An example code for https://blog.oklahome.net/2018/09/protoactor-go-messaging-protocol.html

no-sender.go

package main

import (
"github.com/AsynkronIT/protoactor-go/actor"
"time"
)

type ping struct{}

type pong struct{}

func main() {
	props := actor.FromFunc(func(ctx actor.Context) {
		switch ctx.Message().(type) {
		case *ping:
			// This fails to get sender
			// because the message came
			// from outside of actor system
			//
			// Below execution leads to dead letter
			// 2018/09/14 22:40:02 [ACTOR] [DeadLetter] pid="nil" message=&{} sender="nil"
			ctx.Respond(&pong{})

			// Below execution causes a panic since Sender() returns nil.
			// Actor crashes and that causes supervisor to restart this failing actor.
			// 2018/09/14 22:40:02 [MAILBOX] [ACTOR] Recovering actor="nonhost/$1" reason="runtime error: invalid memory address or nil pointer dereference" stacktrace="github.com/AsynkronIT/protoactor-go/actor.(*PID).ref:26"
			// 2018/09/14 22:40:02 [ACTOR] [SUPERVISION] actor="nonhost/$1" directive="RestartDirective" reason="runtime error: invalid memory address or nil pointer dereference"
			ctx.Sender().Tell(&pong{})

		}
	})

	pid := actor.Spawn(props)
	pid.Tell(&ping{})

	time.Sleep(1 * time.Second) // Just to make sure system ends after actor execution
}