diff -r 000000000000 -r 6474c204b198 addon-sdk/source/lib/sdk/util/dispatcher.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/addon-sdk/source/lib/sdk/util/dispatcher.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,55 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+"use strict";
+
+module.metadata = {
+  "stability": "experimental"
+};
+
+const method = require("method/core");
+
+// Utility function that is just an enhancement over `method` to
+// allow predicate based dispatch in addition to polymorphic
+// dispatch. Unfortunately polymorphic dispatch does not quite
+// cuts it in the world of XPCOM where no types / classes exist
+// and all the XUL nodes share same type / prototype.
+// Probably this is more generic and belongs some place else, but
+// we can move it later once this will be relevant.
+let dispatcher = hint => {
+  const base = method(hint);
+  // Make a map for storing predicate, implementation mappings.
+  let implementations = new Map();
+
+  // Dispatcher function goes through `predicate, implementation`
+  // pairs to find predicate that matches first argument and
+  // returns application of arguments on the associated
+  // `implementation`. If no matching predicate is found delegates
+  // to a `base` polymorphic function.
+  let dispatch = (value, ...rest) => {
+    for (let [predicate, implementation] of implementations) {
+      if (predicate(value))
+        return implementation(value, ...rest);
+    }
+
+    return base(value, ...rest);
+  };
+
+  // Expose base API.
+  dispatch.define = base.define;
+  dispatch.implement = base.implement;
+  dispatch.toString = base.toString;
+
+  // Add a `when` function to allow extending function via
+  // predicates.
+  dispatch.when = (predicate, implementation) => {
+    if (implementations.has(predicate))
+      throw TypeError("Already implemented for the given predicate");
+    implementations.set(predicate, implementation);
+  };
+
+  return dispatch;
+};
+
+exports.dispatcher = dispatcher;