doc parameters for roles

Author: gfldex

doc/Language/typesystem.pod6

@@ -563,6 +563,13 @@ be achieved via C<subset>s.
    role R[A-or-B ::T] {};
    R[A.new].new;
 
+Default parameters can be provided.
+
+    role R[$p = fail("Please provide a parameter to role R")] {};
+    my $i = 1 does R;
+    CATCH { default { say .^name, ': ', .Str} }
+    # OUTPUT«X::AdHoc: Could not instantiate role 'R':␤Please provide a parameter to role R␤»
+
 =head3 Versioning and Authorship
 
 Versioning and authorship can be applied via adverbs X«C«:ver<>»|:ver<> (role)» and X«C«:auth<>»|:auth<> (role)».