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--- at:tutorial:appendix [2008/07/10 16:32] – * tvcutsem
+++ at:tutorial:appendix [2008/07/10 16:39] – added tvcutsem
@@ Line 542: / Line 542: @@
 ==== 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''.