Fix/non bound substitution variables

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).",

x/logic/util/prolog.go

@@ -73,18 +73,39 @@ 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)
 		}
 		results = append(results, types.Result{Substitutions: substitutions})
 	}
 	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
+}