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--- at:tutorial:actors [2007/04/07 20:10] – tvcutsem
+++ at:tutorial:actors [2007/04/07 20:22] – * tvcutsem
@@ Line 332: / Line 332: @@
 ==== Nesting Actors ====
-lexical scoping rules for nested actors
+In AmbientTalk, objects can be nested inside other objects and functions can be defined in other functions. It makes sense to ask whether actors can also be nested. The answer is that yes, actors too can be nested. However, this is not as straightforward as it seems. Consider the following example:
+<code>
+def outer := actor: {
+  def x := 1;
+  def get() { x };
+  def set(v) { x := v };
+  def inner := actor: {
+    def get() { x };
+    def set(v) { x := v };
+  };
+};
+</code>
+If both the ''outer'' and ''inner'' actors lexically see ''x'', they could modify it concurrently, reintroducing race conditions on the internal state of an actor. Therefore, when defining an actor using a block of code, we disallow access to the enclosing lexical scope by the new actor. It is as if it was defined at top-level. Hence, actors behave similarly to [[#isolates|isolates]] in this respect. The above example is incorrect in that ''inner'' will not be able to read or modify ''x''.
+Recall that isolates could be given selective access to their enclosing lexical scope by specifying accessed variables as formal parameters to their initializing closure, which gave rise to copying the variable into the isolate. We allow actors to do the same. Hence, the above example can be written properly as:
+<code>
+def outer := actor: {
+  def x := 1;
+  def get() { x };
+  def set(v) { x := v };
+  def inner := actor: { |x|
+    def get() { x };
+    def set(v) { x := v };
+  };
+};
+</code>
+It still makes sense to nest actors, but each actor will have its own local copy of lexically shared variables. Furthermore, the value bound to the copied variable is parameter-passed according to inter-actor parameter passing rules, so if ''x'' would be bound to an object, then ''outer'' would have a normal reference to the object, while ''inner'' would receive a far reference to the object.