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Kotlin turnstile state machine using fun to implement states
package functional
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.channels.ReceiveChannel
import kotlinx.coroutines.channels.produce
import kotlinx.coroutines.launch
import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.selects.select
@FunctionalInterface
interface State {
/**
* Process the first input from the channel(s) [perform a side-effect] and return a new functional.State
*/
suspend fun process(
pushes: ReceiveChannel<Unit>,
coins: ReceiveChannel<Unit>): State
}
/*
Must explicitly provide fun return type (State) otherwise ::locked ref causes compile error:
"Type checking has run into a recursive problem..."
*/
suspend fun locked(pushes: ReceiveChannel<Unit>, coins: ReceiveChannel<Unit>): State =
select<State> {
pushes.onReceive {
println("push blocked---functional.turnstile is functional.locked")
::locked as State
}
coins.onReceive {
println("coins received---unlocking")
::unlocked as State
}
}
suspend fun unlocked(pushes: ReceiveChannel<Unit>, coins: ReceiveChannel<Unit>): State =
select<State> {
pushes.onReceive {
println("Opened!")
::locked as State
}
coins.onReceive {
println("Already functional.unlocked---returning coins")
::unlocked as State
}
}
private fun CoroutineScope.turnstile(
pushes: ReceiveChannel<Unit>,
coins: ReceiveChannel<Unit>) = launch {
/*
Fails at runtime:
Exception in thread "main" java.lang.ClassCastException: class functional.State_machine_function_statesKt$turnstile$1$state$1 cannot be cast to class functional.State
*/
var state = ::locked as State
while (true) {
state = state.process(pushes, coins)
}
}
fun main() = runBlocking<Unit> {
val pushes = produce {
repeat(10) {
send(Unit)
}
}
val coins = produce {
repeat(10) {
send(Unit)
}
}
turnstile(pushes, coins)
}
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