Arrow Fx. Auto cancels without leaving to user discipline to check for `isAlive`. Examples by @nomisRev

AutoCancellation.kt

import arrow.fx.*
import arrow.fx.extensions.*
import arrow.fx.typeclasses.*
import arrow.fx.extensions.io.bracket.guaranteeCase

fun <A> IOOf<A>.onCancel(token: IOOf<Unit>): IO<A> =
    guaranteeCase { case ->
        when (case) {
            ExitCase.Canceled -> token
            else -> IO.unit
        }
    }

//sampleStart
fun sleeping(i: Int): IO<Unit> =
    IO.effect { println("$i starts sleeping") }
        .followedBy(IO.sleep(i.seconds))
        .onCancel(IO.effect { println("$i is getting cancelled and woken up!") })

suspend fun main(): Unit = IO.fx {
    val (_, cancel) = !listOf(sleeping(1), sleeping(2), sleeping(3))
        .parSequence()
        .effectMap { println("Finished") }
        .fork()
    !sleep(1.seconds)
    !cancel
}.suspended()
//sampleEnd

/**
2 starts sleeping
1 starts sleeping
3 starts sleeping
3 is getting cancelled and woken up!
2 is getting cancelled and woken up!
1 is getting cancelled and woken up!
**/

RecursiveNap.kt

import arrow.fx.IO
import arrow.fx.IOOf
import arrow.fx.extensions.fx
import arrow.fx.extensions.io.bracket.guaranteeCase
import arrow.fx.typeclasses.ExitCase
import arrow.fx.typeclasses.seconds

fun <A> IOOf<A>.onCancel(token: IOOf<Unit>): IO<A> =
    guaranteeCase { case ->
        when (case) {
            ExitCase.Canceled -> token
            else -> IO.unit
        }
    }

fun sleeper(): IO<Unit> = IO.fx {
    !effect { println("I am sleepy. I'm going to nap") }
    !sleep(1.seconds)
    !effect { println("1 second nap.. Going to sleep some more") }
    !sleeper() //stack safe recursion, infinitely sleeps
}

//sampleStart
suspend fun main(): Unit = IO.fx {
    val (_, cancel) = !sleeper()
        .onCancel(IO.effect { println("\nSomeone made me wake up from my nap...") })
        .fork()
    !sleep(3.seconds)
    !cancel
}.suspended()
//sampleEnd

/**
I am sleepy. I'm going to nap
1 second nap.. Going to sleep some more
I am sleepy. I'm going to nap
1 second nap.. Going to sleep some more
I am sleepy. I'm going to nap

Someone made me wake up from my nap...
**/