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--- at:tutorial:appendix [2008/07/10 15:29] – Added tvcutsem
+++ at:tutorial:appendix [2008/07/10 15:53] – added tvcutsem
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 The exception module also exports an auxiliary function ''error(msg)'' which can be used to raise a "quick and dirty" runtime exception with a given message. It also exports the prototypes of a number of standard exceptions that can be raised by the language runtime itself.
+===== Language Extensions =====
+The files in the ''at/lang'' directory define custom language features which mostly use AmbientTalk/2's reflective facilities to extend the language.
+==== Futures and Multifutures ====
+=== Futures ===
+The module ''/.at.lang.futures'' provides support for futures. Futures have already been described as part of the [[concurreny|concurrency]] section in the tutorial.
+The module exports the type tags ''OnewayMessage'', ''FutureMessage'' and ''Due'':
+  * Tagging an asynchronous message with ''FutureMessage'' will attach a future to the message.
+  * Tagging a message with ''OnewayMessage'' ensures no future will ever be attached to the message.
+  * Tagging a message with ''@Due(timeout)'' associates a future with the message that is automatically ruined with a ''TimeoutException'' after the given ''timeout'' period (in milliseconds) has elapsed.
+Messages can be automatically associated with a future by invoking the ''enableFutures()'' function, which enables futures for all messages, except those tagged as a ''OnewayMessage''.
+The futures module also exports the function ''when:becomes:'' to await the resolution of a future, and auxiliary ''when:becomes:catch:using:'' functions.
+Futures can also be created and resolved manually:
+<code>
+import /.at.lang.futures;
+def [fut, res] := makeFuture();
+when: someAsynchronousComputation() becomes: { |value|
+  res.resolve(value); // resolve the future manually
+}
+fut // return the future to a client
+</code>
+Finally, the futures module also provides some auxiliary functions, of which ''group:'' is often a very useful one. The ''group:'' construct groups a table of futures into a single future which is resolved with a table of values or ruined with an exception:
+<code>
+when: (group: [ a<-m(), b<-n() ]) becomes: { |values|
+  def [aResult, bResult] := values;
+  ...
+}
+</code>
+=== Multifutures ===
+The module ''/.at.lang.multifutures'' provides support for multifutures.
+==== Dynamic Variables ====
+==== Ambient References ====
+==== Structural Types ====
+==== Traits ====