print attributes of variables when attribute_goals/2 is not defined

src/prolog/lib/atts.pl

@@ -9,6 +9,17 @@
 
 :- op(1199, fx, attribute).
 
+% represent the list of attributes belonging to a variable,
+% of a particular module, as a list of terms of the form
+% Module:put_atts(V, ListOfAtts).
+'$default_attr_list'(Module, V, ListOfAtts) :-
+    Module:get_atts(V, Attributes),
+    '$default_attr_list'(Attributes, Module, V, ListOfAtts).
+
+'$default_attr_list'([PG | PGs], Module, AttrVar, [Module:put_atts(AttrVar, PG) | Gs]) :-
+    '$default_attr_list'(PGs, Module, AttrVar, Gs).
+'$default_attr_list'([], _, _, []).
+
 '$absent_attr'(V, Attr) :-
     '$get_attr_list'(V, Ls),
     '$absent_from_list'(Ls, Attr).

src/prolog/machine/mod.rs

@@ -187,7 +187,7 @@ impl Machine {
                 self.machine_st.attr_var_init.project_attrs_loc = self.code_repo.code.len();
                 self.code_repo.code.extend(code.into_iter());
             }
-            Err(_) => panic!("Machine::compile_special_forms() failed at PROJECT_ATTRS"),
+            Err(e) => panic!("Machine::compile_special_forms() failed at PROJECT_ATTRS: {}", e),
         }
     }
 

src/prolog/machine/project_attributes.pl

@@ -40,7 +40,7 @@ call_goals([], _).
 call_goals([AttrVar|AttrVars], Module) :-
     (   catch(Module:attribute_goals(AttrVar, Goal),
 	      error(evaluation_error((Module:attribute_goals)/2), attribute_goals/2),
-	      true),
+	      atts:'$default_attr_list'(Module, AttrVar, Goal)),
 	nonvar(Goal) -> enqueue_goal(Goal)
     ;   true
     ),