1 files changed, 26 insertions, 2 deletions
diff --git a/eval.c b/eval.c
index 6fec2b6..f957553 100644
--- a/eval.c
+++ b/eval.c
@@ -40,8 +40,32 @@ evalquery(Term *query)
 		}else{
 			Predicate *pred = findpredicate(module->predicates, goal);
 			if(pred == nil){
-				print("No predicate matches: %S:%S\n", module->name, prettyprint(goal, 0, 0, 0, nil));
-				goto Backtrack;
+				Rune *name;
+				int arity;
+				Term *replacement = nil;
+				Term *procedure;
+				Term *pi;
+				if(goal->tag == CompoundTerm){
+					name = goal->text;
+					arity = goal->arity;
+				}else{
+					name = prettyprint(goal, 0, 0, 0, nil);
+					arity = 0;
+				}
+				switch(flagunknown){
+				case UnknownError:
+					procedure = mkatom(name);
+					procedure->next = mkinteger(arity);
+					pi = mkcompound(L"/", 2, procedure);
+					replacement = existenceerror(L"procedure", pi);
+					break;
+				case UnknownWarning:
+					print("Warning: no such predicate in module %S: %S/%d\n", module->name, name, arity);
+				case UnknownFail:
+					replacement = mkatom(L"fail");
+				}
+				goalstack = addgoals(goalstack, replacement, module);
+				continue;
 			}
 
 			/* Find a clause where the head unifies with the goal */