Actor Model with Go 1.18 Generics

Go 1.18 is scheduled to release in February 2022, this release is a big one because it introduces the highly anticipated (and controversial) Type Parameter feature aka generics; it’s probably going to take some time until we actually migrate to 1.18 at work, so I’ve been looking for excuses to play with it.

A few weeks ago I worked on a feature that could have been implemented succinctly using the actor model, after thinking it through, I realized generics would make it possible to have actors without relying on interface{} too much so I decided to play with how the API could look like.

Generics in Go

Instead of jumping straight into the code, I thought getting a refresher on Go’s flavor of generics and its limitations would be a good idea. I gave the Type Parameter Proposal a quick read, or at least the parts that were relevant for my endeavors.

I’ll assume the reader is familiar with generics in other languages, we’ll go over the bits that are relevant to Go:

Functions and type definitions can have type parameters, using the following syntax:

func SendAndReceive[M, R any](msg M) R {
  // do something with the M and R types
}

type Set[T any] struct {
  /* field(s) that refer to type T */
}

However, receiver functions/methods cannot have type parameters, you can read more about the reasoning here.

type Actor[St any] struct {}

// This is NOT possible
func (a *Actor[St]) SendAndReceive[R any](msg S) R {
  // code here
}

Type parameters can have constraints, the “any” constraint allows any type to be passed; a type argument satisfies a constraint if it implements the constraint (this is an oversimplification, for more details see: Type sets of constraints).

// Only values that implement the `Stringer` interface can be passed 
// to this function

func GenericToString[T Stringer](val T) string {
  return val.String()
}

That’s basically all we need to know to write a simple actor abstraction in Go!

Reducer based actor

At a very high level, an actor is a component we can communicate with by sending messages. Let’s define an interface that conveys this:

type Actor[Msg any] interface {
  Send(msg Msg)
}

Actors usually hold inner state and consume one message at a time, the inner state is modified/queried by reacting to the messages it receives.

Our implementation will need an extra type argument for its state, we’ll also include a channel to send and receive messages; now we need a way to mutate the inner state as we handle messages, to accomplish this we’ll borrow some ideas from F#’s MailboxProcessor and receive a reducer function that will return a new state given the current one and the received message. This is how the first iteration of our actor would look like:

package actor

type reducerActor[Msg, St any] struct {
	state   St
	queue   chan Msg
	reducer func(Msg, St) St
}

func NewFromReducer[Msg, St any](initial St, reducer func(Msg, St) St) Actor[Msg] {
	ga := &reducerActor[Msg, St]{
		state:   initial,
		queue:   make(chan Msg, 1),
		reducer: reducer,
	}
	ga.start()
	return ga
}

func (a *reducerActor[_, _]) start() {
	go a.receiveLoop()
}

func (a *reducerActor[_, _]) receiveLoop() {
	for msg := range a.queue {
		a.state = a.reducer(msg, a.state)
	}
}

func (a *reducerActor[Msg, _]) Send(msg Msg) {
	a.queue <- msg
}

This actor implementation has its shortcomings, they will become obvious when we actually use it, even for something simple, like adding numbers:

type radderAdd struct {
	value int
}

type radderGet struct {
	reply chan int
}

func TestReducerActor(t *testing.T) {

	adder := actor.NewFromReducer(0, func(raw interface{}, sum int) int {
		switch msg := raw.(type) {
		case radderAdd:
			return msg.value + sum
		case radderGet:
			msg.reply <- sum
			return sum
		default:
			panic(fmt.Errorf("unsupported message %T", raw))
		}
	})
	adder.Send(radderAdd{value: 1})
	adder.Send(radderAdd{value: 2})
	adder.Send(radderAdd{value: 3})
	get := radderGet{reply: make(chan int, 1)}
	adder.Send(get)

	actual := <-get.reply
	assert.Equal(t, 6, actual)
}

As you can see, there’s one big, ironic problem with this implementation: Go doesn’t have pattern matching and the closest alternative is a type switch, which forces us to receive interface{} messages; exactly the kind of problem generics (and interfaces) should help us solve.

Second iteration, typed actor

There are plenty of reasons why using interface{} in our API is not ideal, especially if we can do better. We’ll take advantage of Go’s interfaces, instead of relying on the message’s types to mutate the state, let’s allow the messages to determine how the state should change!

Since the flow is pretty much the same, we can define our new actor abstraction using the previous one.

// Message needs to be implemented by actor messages
// Apply should make use of the message's fields and the current state to
//  produce a new one
type Message[St any] interface {
	Apply(St) St
}

func NewTyped[St any](initial St) Actor[Message[St]] {
	return NewFromReducer(initial, func(msg Message[St], state St) St {
		return msg.Apply(state)
	})
}

I don’t like how the message’s type needs to know about the actor’s inner state type, but we’ll have to live with it for now.

One advantage this abstraction provides over the first one is that it allows us to create and share Messages for common actor patterns, like querying (a projection of) the actor’s state:

type getProj[St, Ret any] struct {
	reply      chan Ret
	projection func(St) Ret
}

func (a *getProj[St, _]) Apply(state St) St {
	a.reply <- a.projection(state)
	return state
}

// GetAsync applies the projection func to the actor's state and returns the
// result asynchronously
func GetAsync[St, Ret any](a Actor[Message[St]], projection func(St) Ret) <-chan Ret {
	reply := make(chan Ret, 1)
	gp := &getProj[St, Ret]{reply: reply, projection: projection}
	a.Send(gp)
	return reply
}

// Get applies the projection func to the actor's state and returns the result
func Get[St, Ret any](a Actor[Message[St]], projection func(St) Ret) Ret {
	reply := GetAsync(a, projection)
	return <-reply
}

This is how we would model the previous adder actor using the new API and abstractions:

type adderAdd struct {
	value int
}

func (msg *adderAdd) Apply(st int) int {
	return st + msg.value
}

func add(value int) actor.Message[int] {
	return &adderAdd{value: value}
}

func identity[T any](val T) T { return val }

func TestTypedActor(t *testing.T) {

	adder := actor.NewTyped(0)
	adder.Send(add(1))
	adder.Send(add(2))
	adder.Send(add(3))

	result := actor.Get(adder, identity[int])
	require.Equal(t, 6, result)
}

func TestAddMessage(t *testing.T) {
	require.Equal(t, 0, add(0).Apply(0))
	require.Equal(t, 11, add(1).Apply(10))
}

Using the new API feels cleaner and allows us to unit test our individual messages easily.

Conclusion

The lack of features found commonly in functional languages make it specially challenging to design an ergonomic actor API in Go. The absence of immutability like we have in most functional languages (Elixir, Erlang, F#, Scala, etc …) forces us to think about (not) leaking a reference (pointers, maps, slices) that would allow the calling code to modify the actor’s inner state.

This exercise gave me a good idea of how we can take advantage of Go generics in the future, they will be a valuable tool while designing APIs and libraries, however, like everything else, they should be used in moderation.

You can find the full source code here.

[Bonus] Go generics support in VS Code

VS Code’s Go extension doesn’t support generic code out-of-the box, but worry not, setting it up takes just a few commands and one config change.

In order to make this work, we need to compile the Go language server (gopls) with Go 1.18 (beta1 or beta2). You can find the instructions to install the Go 1.18 beta in the Getting started with generics tutorial. I’m using Go 1.18beta2

If everything went well, go version should output something like go version go1.18beta2 linux/amd64. Now we can install the latest gopls with go install golang.org/x/tools/gopls@latest; please note this will be installed under $GOPATH/bin, if you haven’t explicitly set GOPATH it will live in $HOME/go.

Now we need the full path to the gopls binary, this command should give it to us: echo $(go env GOPATH)/bin/gopls; we need to make note of it, since we’ll need it next.

Open VS Code’s settings.json (Ctrl+Shift+P, select “Preferences: Open Settings (JSON)“) and modify it so it uses our new gopls binary, by adding this fragment:

{
  "go.alternateTools": {
    "gopls": "$NEW_GOPLS_PATH"
  },
}

Restart/reload VS Code so the new config. change takes effect, now it should recognize generic code and shouldn’t report syntax errors anymore.

For more info you can refer to the “Working with generic code” section of this page