From 9250bda2ddca9119c9cb18014f3e2008f30c4e0c Mon Sep 17 00:00:00 2001
From: ccamel <camel.christophe@gmail.com>
Date: Wed, 20 Mar 2024 16:49:25 +0100
Subject: [PATCH 1/2] fix(logic): prevent non-bound substitution variables to
 be returned

---
 x/logic/util/prolog.go | 35 ++++++++++++++++++++++++++++-------
 1 file changed, 28 insertions(+), 7 deletions(-)

diff --git a/x/logic/util/prolog.go b/x/logic/util/prolog.go
index 82dafcd2..aac9bcce 100644
--- a/x/logic/util/prolog.go
+++ b/x/logic/util/prolog.go
@@ -73,14 +73,14 @@ func envsToResults(envs []*engine.Env, vars []engine.ParsedVariable, i *prolog.I
 	for _, rEnv := range envs {
 		substitutions := make([]types.Substitution, 0, len(vars))
 		for _, v := range vars {
-			var expression prolog.TermString
-			err := expression.Scan(&i.VM, v.Variable, rEnv)
-			if err != nil {
-				return nil, err
+			if !isBound(v, rEnv) {
+				// skip parsed variables that are not bound (singletons variables or other)
+				continue
 			}
-			substitution := types.Substitution{
-				Variable:   v.Name.String(),
-				Expression: string(expression),
+
+			substitution, err := scanExpression(i, v, rEnv)
+			if err != nil {
+				return results, err
 			}
 			substitutions = append(substitutions, substitution)
 		}
@@ -88,3 +88,24 @@ func envsToResults(envs []*engine.Env, vars []engine.ParsedVariable, i *prolog.I
 	}
 	return results, nil
 }
+
+func scanExpression(i *prolog.Interpreter, v engine.ParsedVariable, rEnv *engine.Env) (types.Substitution, error) {
+	var expression prolog.TermString
+	err := expression.Scan(&i.VM, v.Variable, rEnv)
+	if err != nil {
+		return types.Substitution{}, err
+	}
+	substitution := types.Substitution{
+		Variable:   v.Name.String(),
+		Expression: string(expression),
+	}
+
+	return substitution, nil
+}
+
+// isBound returns true if the given parsed variable is bound in the given environment.
+func isBound(v engine.ParsedVariable, env *engine.Env) bool {
+	_, ok := env.Resolve(v.Variable).(engine.Variable)
+
+	return !ok
+}

From f327463f8320493c314c5ce37af49812aedb9750 Mon Sep 17 00:00:00 2001
From: ccamel <camel.christophe@gmail.com>
Date: Wed, 20 Mar 2024 16:50:01 +0100
Subject: [PATCH 2/2] test(logic): add test case for non-bound substitution
 variables

---
 x/logic/keeper/grpc_query_ask_test.go | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/x/logic/keeper/grpc_query_ask_test.go b/x/logic/keeper/grpc_query_ask_test.go
index ba353780..7a04b6e1 100644
--- a/x/logic/keeper/grpc_query_ask_test.go
+++ b/x/logic/keeper/grpc_query_ask_test.go
@@ -192,6 +192,16 @@ func TestGRPCAsk(t *testing.T) {
 					}}}},
 				},
 			},
+			{
+				program: "father(bob, X) :- true.",
+				query:   "father(B, X).",
+				expectedAnswer: &types.Answer{
+					Variables: []string{"B", "X"},
+					Results: []types.Result{{Substitutions: []types.Substitution{{
+						Variable: "B", Expression: "bob",
+					}}}},
+				},
+			},
 			{
 				program: "father(bob, alice).",
 				query:   "father(bob, X, O).",