fill in role in /language/typesystem

Author: gfldex

doc/Language/typesystem.pod6

@@ -402,10 +402,6 @@ L<X::Attribute::Required|/type/X::Attribute::Required>.
     CATCH { default { say .^name => .Str } }
     # OUTPUT«X::Attribute::Required => The attribute '$!attr' is required, but you did not provide a value for it.␤»
 
-=head3 trait C<does>
-
-TODO
-
 =head3 trait C<hides>
 
 TODO
@@ -442,6 +438,42 @@ C<.^ver> and C<^.auth>.
 
 =head2 C<role>
 
+Roles are class fragments, which allow the definition of interfaces that are
+shared by classes. The C<role> declarator also introduces a type object that
+can be used for type checks. Roles can be mixed into classes and objects at
+runtime and compile time. The C<role> declarator returns the created type
+object thus allowing the definition of anonymous roles and in-place mixins.
+
+    role Serialize {
+        method to-string { self.Str }
+        method to-number { self.Num }
+    }
+    
+    class A does Serialize {}
+    class B does Serialize {}
+    
+    my Serialize @list;
+    @list.push: A.new;
+    @list.push: B.new;
+    
+    say @list».to-string;
+    # OUTPUT«[A<57192848> B<57192880>]␤»
+
+Use C<...> as the only element of a method body to declare a method to be
+abstract. Any class getting such a method mixed in has to overload it. If the
+method is not overloaded before the end of the compilation unit
+C<X::Comp::AdHoc> will be thrown.
+
+    EVAL 'role R { method overload-this(){...} }; class A does R {}; ';
+    CATCH { default { say .^name, ' ', .Str } }
+    # OUTPUT«X::Comp::AdHoc Method 'overload-this' must be implemented by A because it is required by a role␤»
+
+=head3 trait C<does>
+
+TODO
+
+=head3 trait C<but>
+
 TODO
 
 =head3 Role Arguments