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--- at:tutorial:appendix [2008/07/10 15:19] – added tvcutsem
+++ at:tutorial:appendix [2008/07/10 15:42] – added tvcutsem
@@ Line 344: / Line 344: @@
 The above code defines an isolate object ''i'' which, when passed between actors, becomes a ''some.Object'' on the other side. Note that state (''foo'' in the example) can be transferred as usual via the parameter list of the closure.
+===== Custom Exceptions =====
+The module ''/.at.exceptions'' defines a number of auxiliary methods which can be used to define one's own custom exceptions. Here is how to define a custom exception ''FooException''. First, define a new type tag with which clients of your code can catch the exception:
+<code>
+deftype FooException;
+</code>
+Next, define a prototype exception object using the ''createException'' function exported by the exception module. As a convention, an exception prototype object is prefixed with ''X'':
+<code>
+def XFooException := createException(FooException);
+</code>
+''XFooException'' is now bound to an object which is tagged with the given type tag, and which implements two methods: ''stackTrace'', which returns an AmbientTalk stack trace for the exception, and ''message'', which returns a string indicating what went wrong. The object also has a constructor taking a new message as an argument. You can now raise your custom exception as follows:
+<code>
+raise: XFooException.new("reason for what went wrong");
+</code>
+If your custom exception requires additional state, you can define it as an extension of the prototype exception. If you define a custom constructor, do not forget to initialise the parent object, as follows:
+<code>
+deftype IndexOutOfBounds;
+def XIndexOutOfBounds := createException(IndexOutOfBounds) with: {
+  def min;
+  def max;
+  def idx;
+  def init(min, max, idx) {
+    super^init("Index out of bounds: given " + idx + " allowed: [" + min + "," + max + "]");
+    self.min := min;
+    self.max := max;
+    self.idx := idx;
+  };
+}
+</code>
+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 ====
+==== Dynamic Variables ====
+==== Ambient References ====
+==== Structural Types ====
+==== Traits ====