static bool Eval(WrenVM vm, Obj[] args, int stackStart)
        {
            if (args[stackStart + 1] is ObjString)
            {

                // Eval the code in the module where the calling function was defined.
                Obj callingFn = vm.Fiber.GetFrame().Fn;
                ObjModule module = (callingFn is ObjFn)
                    ? ((ObjFn)callingFn).Module
                    : ((ObjClosure)callingFn).Function.Module;

                // Compile it.
                ObjFn fn = Compiler.Compile(vm, module, "", args[stackStart + 1].ToString(), false);

                if (fn == null)
                {
                    vm.Fiber.Error = Obj.MakeString("Could not compile source code.");
                    return false;
                }

                // TODO: Include the compile errors in the runtime error message.

                // Create a fiber to run the code in.
                ObjFiber evalFiber = new ObjFiber(fn) { Caller = vm.Fiber };

                // Switch to the fiber.
                args[stackStart] = evalFiber;

                return false;
            }

            vm.Fiber.Error = Obj.MakeString("Source code must be a string.");
            return false;
        }

 public Obj GetKey(int index)
 {
     if (index < 0 || index >= _entries.Count)
         return Undefined;
     Obj[] v = new Obj[_entries.Count];
     _entries.Keys.CopyTo(v, 0);
     return v[index];
 }

        // Removes [key] from [map], if present. Returns the value for the key if found
        // or `NULL_VAL` otherwise.
        public Obj Remove(Obj key)
        {
            Obj v;
            if (!_entries.TryGetValue(key, out v)) 
                return Null;

            _entries.Remove(key);
            return v;
        }

        // Resets [fiber] back to an initial state where it is ready to invoke [fn].
        private void ResetFiber(Obj fn)
        {
            Stack = new Obj[InitialStackSize];
            Capacity = InitialStackSize;
            Frames = new List<CallFrame>();

            // Push the stack frame for the function.
            StackTop = 0;
            NumFrames = 1;
            OpenUpvalues = null;
            Caller = null;
            Error = null;
            CallerIsTrying = false;

            CallFrame frame = new CallFrame { Fn = fn, StackStart = 0, Ip = 0 };
            Frames.Add(frame);
        }

        // TODO: The argument list here is getting a bit gratuitous.
        // Creates a new function object with the given code and constants. The new
        // function will take over ownership of [bytecode] and [sourceLines]. It will
        // copy [constants] into its own array.
        public ObjFn(ObjModule module,
            Obj[] constants,
            int numUpvalues, int arity,
            byte[] bytecode, ObjString debugSourcePath,
            string debugName, int[] sourceLines)
        {
            Bytecode = bytecode;
            Constants = constants;
            Module = module;
            NumUpvalues = numUpvalues;
            NumConstants = constants.Length;
            Arity = arity;

            /* Debug Information */
            SourcePath = debugSourcePath;
            Name = debugName;
            SourceLines = sourceLines;

            ClassObj = WrenVM.FnClass;
        }

        static bool prim_map_containsKey(WrenVM vm, Obj[] args, int stackStart)
        {
            ObjMap map = (ObjMap)args[stackStart];

            if (ValidateKey(args[stackStart + 1]))
            {
                Obj v = map.Get(args[stackStart + 1]);

                args[stackStart] = Obj.Bool(v != Obj.Undefined);
                return true;
            }

            vm.Fiber.Error = Obj.MakeString("Key must be a value type or fiber.");
            return false;
        }

 static bool prim_map_count(WrenVM vm, Obj[] args, int stackStart)
 {
     ObjMap m = (ObjMap)args[stackStart];
     args[stackStart] = new Obj(m.Count());
     return true;
 }

        static bool prim_map_subscriptSetter(WrenVM vm, Obj[] args, int stackStart)
        {
            if (ValidateKey(args[stackStart + 1]))
            {
                ObjMap map = args[stackStart] as ObjMap;

                if (map != null)
                {
                    map.Set(args[stackStart + 1], args[stackStart + 2]);
                }
                args[stackStart] = args[stackStart + 2];
                return true;
            }

            vm.Fiber.Error = Obj.MakeString("Key must be a value type or fiber.");
            return false;
        }

 static bool prim_map_clear(WrenVM vm, Obj[] args, int stackStart)
 {
     ObjMap m = args[stackStart] as ObjMap;
     if (m != null)
         m.Clear();
     args[stackStart] = Obj.Null;
     return true;
 }

        // Defines [methodValue] as a method on [classObj].
        private static bool BindMethod(bool isStatic, int symbol, ObjClass classObj, Obj methodContainer)
        {
            // If we are binding a foreign method, just return, as this will be handled later
            if (methodContainer is ObjString)
                return true;

            ObjFn methodFn = methodContainer as ObjFn ?? ((ObjClosure)methodContainer).Function;

            Method method = new Method { MType = MethodType.Block, Obj = methodContainer };

            if (isStatic)
                classObj = classObj.ClassObj;

            // Methods are always bound against the class, and not the metaclass, even
            // for static methods, because static methods don't have instance fields
            // anyway.
            Compiler.BindMethodCode(classObj, methodFn);

            classObj.BindMethod(symbol, method);
            return true;
        }

 static bool prim_fn_toString(WrenVM vm, Obj[] args, int stackStart)
 {
     args[stackStart] = Obj.MakeString("<fn>");
     return true;
 }

        private ObjFiber LoadModule(Obj name, string source)
        {
            ObjModule module = GetModule(name);

            // See if the module has already been loaded.
            if (module == null)
            {
                module = new ObjModule(name as ObjString);

                // Store it in the VM's module registry so we don't load the same module
                // multiple times.
                _modules.Set(name, module);

                // Implicitly import the core module.
                ObjModule coreModule = GetCoreModule();
                foreach (ModuleVariable t in coreModule.Variables)
                {
                    DefineVariable(module, t.Name, t.Container);
                }
            }

            ObjFn fn = Compiler.Compile(this, module, name.ToString(), source, true);
            if (fn == null)
            {
                // TODO: Should we still store the module even if it didn't compile?
                return null;
            }

            ObjFiber moduleFiber = new ObjFiber(fn);

            // Return the fiber that executes the module.
            return moduleFiber;
        }

 static bool prim_list_add(WrenVM vm, Obj[] args, int stackStart)
 {
     ObjList list = args[stackStart] as ObjList;
     if (list == null)
     {
         vm.Fiber.Error = Obj.MakeString("Trying to add to a non-list");
         return false;
     }
     list.Add(args[stackStart + 1]);
     args[stackStart] = args[stackStart + 1];
     return true;
 }

        static bool prim_list_iterate(WrenVM vm, Obj[] args, int stackStart)
        {
            ObjList list = (ObjList)args[stackStart];

            // If we're starting the iteration, return the first index.
            if (args[stackStart + 1] == Obj.Null)
            {
                if (list.Count() != 0)
                {
                    args[stackStart] = new Obj(0.0);
                    return true;
                }

                args[stackStart] = Obj.False;
                return true;
            }

            if (args[stackStart + 1].Type == ObjType.Num)
            {
                int index = (int)args[stackStart + 1].Num;
                if (args[stackStart + 1].Num == index)
                {
                    if (!(index < 0) && !(index >= list.Count() - 1))
                    {
                        // Move to the next index.
                        args[stackStart] = new Obj(index + 1);
                        return true;
                    }

                    // Stop if we're out of bounds.
                    args[stackStart] = Obj.False;
                    return true;
                }

                vm.Fiber.Error = Obj.MakeString("Iterator must be an integer.");
                return false;
            }

            vm.Fiber.Error = Obj.MakeString("Iterator must be a number.");
            return false;
        }

        bool CheckArity(Obj[] args, int numArgs, int stackStart)
        {
            ObjFn fn = args[stackStart] as ObjFn;
            ObjClosure c = args[stackStart] as ObjClosure;

            if (c != null)
            {
                fn = c.Function;
            }

            if (fn == null)
            {
                Fiber.Error = Obj.MakeString("Receiver must be a function or closure.");
                return false;
            }

            if (numArgs - 1 < fn.Arity)
            {
                Fiber.Error = Obj.MakeString("Function expects more arguments.");
                return false;
            }

            return true;
        }

        static bool prim_list_insert(WrenVM vm, Obj[] args, int stackStart)
        {
            ObjList list = (ObjList)args[stackStart];

            if (args[stackStart + 1].Type == ObjType.Num)
            {
                int index = (int)args[stackStart + 1].Num;
                if (args[stackStart + 1].Num == index)
                {
                    if (index < 0)
                        index += list.Count() + 1;
                    if (index >= 0 && index <= list.Count())
                    {
                        list.Insert(args[stackStart + 2], index);
                        args[stackStart] = args[stackStart + 2];
                        return true;
                    }
                    vm.Fiber.Error = Obj.MakeString("Index out of bounds.");
                    return false;
                }

                // count + 1 here so you can "insert" at the very end.
                vm.Fiber.Error = Obj.MakeString("Index must be an integer.");
                return false;
            }

            vm.Fiber.Error = Obj.MakeString("Index must be a number.");
            return false;
        }

        static bool prim_list_count(WrenVM vm, Obj[] args, int stackStart)
        {
            ObjList list = args[stackStart] as ObjList;
            if (list != null)
            {
                args[stackStart] = new Obj(list.Count());
                return true;
            }

            vm.Fiber.Error = Obj.MakeString("Trying to clear a non-list");
            return false;
        }

        static bool prim_list_clear(WrenVM vm, Obj[] args, int stackStart)
        {
            ObjList list = args[stackStart] as ObjList;
            if (list == null)
            {
                vm.Fiber.Error = Obj.MakeString("Trying to clear a non-list");
                return false;
            }
            list.Clear();

            args[stackStart] = Obj.Null;
            return true;
        }

        private static bool prim_map_iterate(WrenVM vm, Obj[] args, int stackStart)
        {
            ObjMap map = (ObjMap)args[stackStart];

            if (map.Count() == 0)
            {
                args[stackStart] = Obj.False;
                return true;
            }

            // Start one past the last entry we stopped at.
            if (args[stackStart + 1].Type == ObjType.Num)
            {
                if (args[stackStart + 1].Num < 0)
                {
                    args[stackStart] = Obj.False;
                    return true;
                }
                int index = (int)args[stackStart + 1].Num;

                if (index == args[stackStart + 1].Num)
                {
                    args[stackStart] = index > map.Count() || map.Get(index) == Obj.Undefined ? Obj.False : new Obj(index + 1);
                    return true;
                }

                vm.Fiber.Error = Obj.MakeString("Iterator must be an integer.");
                return false;
            }

            // If we're starting the iteration, start at the first used entry.
            if (args[stackStart + 1] == Obj.Null)
            {
                args[stackStart] = new Obj(1);
                return true;
            }

            vm.Fiber.Error = Obj.MakeString("Iterator must be a number.");
            return false;
        }

        private bool ImportVariable(Obj moduleName, Obj variableName, out Obj result)
        {
            ObjModule module = GetModule(moduleName);
            if (module == null)
            {
                result = Obj.MakeString("Could not load module");
                return false; // Should only look up loaded modules
            }

            ObjString variable = variableName as ObjString;
            if (variable == null)
            {
                result = Obj.MakeString("Variable name must be a string");
                return false;
            }

            int variableEntry = module.Variables.FindIndex(v => v.Name == variable.ToString());

            // It's a runtime error if the imported variable does not exist.
            if (variableEntry == -1)
            {
                result = Obj.MakeString(string.Format("Could not find a variable named '{0}' in module '{1}'.", variableName, moduleName));
                return false;
            }

            result = module.Variables[variableEntry].Container;
            return true;
        }