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--- at:tutorial:appendix [2008/07/10 16:23] – * tvcutsem
+++ at:tutorial:appendix [2008/07/10 16:39] – added tvcutsem
@@ Line 392: / Line 392: @@
 === 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 ''/.at.lang.futures'' provides support for futures. Futures have already been described as part of the [[:at:tutorial:actors#futures|concurrency]] section in the tutorial.
 The module exports the type tags ''OnewayMessage'', ''FutureMessage'' and ''Due'':
@@ Line 505: / Line 505: @@
 ==== Structural Types ====
+The module ''/.at.lang.structuraltypes'' implements a small library to use structural typing. The library allows for the creation of 'protocols', which are first-class structural types. A structural type is simply a set of selectors. An object o conforms to a protocol P <=> for all selectors s of P, o respondsTo s where respondsTo is determined by o's mirror.
+A structural type can be branded with type tags. In this case, objects only conform to the type if they are structurally conformant **and** if they are tagged with the structural type's brands.
+Use the ''protocol:'' function to create a new protocol:
+<code>
+def PersonProtocol := protocol: {
+  def name;
+  def age;
+} named: `Person;
+</code>
+The ''`Person'' argument is used to give the protocol a name simply for display purposes. The ''object:implements:'' function automatically checks whether an object conforms to any declared types:
+<code>
+def tom := object: {
+  def name := "Tom";
+  def age() { 24 };
+} implements: PersonProtocol;
+</code>
+You can also create a protocol from an object:
+<code>def tomsProtocol := protocolOf: tom;</code>
+You can test protocol conformance in either of two styles:
+  * ''does: tom implement: PersonProtocol'' => true or false
+  * ''PersonProtocol <? tom'' => true or false
+You can also force a ''StructuralTypeMismatch'' exception to be raised if the object does not conform to the type:
+  *  ''ensure: tom implements: PersonProtocol'' => true or exception
+  *  ''PersonProtocol.checkConformance(tom)'' => true or exception
+More usage examples of structural types can be found in the unit test defined in the file ''at/lang/structuraltypes.at''.
 ==== Traits ====
+The module ''/.at.lang.traits'' exports a small library to use AmbientTalk's traits in a more structured manner. In the literature, traits are described as reusable components with two interfaces: an interface of methods that are //provided// by the trait //to// the composite and an interface of methods that are //required// by the trait //from// the composite. AmbientTalk's traits only make the provided interface explicit. The required interface remains implicit and unchecked at composition time.
+Using the ''traits'' module, a trait can specify that it requires its composite to adhere to a certain protocol (i.e. a structural type, cf. the previous section). Using traits in this way requires an explicit composition step where the trait's requirements are checked.
+To define a "structured" trait, define your  trait objects as follows:
+<code>
+trait: {
+  ...
+} requiring: Protocol;
+</code>
+The above code creates a trait that can only be composed into an object adhering to the specified protocol. To compose traits, use the following language construct:
+<code>
+object: {
+  use: {
+    import T1 exclude ...;
+    import T2 alias ...;
+  }
+}
+</code>
+The ''use:'' block can **only** include ''import'' statements. It simply executes the ''import'' statements, but in addition checks whether the composite, //after// having imported all of its traits, provides all of the methods specified in the required protocol of its imported traits.
+Note that the place where ''use:'' is used inside an object matters: if one of the traits requires a method ''m()'' that is defined only later in the composite, the check will fail. To avoid this, place the ''use:'' block at the bottom of the object declaration.
+Usage examples can be found in the unit tests in the file ''at/lang/traits.at''.