Differences

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--- at:tutorial:actors [2007/04/01 12:50] – added tvcutsem
+++ at:tutorial:actors [2007/04/06 20:00] – added tvcutsem
@@ Line 73: / Line 73: @@
     system.println("sum = " + sum);
   };
-};
+});
 >>nil
 </code>
@@ Line 114: / Line 114: @@
   - it is parameter-passed by-copy rather than by-reference in inter-actor message sends. The copy of the isolate received by the remote actor can only access that actor's global lexical scope, no longer the global scope of its original host.
   - external method definitions on isolates are disallowed. The reason for this is that external method definitions implicitly carry a lexical scope (the scope of their definition). Hence, if an isolate with external methods has to be copied, those scopes would have to be copied as well. Following the rule that objects  encapsulating a lexical scope are pass-by-reference, we chose to disallow external methods on isolates.
+Returning to the calculator example, the calculator can now add complex numbers locally and send (a copy of) the resulting complex number back to the customer:
+<code>
+>calculator<-add(
+  complexNumber.new(1,1),
+  complexNumber.new(2,2),
+  object: {
+    def result(sum) {
+      system.println("sum=("+sum.re+","+sum.im+")");
+    };
+  });
+>>nil
+sum=(3,3)
+</code>
+<note>
+A word of warning: isolates are objects that are copied freely between actors. As a result, they should be objects whose actual object identity is of little importance. Usually, the identity of by-copy objects is determined by the value of some of the object's fields. Therefore, it is good practice to override the ''=='' method on isolates to compare isolates based on their semantic identity, rather than on their object identity. For example, equality for complex numbers should be defined as:
+<code>
+def ==(other) {
+  (re == other.re).and: { im == other.im }
+}
+</code>
+</note>
+It is important to note that an isolate has no access whatsoever to its encompassing scope. The following code results in an exception:
+<code>
+>def x := 1;
+def adder := isolate: {
+  def add(n) { x + n };
+};
+adder.add(3)
+>>Undefined variable access: x
+origin:
+at adder.add(3)
+</code>
+Sometimes it is useful to initialize an isolate with the values of lexically visible variables. In that case, AmbientTalk allows the programmer to specify which lexical variables should be //copied into// the isolate itself, such that the isolate has its own, local copy of the variable. Lexical variables that need to be copied like this are specified as formal parameters to the closure passed to the ''isolate:'' primitive, as follows:
+<code>
+>def x := 1;
+def adder := isolate: { |x|
+  def add(n) { x + n };
+};
+adder.add(3)
+>>4
+</code>
 === Futures ===
-futures language construct
+As you may have noticed previously, asynchronous message sends do not return any value (that is, they return ''nil''). Quite often, the developer is required to work around this lack of return values by means of e.g. explicit customer objects, as shown previously. This, however, leads to less expressive, more difficult to understand code, where the control flow quickly becomes implicit.
+The most well-known language feature to reconcile return values with asynchronous message sends is the notion of a //future//. Futures are objects that represent return values that may not yet have been computed. Once the asynchronously invoked method has completed, the future is replaced with the actual return value, and objects that referred to the future transparently refer to the return value.
+Using futures, it is possible to re-implement the previous example of requesting our calculator actor to add two numbers as follows:
+<code>
+def sum := calculator<-add(1,2);
+</code>
+Futures are a frequently recurring language feature in concurrent and distributed languages (for example, in ABCL, the actor-based concurrent language). They are also commonly known by the name of //promises// (this is how they are called in the [[http://www.erights.org|E language]] and in Argus). In AmbientTalk, futures are not native to the language. However, because of AmbientTalk's reflective infrastructure, it is possible to build futures on top of the language. The system library shipped with AmbientTalk contains exactly this: a reflective implementation that adds futures to the language kernel. This implementation can be found in the file ''at/lang/futures.at''.
+To enable futures, it suffices to import the futures module and to enable it, as follows:
+<code>
+import /.at.lang.futures;
+enableFutures(true);
+</code>
+The first statement imports the futures module into the current lexical scope. This enables you as a developer to use some additional language constructs exported by the futures module, as will be explained later. The second statement enables the futures behaviour, causing any asynchronous message send to return a future rather than ''nil''.
 === Actor Mirrors ===