diff --git a/role_manager.go b/role_manager.go
index b320c54..faf2d4f 100644
--- a/role_manager.go
+++ b/role_manager.go
@@ -15,6 +15,7 @@
 package sessionrolemanager
 
 import (
+	"errors"
 	"sort"
 
 	"github.com/casbin/casbin/rbac"
@@ -48,16 +49,17 @@ func (rm *RoleManager) createRole(name string) *SessionRole {
 }
 
 // Clear clears all stored data and resets the role manager to the initial state.
-func (rm *RoleManager) Clear() {
+func (rm *RoleManager) Clear() error {
 	rm.allRoles = make(map[string]*SessionRole)
+	return nil
 }
 
 // AddLink adds the inheritance link between role: name1 and role: name2.
 // aka role: name1 inherits role: name2.
 // timeRange is the time range when the role inheritance link is active.
-func (rm *RoleManager) AddLink(name1 string, name2 string, timeRange ...string) {
+func (rm *RoleManager) AddLink(name1 string, name2 string, timeRange ...string) error {
 	if len(timeRange) != 2 {
-		return
+		return errors.New("error: timeRange should be 2 parameters")
 	}
 	startTime := timeRange[0]
 	endTime := timeRange[1]
@@ -67,62 +69,64 @@ func (rm *RoleManager) AddLink(name1 string, name2 string, timeRange ...string)
 
 	session := Session{role2, startTime, endTime}
 	role1.addSession(session)
+	return nil
 }
 
 // DeleteLink deletes the inheritance link between role: name1 and role: name2.
 // aka role: name1 does not inherit role: name2 any more.
 // unused is not used.
-func (rm *RoleManager) DeleteLink(name1 string, name2 string, unused ...string) {
+func (rm *RoleManager) DeleteLink(name1 string, name2 string, unused ...string) error {
 	if !rm.hasRole(name1) || !rm.hasRole(name2) {
-		return
+		return errors.New("error: name1 or name2 does not exist")
 	}
 
 	role1 := rm.createRole(name1)
 	role2 := rm.createRole(name2)
 
 	role1.deleteSessions(role2.name)
+	return nil
 }
 
 // HasLink determines whether role: name1 inherits role: name2.
 // requestTime is the querying time for the role inheritance link.
-func (rm *RoleManager) HasLink(name1 string, name2 string, requestTime ...string) bool {
+func (rm *RoleManager) HasLink(name1 string, name2 string, requestTime ...string) (bool, error) {
 	if len(requestTime) != 1 {
-		return false
+		return false, errors.New("error: requestTime should be 1 parameter")
 	}
 
 	if name1 == name2 {
-		return true
+		return true, nil
 	}
 
 	if !rm.hasRole(name1) || !rm.hasRole(name2) {
-		return false
+		return false, nil
 	}
 
 	role1 := rm.createRole(name1)
-	return role1.hasValidSession(name2, rm.maxHierarchyLevel, requestTime[0])
+	return role1.hasValidSession(name2, rm.maxHierarchyLevel, requestTime[0]), nil
 }
 
 // GetRoles gets the roles that a subject inherits.
 // currentTime is the querying time for the role inheritance link.
-func (rm *RoleManager) GetRoles(name string, currentTime ...string) []string {
+func (rm *RoleManager) GetRoles(name string, currentTime ...string) ([]string, error) {
 	if len(currentTime) != 1 {
-		return nil
+		return nil, errors.New("error: currentTime should be 1 parameter")
 	}
 	requestTime := currentTime[0]
 
 	if !rm.hasRole(name) {
-		return nil
+		return nil, errors.New("error: name does not exist")
 	}
 
 	sessionRoles := rm.createRole(name).getSessionRoles(requestTime)
-	return sessionRoles
+	return sessionRoles, nil
 }
 
 // GetUsers gets the users that inherits a subject.
 // currentTime is the querying time for the role inheritance link.
-func (rm *RoleManager) GetUsers(name string, currentTime ...string) []string {
+func (rm *RoleManager) GetUsers(name string, currentTime ...string) ([]string, error) {
 	if len(currentTime) != 1 {
-		return nil
+		return nil, errors.New("error: currentTime should be 1 parameter")
 	}
 	requestTime := currentTime[0]
 
@@ -133,14 +137,15 @@ func (rm *RoleManager) GetUsers(name string, currentTime ...string) []string {
 		}
 	}
 	sort.Strings(users)
-	return users
+	return users, nil
 }
 
 // PrintRoles prints all the roles to log.
-func (rm *RoleManager) PrintRoles() {
+func (rm *RoleManager) PrintRoles() error {
 	for _, role := range rm.allRoles {
 		util.LogPrint(role.toString())
 	}
+	return nil
 }
 
 // SessionRole is a modified version of the default role.
diff --git a/role_manager_test.go b/role_manager_test.go
index e1f46b8..3e0b725 100644
--- a/role_manager_test.go
+++ b/role_manager_test.go
@@ -20,7 +20,7 @@ import (
 	"time"
 
 	"github.com/casbin/casbin"
-	"github.com/casbin/casbin/file-adapter"
+	"github.com/casbin/casbin/persist/file-adapter"
 	"github.com/casbin/casbin/rbac"
 	"github.com/casbin/casbin/util"
 )
@@ -34,7 +34,7 @@ func testEnforce(t *testing.T, e *casbin.Enforcer, sub string, obj interface{},
 
 func testSessionRole(t *testing.T, rm rbac.RoleManager, name1 string, name2 string, requestTime string, res bool) {
 	t.Helper()
-	myRes := rm.HasLink(name1, name2, requestTime)
+	myRes, _ := rm.HasLink(name1, name2, requestTime)
 
 	if myRes != res {
 		t.Errorf("%s < %s at time %s: %t, supposed to be %t", name1, name2, requestTime, !res, res)
@@ -43,7 +43,7 @@ func testSessionRole(t *testing.T, rm rbac.RoleManager, name1 string, name2 stri
 
 func testPrintSessionRoles(t *testing.T, rm rbac.RoleManager, name1 string, requestTime string, res []string) {
 	t.Helper()
-	myRes := rm.GetRoles(name1, requestTime)
+	myRes, _ := rm.GetRoles(name1, requestTime)
 
 	if !util.ArrayEquals(myRes, res) {
 		t.Errorf("%s should have the roles %s at time %s, but has: %s", name1, res, requestTime, myRes)
@@ -137,18 +137,18 @@ func TestHasLink(t *testing.T) {
 
 	alpha := "alpha"
 	bravo := "bravo"
-	if rm.HasLink(alpha, bravo) {
+	if res, _ := rm.HasLink(alpha, bravo); res {
 		t.Errorf("Role manager should not have link %s < %s", alpha, bravo)
 	}
-	if !rm.HasLink(alpha, alpha, getCurrentTime()) {
+	if res, _ := rm.HasLink(alpha, alpha, getCurrentTime()); !res {
 		t.Errorf("Role manager should have link %s < %s", alpha, alpha)
 	}
-	if rm.HasLink(alpha, bravo, getCurrentTime()) {
+	if res, _ := rm.HasLink(alpha, bravo, getCurrentTime()); res {
 		t.Errorf("Role manager should not have link %s < %s", alpha, bravo)
 	}
 
 	rm.AddLink(alpha, bravo, getCurrentTime(), getInOneHour())
-	if !rm.HasLink(alpha, bravo, getCurrentTime()) {
+	if res, _ := rm.HasLink(alpha, bravo, getCurrentTime()); !res {
 		t.Errorf("Role manager should have link %s < %s", alpha, bravo)
 	}
 }
@@ -163,14 +163,14 @@ func TestDeleteLink(t *testing.T) {
 	rm.AddLink(alpha, charlie, getOneHourAgo(), getInOneHour())
 
 	rm.DeleteLink(alpha, bravo)
-	if rm.HasLink(alpha, bravo, getCurrentTime()) {
+	if res, _ := rm.HasLink(alpha, bravo, getCurrentTime()); res {
 		t.Errorf("Role manager should not have link %s < %s", alpha, bravo)
 	}
 
 	rm.DeleteLink(alpha, "delta")
 	rm.DeleteLink(bravo, charlie)
 
-	if !rm.HasLink(alpha, charlie, getCurrentTime()) {
+	if res, _ := rm.HasLink(alpha, charlie, getCurrentTime()); !res {
 		t.Errorf("Role manager should have link %s < %s", alpha, charlie)
 	}
 }
@@ -180,7 +180,7 @@ func TestHierarchieLevel(t *testing.T) {
 
 	rm.AddLink("alpha", "bravo", getOneHourAgo(), getInOneHour())
 	rm.AddLink("bravo", "charlie", getOneHourAgo(), getInOneHour())
-	if rm.HasLink("alpha", "charlie", getCurrentTime()) {
+	if res, _ := rm.HasLink("alpha", "charlie", getCurrentTime()); res {
 		t.Error("Role manager should not have link alpha < charlie")
 	}
 }
@@ -191,10 +191,10 @@ func TestOutdatedSessions(t *testing.T) {
 	rm.AddLink("alpha", "bravo", getOneHourAgo(), getCurrentTime())
 	rm.AddLink("bravo", "charlie", getOneHourAgo(), getInOneHour())
 
-	if rm.HasLink("alpha", "bravo", getInOneHour()) {
+	if res, _ := rm.HasLink("alpha", "bravo", getInOneHour()); res {
 		t.Error("Role manager should not have link alpha < bravo")
 	}
-	if !rm.HasLink("alpha", "charlie", getOneHourAgo()) {
+	if res, _ := rm.HasLink("alpha", "charlie", getOneHourAgo()); !res {
 		t.Error("Role manager should have link alpha < charlie")
 	}
 }
@@ -202,11 +202,11 @@ func TestOutdatedSessions(t *testing.T) {
 func TestGetRoles(t *testing.T) {
 	rm := NewRoleManager(3)
 
-	if rm.GetRoles("alpha") != nil {
+	if res, _ := rm.GetRoles("alpha"); res != nil {
 		t.Error("Calling GetRoles without a time should return nil.")
 	}
 
-	if rm.GetRoles("bravo", getCurrentTime()) != nil {
+	if res, _ := rm.GetRoles("bravo", getCurrentTime()); res != nil {
 		t.Error("bravo should not exist")
 	}
 
@@ -236,27 +236,27 @@ func TestGetUsers(t *testing.T) {
 	rm.AddLink("charlie", "alpha", getOneHourAgo(), getInOneHour())
 	rm.AddLink("delta", "alpha", getOneHourAgo(), getInOneHour())
 
-	myRes := rm.GetUsers("alpha")
+	myRes, _ := rm.GetUsers("alpha")
 	if myRes != nil {
 		t.Errorf("Calling GetUsers without a time should return nil.")
 	}
 
-	myRes = rm.GetUsers("alpha", getCurrentTime())
+	myRes, _ = rm.GetUsers("alpha", getCurrentTime())
 	if !util.ArrayEquals(myRes, []string{"bravo", "charlie", "delta"}) {
 		t.Errorf("Alpha should have the using roles [bravo charlie delta] but has %s", myRes)
 	}
 
-	myRes = rm.GetUsers("alpha", getOneHourAgo())
+	myRes, _ = rm.GetUsers("alpha", getOneHourAgo())
 	if !util.ArrayEquals(myRes, []string{"bravo", "charlie", "delta"}) {
 		t.Errorf("Alpha should have the using roles [bravo charlie delta] but has %s", myRes)
 	}
 
-	myRes = rm.GetUsers("alpha", getAfterOneHour())
+	myRes, _ = rm.GetUsers("alpha", getAfterOneHour())
 	if !util.ArrayEquals(myRes, []string{}) {
 		t.Errorf("Alpha should not have any using roles but has %s", myRes)
 	}
 
-	myRes = rm.GetUsers("bravo", getCurrentTime())
+	myRes, _ = rm.GetUsers("bravo", getCurrentTime())
 	if !util.ArrayEquals(myRes, []string{}) {
 		t.Error("bravo should have no using roles")
 	}