is_list/1 is now a builtin.

C/utilpreds.c

@@ -2821,6 +2821,23 @@ p_unifiable(void)
   return Yap_unify(ARG3, tf);
 }
 
+int
+Yap_IsListTerm(Term t)
+{
+  while (!IsVarTerm(t) && IsPairTerm(t)) {
+    t = TailOfTerm(t);
+  }
+  return t == TermNil;
+}
+
+static Int
+p_is_list(void)
+{
+  return Yap_IsListTerm(Deref(ARG1));
+}
+
+
+
 void Yap_InitUtilCPreds(void)
 {
   Term cm = CurrentModule;
@@ -2832,6 +2849,7 @@ void Yap_InitUtilCPreds(void)
   Yap_InitCPred("$non_singletons_in_term", 3, p_non_singletons_in_term, SafePredFlag|HiddenPredFlag);
   Yap_InitCPred("term_variables", 2, p_term_variables, 0);
   Yap_InitCPred("term_variables", 3, p_term_variables3, 0);
+  Yap_InitCPred("is_list", 1, p_is_list, SafePredFlag);
   CurrentModule = TERMS_MODULE;
   Yap_InitCPred("variable_in_term", 2, p_var_in_term, SafePredFlag);
   Yap_InitCPred("term_hash", 4, p_term_hash, SafePredFlag);

H/Yapproto.h

@@ -353,6 +353,7 @@ void	STD_PROTO(Yap_InitUserBacks,(void));
 
 /* utilpreds.c */
 Term	STD_PROTO(Yap_CopyTerm,(Term));
+int	STD_PROTO(Yap_IsListTerm,(Term));
 Term	STD_PROTO(Yap_CopyTermNoShare,(Term));
 int	STD_PROTO(Yap_SizeGroundTerm,(Term, int));
 void	STD_PROTO(Yap_InitUtilCPreds,(void));

library/lists.yap

@@ -8,7 +8,6 @@
 	   append/3,
 	   append/2,
 	   delete/3,
-	   is_list/1,
 	   last/2,
 	   member/2,
 	   memberchk/2,
@@ -83,14 +82,6 @@ delete([Head|List], Elem, [Head|Residue]) :-
 	delete(List, Elem, Residue).
 
 
-% is_list(List)
-% is true when List is a proper List
-%
-is_list(L) :- var(L), !, fail.
-is_list([]).
-is_list([_|List]) :- is_list(List).
-
- 
 %   last(List, Last)
 %   is true when List is a List and Last is identical to its last element.
 %   This could be defined as last(L, X) :- append(_, [X], L).

library/yap2swi/yap2swi.c

@@ -1403,12 +1403,7 @@ X_API int PL_is_integer(term_t ts)
 X_API int PL_is_list(term_t ts)
 {
   YAP_Term t = Yap_GetFromSlot(ts);
-  if (YAP_IsPairTerm(t)) {
-    return 1;
-  } else if (IsAtomTerm(t)) {
-    return t == TermNil;
-  } else
-    return 0;
+  return Yap_IsListTerm(t);
 }
 
 X_API int PL_is_number(term_t ts)