Remove mentions of obsolete actors from our quickstart guide (#3786)

Fixes #3785

Author: jakzal

docs/topics/shared-mutable-state-and-concurrency.md

@@ -370,133 +370,11 @@ The locking in this example is fine-grained, so it pays the price. However, it i
 where you absolutely must modify some shared state periodically, but there is no natural thread that this state
 is confined to.
 
-## Actors
-
-An [actor](https://en.wikipedia.org/wiki/Actor_model) is an entity made up of a combination of a coroutine,
-the state that is confined and encapsulated into this coroutine,
-and a channel to communicate with other coroutines. A simple actor can be written as a function, 
-but an actor with a complex state is better suited for a class. 
-
-There is an [actor] coroutine builder that conveniently combines actor's mailbox channel into its 
-scope to receive messages from and combines the send channel into the resulting job object, so that a 
-single reference to the actor can be carried around as its handle.
-
-The first step of using an actor is to define a class of messages that an actor is going to process.
-Kotlin's [sealed classes](https://kotlinlang.org/docs/reference/sealed-classes.html) are well suited for that purpose.
-We define `CounterMsg` sealed class with `IncCounter` message to increment a counter and `GetCounter` message
-to get its value. The latter needs to send a response. A [CompletableDeferred] communication
-primitive, that represents a single value that will be known (communicated) in the future,
-is used here for that purpose.
-
-```kotlin
-// Message types for counterActor
-sealed class CounterMsg
-object IncCounter : CounterMsg() // one-way message to increment counter
-class GetCounter(val response: CompletableDeferred<Int>) : CounterMsg() // a request with reply
-```
-
-Then we define a function that launches an actor using an [actor] coroutine builder:
-
-```kotlin
-// This function launches a new counter actor
-fun CoroutineScope.counterActor() = actor<CounterMsg> {
-    var counter = 0 // actor state
-    for (msg in channel) { // iterate over incoming messages
-        when (msg) {
-            is IncCounter -> counter++
-            is GetCounter -> msg.response.complete(counter)
-        }
-    }
-}
-```
-
-The main code is straightforward:
-
-<!--- CLEAR -->
-
-```kotlin
-import kotlinx.coroutines.*
-import kotlinx.coroutines.channels.*
-import kotlin.system.*
-
-suspend fun massiveRun(action: suspend () -> Unit) {
-    val n = 100  // number of coroutines to launch
-    val k = 1000 // times an action is repeated by each coroutine
-    val time = measureTimeMillis {
-        coroutineScope { // scope for coroutines 
-            repeat(n) {
-                launch {
-                    repeat(k) { action() }
-                }
-            }
-        }
-    }
-    println("Completed ${n * k} actions in $time ms")    
-}
-
-// Message types for counterActor
-sealed class CounterMsg
-object IncCounter : CounterMsg() // one-way message to increment counter
-class GetCounter(val response: CompletableDeferred<Int>) : CounterMsg() // a request with reply
-
-// This function launches a new counter actor
-fun CoroutineScope.counterActor() = actor<CounterMsg> {
-    var counter = 0 // actor state
-    for (msg in channel) { // iterate over incoming messages
-        when (msg) {
-            is IncCounter -> counter++
-            is GetCounter -> msg.response.complete(counter)
-        }
-    }
-}
-
-//sampleStart
-fun main() = runBlocking<Unit> {
-    val counter = counterActor() // create the actor
-    withContext(Dispatchers.Default) {
-        massiveRun {
-            counter.send(IncCounter)
-        }
-    }
-    // send a message to get a counter value from an actor
-    val response = CompletableDeferred<Int>()
-    counter.send(GetCounter(response))
-    println("Counter = ${response.await()}")
-    counter.close() // shutdown the actor
-}
-//sampleEnd    
-```
-{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
-
-> You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-sync-07.kt).
->
-{type="note"}
-
-<!--- TEST ARBITRARY_TIME
-Completed 100000 actions in xxx ms
-Counter = 100000
--->
-
-It does not matter (for correctness) what context the actor itself is executed in. An actor is
-a coroutine and a coroutine is executed sequentially, so confinement of the state to the specific coroutine
-works as a solution to the problem of shared mutable state. Indeed, actors may modify their own private state, 
-but can only affect each other through messages (avoiding the need for any locks).
-
-Actor is more efficient than locking under load, because in this case it always has work to do and it does not 
-have to switch to a different context at all.
-
-> Note that an [actor] coroutine builder is a dual of [produce] coroutine builder. An actor is associated 
-> with the channel that it receives messages from, while a producer is associated with the channel that it 
-> sends elements to.
->
-{type="note"}
-
 <!--- MODULE kotlinx-coroutines-core -->
 <!--- INDEX kotlinx.coroutines -->
 
 [Dispatchers.Default]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-dispatchers/-default.html
 [withContext]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-context.html
-[CompletableDeferred]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-completable-deferred/index.html
 
 <!--- INDEX kotlinx.coroutines.sync -->
 
@@ -505,9 +383,4 @@ have to switch to a different context at all.
 [Mutex.unlock]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.sync/-mutex/unlock.html
 [withLock]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.sync/with-lock.html
 
-<!--- INDEX kotlinx.coroutines.channels -->
-
-[actor]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/actor.html
-[produce]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/produce.html
-
 <!--- END -->

kotlinx-coroutines-core/jvm/test/guide/test/SharedStateGuideTest.kt

@@ -56,12 +56,4 @@ class SharedStateGuideTest {
             "Counter = 100000"
         )
     }
-
-    @Test
-    fun testExampleSync07() {
-        test("ExampleSync07") { kotlinx.coroutines.guide.exampleSync07.main() }.verifyLinesArbitraryTime(
-            "Completed 100000 actions in xxx ms",
-            "Counter = 100000"
-        )
-    }
 }