fix length/2 error handling.

C/utilpreds.c

@@ -5148,10 +5148,41 @@ static Int
 p_skip_list( USES_REGS1 ) {
   Term *tail;
   Int len = Yap_SkipList(XREGS+2, &tail);
+
   return Yap_unify(MkIntegerTerm(len), ARG1) &&
     Yap_unify(*tail, ARG3);
 }
 
+static Int 
+p_skip_list4( USES_REGS1 ) {
+  Term *tail;
+  Int len = Yap_SkipList(XREGS+1, &tail);
+  Term t2 = Deref(ARG2), t = *tail;
+
+  if (!IsVarTerm(t2)) {
+    Int len1;
+    if (!IsIntegerTerm(t2)) {
+      Yap_Error(TYPE_ERROR_INTEGER, t2, "length/2");
+      return FALSE;
+    }
+    if ((len1 = IntegerOfTerm(t2)) < 0) {
+      Yap_Error(DOMAIN_ERROR_NOT_LESS_THAN_ZERO, t2, "length/2");
+      return FALSE;
+    }
+    if (t == TermNil)
+      return
+	len1 == len &&
+	Yap_unify(*tail, ARG4);
+  }
+  /* don't set M0 if full list, just check M */
+  if (t == TermNil) {
+    return Yap_unify_constant(ARG4, TermNil) &&
+      Yap_unify_constant(ARG2, MkIntegerTerm(len));
+  }
+  return Yap_unify(MkIntegerTerm(len), ARG3) &&
+    Yap_unify(t, ARG4);
+}
+
 void Yap_InitUtilCPreds(void)
 {
   CACHE_REGS
@@ -5174,6 +5205,7 @@ void Yap_InitUtilCPreds(void)
   Yap_InitCPred("unnumbervars", 2, p_unnumbervars, 0);
   /* use this carefully */
   Yap_InitCPred("$skip_list", 3, p_skip_list, SafePredFlag|TestPredFlag);
+  Yap_InitCPred("$skip_list", 4, p_skip_list4, SafePredFlag|TestPredFlag);
   CurrentModule = TERMS_MODULE;
   Yap_InitCPred("variable_in_term", 2, p_var_in_term, 0);
   Yap_InitCPred("term_hash", 4, p_term_hash, 0);

pl/sort.yap

@@ -25,10 +25,10 @@
 %   length of a list.
 
 length(L, M) :-
-	  '$skip_list'(M0, L, R),
+	  '$skip_list'(L, M, M0, R),
-	  ( R == [] -> M = M0 ;
+	  ( R == [] -> true ;
 	    var(R) -> '$$_length'(R, M, M0) ;
-	    '$do_error'(type_error(list,L),length(L,M))
+	    L \= [_|_], '$do_error'(type_error(list,L),length(L,M))
 	  ).
 
 %