Make #visit method work for null and undefined

lib/core/util/visitor.coffee

@@ -1,16 +1,24 @@
 Monarch.Util.Visitor =
   visit: (object, args...) ->
-    throw new Error("Cannot visit #{object}") unless object
-    if object.acceptVisitor?
-      object.acceptVisitor(this, args...)
+    if object?.acceptVisitor?
+      object.acceptVisitor(this, args)
     else
-      name = object.constructor.name
-      method = this['visit_' + name]
-      throw new Error("Cannot visit #{name}") unless method
-      method.apply(this, arguments)
+      visitPrimitive.apply(this, arguments)
 
 for moduleName in ["Expressions", "Relations"]
   _.each Monarch[moduleName], (klass, klassName) ->
     methodName = "visit_#{moduleName}_#{klassName}"
-    klass.prototype.acceptVisitor = (visitor, args...) ->
+    klass.prototype.acceptVisitor = (visitor, args) ->
       visitor[methodName](this, args...)
+
+visitPrimitive = (object) ->
+  name = visiteeName(object)
+  method = this['visit_' + name]
+  throw new Error("Cannot visit #{name}") unless method
+  method.apply(this, arguments)
+
+visiteeName = (object) ->
+  switch object
+    when null then "null"
+    when undefined then "undefined"
+    else object.constructor.name

spec/server/util/visitor_spec.coffee

@@ -10,6 +10,8 @@ describe "Visitor", ->
   SomeVisitor =
     name: "Joe-Visitor"
     visit: Monarch.Util.Visitor.visit,
+    visit_null: (obj, arg) ->
+      "#{@name} visited null with #{arg}"
     visit_SomeModule_AcceptingClass: (obj, arg) ->
       "#{@name} visited accepting class with #{arg}"
     visit_NonAcceptingClass: (obj, arg) ->
@@ -33,8 +35,13 @@ describe "Visitor", ->
             SomeVisitor.visit(new OtherClass)
           ).toThrow(new Error("Cannot visit OtherClass"))
 
+    describe "when the visitee is null", ->
+      it "calls the visit_Null method", ->
+        expect(SomeVisitor.visit(null, 2)).toBe(
+          'Joe-Visitor visited null with 2')
+
     describe "when no visitee is passed", ->
       it "throws an exception if no object is passed", ->
-        expect(-> SomeVisitor.visit(undefined)).toThrow(new Error("Cannot visit undefined"))
-        expect(-> SomeVisitor.visit(null)).toThrow(new Error("Cannot visit null"))
+        expect(-> SomeVisitor.visit(undefined)).toThrow(
+          new Error("Cannot visit undefined"))