diff --git a/prototype/UI/index.html b/prototype/UI/index.html
index 125f14f87..21d8c1065 100644
--- a/prototype/UI/index.html
+++ b/prototype/UI/index.html
@@ -223,9 +223,13 @@
         return this;
     }
 
-    function Leaf(object) {
-        this.value = object;
-        return this;
+    // forall a b. Dictionary String a -> (a -> b) -> Dictionary String b
+    Dictionary.prototype.map = function(transform) {
+        var copy = new Dictionary();
+        for (key in this.values) {
+            copy.values[key] = transform(this.values[key]);
+        }
+        return copy;
     }
 
     function Syntax(json, continuation) {