
+        '';
+      };
+    })
+
+    {
+      _file = "default-1.nix";
+      default = { config, ... }: {
+        options.settingsDict = lib.mkOption { type = lazyAttrsOf str; default = {}; };
+        options.bottom = lib.mkOption { type = enum []; };
+      };
+    }
+
+    {
+      _file = "default-a-is-b.nix";
+      default = ./define-settingsDict-a-is-b.nix;
+    }
+
+    {
+      _file = "nodes-foo.nix";
+      nodes.foo.settingsDict.b = "beta";
+    }
+
+    {
+      _file = "the-file-that-contains-the-bad-config.nix";
+      default.bottom = "bogus";
+    }
+
+    {
+      _file = "nodes-foo-c-is-a.nix";
+      nodes.foo = { config, ... }: {
+        settingsDict.c = config.settingsDict.a;
+      };
+    }
+
+  ];
+}

lib/tests/modules/define-settingsDict-a-is-b.nix

@@ -0,0 +1,3 @@
+{ config, ... }: {
+  settingsDict.a = config.settingsDict.b;
+}

lib/types.nix

@@ -539,6 +539,36 @@ rec {
       modules = toList modules;
     };
 
+    # A module to be imported in some other part of the configuration.
+    deferredModule = deferredModuleWith { };
+
+    # A module to be imported in some other part of the configuration.
+    # `staticModules`' options will be added to the documentation, unlike
+    # options declared via `config`.
+    deferredModuleWith = attrs@{ staticModules ? [] }: mkOptionType {
+      name = "deferredModule";
+      description = "module";
+      check = x: isAttrs x || isFunction x || path.check x;
+      merge = loc: defs: {
+        imports = staticModules ++ map (def: lib.setDefaultModuleLocation "${def.file}, via option ${showOption loc}" def.value) defs;
+      };
+      inherit (submoduleWith { modules = staticModules; })
+        getSubOptions
+        getSubModules;
+      substSubModules = m: deferredModuleWith (attrs // {
+        staticModules = m;
+      });
+      functor = defaultFunctor "deferredModuleWith" // {
+        type = types.deferredModuleWith;
+        payload = {
+          inherit staticModules;
+        };
+        binOp = lhs: rhs: {
+          staticModules = lhs.staticModules ++ rhs.staticModules;
+        };
+      };
+    };
+
     # The type of a type!
     optionType = mkOptionType {
       name = "optionType";

nixos/doc/manual/development/option-types.section.md

@@ -220,6 +220,25 @@ Value types are types that take a value parameter.
         requires using a function:
         `the-submodule = { ... }: { options = { ... }; }`.
 
+`types.deferredModule`
+
+:   Whereas `submodule` represents an option tree, `deferredModule` represents
+    a module value, such as a module file or a configuration.
+
+    It can be set multiple times.
+
+    Module authors can use its value in `imports`, in `submoduleWith`'s `modules`
+    or in `evalModules`' `modules` parameter, among other places.
+
+    Note that `imports` must be evaluated before the module fixpoint. Because
+    of this, deferred modules can only be imported into "other" fixpoints, such
+    as submodules.
+
+    One use case for this type is the type of a "default" module that allow the
+    user to affect all submodules in an `attrsOf submodule` at once. This is
+    more convenient and discoverable than expecting the module user to
+    type-merge with the `attrsOf submodule` option.
+
 ## Composed Types {#sec-option-types-composed}
 
 Composed types are types that take a type as parameter. `listOf

nixos/doc/manual/from_md/development/option-types.section.xml

@@ -427,6 +427,43 @@
           </itemizedlist>
         </listitem>
       </varlistentry>
+      <varlistentry>
+        <term>
+          <literal>types.deferredModule</literal>
+        </term>
+        <listitem>
+          <para>
+            Whereas <literal>submodule</literal> represents an option
+            tree, <literal>deferredModule</literal> represents a module
+            value, such as a module file or a configuration.
+          </para>
+          <para>
+            It can be set multiple times.
+          </para>
+          <para>
+            Module authors can use its value in
+            <literal>imports</literal>, in
+            <literal>submoduleWith</literal><quote>s
+            <literal>modules</literal> or in
+            <literal>evalModules</literal></quote>
+            <literal>modules</literal> parameter, among other places.
+          </para>
+          <para>
+            Note that <literal>imports</literal> must be evaluated
+            before the module fixpoint. Because of this, deferred
+            modules can only be imported into <quote>other</quote>
+            fixpoints, such as submodules.
+          </para>
+          <para>
+            One use case for this type is the type of a
+            <quote>default</quote> module that allow the user to affect
+            all submodules in an <literal>attrsOf submodule</literal> at
+            once. This is more convenient and discoverable than
+            expecting the module user to type-merge with the
+            <literal>attrsOf submodule</literal> option.
+          </para>
+        </listitem>
+      </varlistentry>
     </variablelist>
   </section>
   <section xml:id="sec-option-types-composed">