feat: return last and current state on transition

README.md

@@ -21,7 +21,7 @@ Basic usage works as follows:
 Here is an example code based on the very simple use-case of a door:
 
 ```go
-// Signature: <initial state>
+// Signature: <machine> = <initial state>
 machine := fsm.NewMachine("close")
 
 // Signature: <current state, input event, next state>
@@ -40,8 +40,8 @@ machine.SetExitAction(func(current, next string) error {
 
 fmt.Printf("Current state is %s\n", machine.State())
 
-// Signature: <input event>
-if err := machine.Transition("open-door"); err != nil {
+// Signature: <last, current, err> = <input event>
+if _, _, err := machine.Transition("open-door"); err != nil {
     panic(err)
 }
 

examples/door-complex-types/main.go

@@ -22,14 +22,14 @@ func main() {
 		return nil
 	})
 
-	machine.SetExitAction(func(old, next *DoorState) error {
-		fmt.Printf("Leaving '%s' going to '%s'\n", old.Id, next.Id)
+	machine.SetExitAction(func(current, next *DoorState) error {
+		fmt.Printf("Leaving '%s' going to '%s'\n", current.Id, next.Id)
 		return nil
 	})
 
 	fmt.Printf("Current state is %+v\n", machine.State())
 
-	if err := machine.Transition("open-door"); err != nil {
+	if _, _, err := machine.Transition("open-door"); err != nil {
 		panic(err)
 	}
 

examples/door/main.go

@@ -23,7 +23,7 @@ func main() {
 
 	fmt.Printf("Current state is %s\n", machine.State())
 
-	if err := machine.Transition("open-door"); err != nil {
+	if _, _, err := machine.Transition("open-door"); err != nil {
 		panic(err)
 	}
 

state-machine.go

@@ -73,29 +73,30 @@ func (machine *Machine[T]) SetExitAction(f StateActionFunc[T]) {
 // possible, an error is returned.
 // When leaving the old state, the exit action is called (if provided). When
 // entering the new state, the enter action is called (if provided).
-func (machine *Machine[T]) Transition(input Event) error {
+func (machine *Machine[T]) Transition(input Event) (*T, *T, error) {
 	transition := machine.findTransition(machine.current, input)
 	if transition == nil {
-		return fmt.Errorf("there is no state to transition to from state '%+v' on event '%s'", machine.current, input)
+		return nil, nil, fmt.Errorf("there is no state to transition to from state '%+v' on event '%s'", machine.current, input)
 	}
 
 	next := transition.Next
 
 	if machine.exitAction != nil {
 		if err := machine.exitAction(machine.current, next); err != nil {
-			return err
+			return nil, nil, err
 		}
 	}
 
 	if machine.enterAction != nil {
 		if err := machine.enterAction(machine.current, next); err != nil {
-			return err
+			return nil, nil, err
 		}
 	}
 
 	// We will only enter the next state if the enter action was successful.
+	last := copy(machine.current)
 	machine.current = next
-	return nil
+	return &last, &machine.current, nil
 }
 
 // CanTransition checks if the machine can transition to a new state when the
@@ -117,3 +118,7 @@ func (machine *Machine[T]) findTransition(current T, input Event) *Transition[T]
 
 	return nil
 }
+
+func copy[T any](t T) T {
+	return t
+}

state-machine_test.go

@@ -58,11 +58,11 @@ func TestState(t *testing.T) {
 func TestTransition(t *testing.T) {
 	machine := getMachine()
 
-	if err := machine.Transition(OpenDoor); err != nil {
+	if _, _, err := machine.Transition(OpenDoor); err != nil {
 		t.Error(err)
 	}
 
-	if err := machine.Transition(OpenDoor); err == nil {
+	if _, _, err := machine.Transition(OpenDoor); err == nil {
 		t.Error("expecting door to be opened")
 	}
 }
@@ -82,16 +82,30 @@ func TestCanTransition(t *testing.T) {
 func TestComplexTransition(t *testing.T) {
 	machine := getComplexMachine()
 
-	if err := machine.Transition(OpenDoor); err != nil {
+	var last *ComplexDoorState
+	var current *ComplexDoorState
+	if l, c, err := machine.Transition(OpenDoor); err != nil {
 		t.Error(err)
+	} else {
+		last = *l
+		current = *c
 	}
 
-	if err := machine.Transition(OpenDoor); err == nil {
+	if _, _, err := machine.Transition(OpenDoor); err == nil {
 		t.Error("expecting door to be opened")
 	}
 
 	state := machine.State()
 	if state.OpenedBy != "Peter" {
 		t.Errorf("expecting state %s to be 'Peter', not %s", state.Id, state.OpenedBy)
 	}
+
+	if state.Id != current.Id {
+		t.Errorf("States are not equal (%s vs %s)", state.Id, current.Id)
+	}
+
+	last.Id = "this should have no effect"
+	if state.Id == last.Id {
+		t.Errorf("State and last state are equal")
+	}
 }