diff --git a/lib/modules.nix b/lib/modules.nix
index 31134c66276..1d428311cd1 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -144,7 +144,8 @@ rec {
          opt.options ? example && res ? example ||
          opt.options ? description && res ? description ||
          opt.options ? apply && res ? apply ||
-         opt.options ? type && res ? type
+         # Accept to merge options which have identical types.
+         opt.options ? type && res ? type && opt.options.type.name != res.type.name
       then
         throw "The option `${showOption loc}' in `${opt.file}' is already declared in ${showFiles res.declarations}."
       else
@@ -155,12 +156,16 @@ rec {
              current option declaration as the file use for the submodule.  If the
              submodule defines any filename, then we ignore the enclosing option file. */
           options' = toList opt.options.options;
+          addModuleFile = m:
+            if isFunction m then args: { _file = opt.file; } // (m args)
+            else { _file = opt.file; } // m;
           coerceOption = file: opt:
             if isFunction opt then args: { _file = file; } // (opt args)
-            else args: { _file = file; options = opt; };
+            else { _file = file; options = opt; };
+          getSubModules = opt.options.type.getSubModules or null;
           submodules =
-            if opt.options ? options
-            then map (coerceOption opt.file) options' ++ res.options
+            if getSubModules != null then map addModuleFile getSubModules ++ res.options
+            else if opt.options ? options then map (coerceOption opt.file) options' ++ res.options
             else res.options;
         in opt.options // res //
           { declarations = [opt.file] ++ res.declarations;
@@ -292,7 +297,8 @@ rec {
     in sort compare defs';
 
   /* Hack for backward compatibility: convert options of type
-     optionSet to configOf.  FIXME: remove eventually. */
+     optionSet to options of type submodule.  FIXME: remove
+     eventually. */
   fixupOptionType = loc: opt:
     let
       options = opt.options or
@@ -305,7 +311,10 @@ rec {
         else if tp.name == "list of option sets" then types.listOf (types.submodule options)
         else if tp.name == "null or option set" then types.nullOr (types.submodule options)
         else tp;
-    in opt // { type = f (opt.type or types.unspecified); };
+    in
+      if opt.type.getSubModules or null == null
+      then opt // { type = f (opt.type or types.unspecified); }
+      else opt // { type = opt.type.substSubModules opt.options; options = []; };
 
 
   /* Properties. */
diff --git a/lib/types.nix b/lib/types.nix
index 0e2b6515e16..783e07cdc72 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -33,9 +33,14 @@ rec {
     , # Return a flat list of sub-options.  Used to generate
       # documentation.
       getSubOptions ? prefix: {}
+    , # List of modules if any, or null if none.
+      getSubModules ? null
+    , # Function for building the same option type  with a different list of
+      # modules.
+      substSubModules ? m: null
     }:
     { _type = "option-type";
-      inherit name check merge getSubOptions;
+      inherit name check merge getSubOptions getSubModules substSubModules;
     };
 
 
@@ -110,6 +115,8 @@ rec {
           elemType.merge (loc ++ ["[${toString n}-${toString m}]"])
             [{ inherit (def) file; value = def'; }]) def.value) defs);
       getSubOptions = prefix: elemType.getSubOptions (prefix ++ ["*"]);
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: listOf (elemType.substSubModules m);
     };
 
     attrsOf = elemType: mkOptionType {
@@ -121,6 +128,8 @@ rec {
           (map (def: listToAttrs (mapAttrsToList (n: def':
             { name = n; value = { inherit (def) file; value = def'; }; }) def.value)) defs);
       getSubOptions = prefix: elemType.getSubOptions (prefix ++ ["<name>"]);
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: attrsOf (elemType.substSubModules m);
     };
 
     # List or attribute set of ...
@@ -147,12 +156,16 @@ rec {
           else false;
         merge = loc: defs: attrOnly.merge loc (imap convertIfList defs);
         getSubOptions = prefix: elemType.getSubOptions (prefix ++ ["<name?>"]);
+        getSubModules = elemType.getSubModules;
+        substSubModules = m: loaOf (elemType.substSubModules m);
       };
 
     uniq = elemType: mkOptionType {
       inherit (elemType) name check;
       merge = mergeOneOption;
       getSubOptions = elemType.getSubOptions;
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: uniq (elemType.substSubModules m);
     };
 
     nullOr = elemType: mkOptionType {
@@ -165,6 +178,8 @@ rec {
           throw "The option `${showOption loc}' is defined both null and not null, in ${showFiles (getFiles defs)}."
         else elemType.merge loc defs;
       getSubOptions = elemType.getSubOptions;
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: nullOr (elemType.substSubModules m);
     };
 
     functionTo = elemType: mkOptionType {
@@ -173,6 +188,8 @@ rec {
       merge = loc: defs:
         fnArgs: elemType.merge loc (map (fn: { inherit (fn) file; value = fn.value fnArgs; }) defs);
       getSubOptions = elemType.getSubOptions;
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: functionTo (elemType.substSubModules m);
     };
 
     submodule = opts:
@@ -192,6 +209,8 @@ rec {
           { modules = opts'; inherit prefix;
             # FIXME: hack to get shit to evaluate.
             args = { name = ""; }; }).options;
+        getSubModules = opts';
+        substSubModules = m: submodule m;
       };
 
     enum = values: mkOptionType {