fix nb_getval to handle exceptions.

C/globals.c

@@ -1286,8 +1286,7 @@ p_nb_getval(void)
   }
   ge = FindGlobalEntry(AtomOfTerm(t));
   if (!ge) {
-    Yap_Error(EXISTENCE_ERROR_VARIABLE,t,"nb_getval");
-    return FALSE;
+    return Yap_unify(TermNil, ARG3);
   }
   READ_LOCK(ge->GRWLock);
   to = ge->global;
@@ -2636,11 +2635,10 @@ void Yap_InitGlobals(void)
   Yap_InitCPred("$allocate_arena", 2, p_allocate_arena, 0);
   Yap_InitCPred("arena_size", 1, p_default_arena_size, 0);
   Yap_InitCPred("b_setval", 2, p_b_setval, SafePredFlag);
-  Yap_InitCPred("b_getval", 2, p_nb_getval, SafePredFlag);
   Yap_InitCPred("nb_setval", 2, p_nb_setval, 0L);
   Yap_InitCPred("nb_set_shared_val", 2, p_nb_set_shared_val, 0L);
   Yap_InitCPred("nb_linkval", 2, p_nb_linkval, 0L);
-  Yap_InitCPred("nb_getval", 2, p_nb_getval, SafePredFlag);
+  Yap_InitCPred("$nb_getval", 3, p_nb_getval, SafePredFlag);
   Yap_InitCPred("nb_setarg", 3, p_nb_setarg, 0L);
   Yap_InitCPred("nb_set_shared_arg", 3, p_nb_set_shared_arg, 0L);
   Yap_InitCPred("nb_linkarg", 3, p_nb_linkarg, 0L);

pl/boot.yap

@@ -161,10 +161,10 @@ true :- true.
 	'$system_catch'('$do_yes_no'((G->true),user),user,Error,user:'$Error'(Error)),
 	fail.
 '$enter_top_level' :-
-	nb_getval('$break',BreakLevel),
+	'$nb_getval'('$break',BreakLevel,fail),
 	 '$debug_on'(DBON),
 	(
-	 nb_getval('$trace',on)
+	 '$nb_getval'('$trace', on, fail)
 	->
 	 TraceDebug = trace
 	;
@@ -349,7 +349,7 @@ true :- true.
 	 '$do_error'(type_error(callable,R),meta_call(Source)).
  '$execute_command'(end_of_file,_,_,_,_) :- !.
  '$execute_command'(Command,_,_,_,_) :-
-	 nb_getval('$if_skip_mode',skip),
+	 '$nb_getval'('$if_skip_mode', skip, fail),
 	 \+ '$if_directive'(Command),
 	 !.
  '$execute_command'((:-G),_,_,Option,_) :- !,
@@ -1296,3 +1296,28 @@ throw(Ball) :-
 '$run_at_thread_start'.
 
 
+nb_getval(GlobalVariable, Val) :-
+	'$nb_getval'(GlobalVariable, Val, Error),
+	(var(Error)
+	->
+	 true
+	;
+	 '$getval_exception'(GlobalVariable, Val, nb_getval(GlobalVariable, Val)) ->
+	 nb_getval(GlobalVariable, Val)
+	;
+	 '$do_error'(existence_error(variable, GlobalVariable),nb_getval(GlobalVariable, Val))
+	).
+		    
+
+b_getval(GlobalVariable, Val) :-
+	'$nb_getval'(GlobalVariable, Val, Error),
+	(var(Error)
+	->
+	 true
+	;
+	 '$getval_exception'(GlobalVariable, Val, b_getval(GlobalVariable, Val)) ->
+	 true
+	;
+	 '$do_error'(existence_error(variable, GlobalVariable),b_getval(GlobalVariable, Val))
+	).
+		    

pl/init.yap

@@ -202,6 +202,8 @@ yap_hacks:cut_by(CP) :- '$$cut_by'(CP).
 
 :- multifile user:portray_message/2.
 
+:- multifile user:exception/3.
+
 file_search_path(library, Dir) :-
 	library_directory(Dir).
 file_search_path(commons, Dir) :-

pl/utils.yap

@@ -580,3 +580,23 @@ nb_current(GlobalVariable, Val) :-
 	'$nb_current'(GlobalVariable),
 	nb_getval(GlobalVariable, Val).
 
+'$getval_exception'(GlobalVariable, Val, Caller) :-
+	user:exception(undefined_global_variable, GlobalVariable, Action),
+	!,
+	(
+	 Action == fail
+	->
+	 fail
+	;
+	 Action == retry
+	->
+	 b_getval(GlobalVariable, Val)
+	;
+	 Action == error
+	->
+	 '$do_error'(existence_error(variable, GlobalVariable),Caller)
+	;
+	 '$do_error'(type_error(atom, Action),Caller)
+	).
+
+