I’ve been investigating alternatives to `runBlocking {}`in Kotlin/JS, and I’m wondering something. C...

# javascript

[Adam S]

I’ve been investigating alternatives to `runBlocking {}`in Kotlin/JS, and I’m wondering something. Correct me if I’m wrong, but the restriction is basically because in JS bridging between blocking/non-blocking requires using await, which is only possible inside async functions. This is incongruous with JVM and Native, where it’s always possible to block the thread, and no async function keyword is required.

So, I was thinking. would it be possible for a Kotlin compiler plugin to edit the Kotlin/JS compiler so it generates a async function instead of a regular function wherever runBlocking {} is used in a Kotlin/JS function?

[Adam S]

so this Kotlin

fun main() {
  println(getBlahs());
}

fun getBlahs(): String {
  return "${blah()} and ${blah(3)}"
}

fun blah(): String = "blah"
fun blah(times: Int): String = runBlocking { "blah".repeat(times) }

would get compiled to JavaScript like this:

async function main() {
    println(await getBlahs());
}
async function getBlahs() {
    return blah() + ' and ' + (await blah_0(3));
}
function blah() {
    return 'blah';
}
async function blah_0(times) { // 'async' is added to function definition
    return Promise(repeat('blah', times));
}

[Edoardo Luppi]

I think if it was possible to implement runBlocking by using async/await it would have been done already.
I recall that this discussion for KotlinJS has been going on for quite a while, like years probably

[Edoardo Luppi]

Also, in this case it seems you're using a top level async function, I think this approach has several limitations

[Edoardo Luppi]

https://github.com/tc39/proposal-top-level-await
Maybe related?

[Adam S]

at the moment I’m just curious whether a compiler plugin is able to change the compiled JavaScript code, so Kotlin will compile all Kotlin functions into async functions

[CLOVIS]

The concept of "blocking" doesn't really exist in JS, so there can be no runBlocking

[CLOVIS]

async/await is non-blocking, like coroutines

[CLOVIS]

The best way to convert between both worlds is to convert through Promise

[Adam S]

The concept of “blocking” doesn’t really exist in JS

isn’t await effectively the same as runBlocking {}?

What’s the difference between this JavaScript function

async function blah() {
  const data = await foo()
  return data
}

and this Kotlin function?

fun blah(): String {
  val data = runBlocking { foo() }
  return data
}

[CLOVIS]

No, await is the same as calling a suspend function normally, or as calling KotlinX.coroutines' await() function

[CLOVIS]

In both cases, it means "I'm waiting for this function to end, you can do something else in the meantime".

runBlocking means "I'm waiting for this function to end, but you are NOT allowed to do anything else. You're waiting with me."

On JS, that would make the entire tab unresponsive: no events could be sent anymore, so all buttons etc would do nothing.

[Adam S]

that sounds like the same thing to me :)

[CLOVIS]

It really isn't. One of them breaks your entire website, the other lets it run normally.

[CLOVIS]

Same reason why you are not allowed to use runBlocking on the main thread on Android, except on JS there is nothing but the main thread, so it doesn't even compile

[Adam S]

sorry, I’m not getting what you mean

[Adam S]

if Kotlin/JS compiled all functions to async and converted runBlocking { foo() } to await foo() then (assuming a little more magic 🪄 to create promises?) then it looks like it would work to me

[CLOVIS]

Let's imagine you have a simple website with a button that does window.alert("clicked")

When the website loads, you do

GlobalScope.launch {
    delay(10_000)
    window.alert("stopped")
}

If you click on the button, you get the alert immediately. 10 seconds later, the "stopped" alert appears.

Now, let's imagine runBlocking existed. When the page loads, you do:

runBlocking {
    delay(10_000)
    window.alert("stopped")
}

You click on the button. Nothing happens. After 10 seconds, the "stopped" alter appears. When you close it, the "clicked" alert appears.

When you use runBlocking , everything else in the page must wait for it to end before anything else can happen.

[Adam S]

When you use runBlocking , everything else in the page must wait for it to end before anything else can happen.

yes, that’s fine by me, that’s what I want

[CLOVIS]

It's really not what you want. It means each time you do a network call, all buttons in the app stop working until the network call ends.

[CLOVIS]

You can never have two HTTP requests at the same time

[Adam S]

that’s fine by me

[Adam S]

I want that

[CLOVIS]

Just have a global Mutex then, but seriously your users don't want that

[Adam S]

I’m my own user :)

[CLOVIS]

What are you making?

[Adam S]

a test utility to load test resources for multiplatform testing over a network. I want to implement the Okio Filesystem, and the functions aren’t marked as suspend

[CLOVIS]

I don't think that's possible. The browser does not have synchronous APIs to do that. The native APIs are callback/promise based, so you can't use them in regular functions

[CLOVIS]

That's not a Kotlin thing btw, you can't do it in JS either

[CLOVIS]

(and it's on purpose by the browser APIs : if you could do it, everyone would make websites that constantly block everything—Android forbids blocking the main thread for the same reason)

[Adam S]

The native APIs are callback/promise based, so you can’t use them in regular functions

yeah exactly, so I wanted to find out if there’s a way to tell Kotlin/JS to generate async functions

[CLOVIS]

You said you want to implement an interface that is just regular functions? You can't await inside of them

[CLOVIS]

Even in JS

[Adam S]

that’s true at the Kotlin level, but in JavaScript I don’t think the interface exists

[CLOVIS]

The signature must watch, and it can never be async

[CLOVIS]

The only exception is if your original interface always returns values by callback/promise/deferred/future/whatever, in which case you can do stuff. But if it returns plain values, it's just not possible (again, on purpose by the W3C)

[Adam S]

as I understand JavaScript doesn’t have interfaces, though I’m no expert! So as long as the Kotlin code implements the interface, and the JavaScript code works, then it’s okay if the JavaScript code differs and returns a Promise

[CLOVIS]

True, but the JS code will have to be async /return a promise. Regular functions in your Kotlin interface must be regular JS functions, so you can't call async functions inside of them.

[Adam S]

Regular functions in your Kotlin interface must be regular JS functions

but what if a Kotlin compiler plugin added async to all generated JavaScript functions?

[CLOVIS]

Then you wouldn't be able to call any Kotlin code in any non-async JS function, so you would never be able to do any kind of interop with JS

[Adam S]

mmm yes, good point. So what if async were added selectively, only where necessary?

[CLOVIS]

How do you know where it is necessary?

[Adam S]

even if it was manually, like a @JsAsync annotation

[CLOVIS]

You still wouldn't be able to add it to an interface implementation, it would need to be on the interface itself

[Adam S]

there’s probably a way to be clever and do some analysis to determine all call routes, but just running it and see if it works would be fine initially

[CLOVIS]

Example:

interface Foo {
    fun foo(): String
]

class FooImpl1 {
    override fun foo(): String = "1"
}

class FooImpl2 {
    @JsAsync
    override fun foo(): String = "2"
}

fun bar(impl: Foo): String {
    return impl.foo()
}

How do you compile bar ? If the argument is FooImpl1 , it must be a regular JS function (not async), but if the argument is FooImpl2 , then it must be async .

[CLOVIS]

(same reason you can't add suspend to overriden functions, so we just circled to the same thing)

[Adam S]

yeah, that’s a good edge case

[Adam S]

in my application it wouldn’t happen because I wouldn’t use Foo, I’d be using my custom expect class FooImpl, which in jsMain would have the annotations

[CLOVIS]

Same problem. What about common code that calls your implementation? It needs to magically know your implementation is @JsAsync and add it to somehow. Which means whenever you add/remove JsAsync to any function anywhere, your entire codebase changes signature

[Adam S]

the Kotlin/JS compiler needs to know, but the whole application doesn’t need to know, does it?

[Adam S]

anyway, even if there is no annotation there’ll be some way of getting this information to the Kotlin/JS compiler plugin. Even if I hard code it so that it will add async to any usage of Foo.foo()

[Adam S]

I’d like to be able to experiment and create a compiler plugin, hard code the functions that need to be made async, but I’m not sure if a compiler plugin would be able to add async to a function signature…

[CLOVIS]

But that's impossible. The functions that need to be async are the functions you want, and all the functions that appear above on the call chain. If at any point you call

list.forEach {
    yourAsyncFunction()
}

then List.forEach must be async , and therefore all functions that call it must be too. And now, everything has to be async .

[Adam S]

yeah, that’s right, so the compiler would need to figure out which functions need to be made async. Again, that can either be done in a clever way, or just hard code it

[CLOVIS]

I mean, it's not "figure out", it's the entire codebase. The entire standard library needs to be recompiled just for your project, as well as any other library you use.

[Adam S]

how so? I use the stdlib, the stdlib doesn’t use my library

[Adam S]

e.g. forEach {} doesn’t need to be made async, because I’d be using it inside one of my own library’s functions

[CLOVIS]

All lambdas passed must be async too. All functions you call which accept lambdas must be async if you call async stuff inside of them, so forEach , filter , map etc need to be async as soon as a single of their usage calls async in them

[CLOVIS]

And of course, if they become async , then all their usages must be async too, etc

[CLOVIS]

I guess you can find a way not to need that for inline functions (same way they don't need to suspend), but that's only a small portion of functions

[Adam S]

sorry, I don’t follow… it wouldn’t be changing the signatures of those functions to change them from fun operation(block: () -> Unit) to fun operation(block: () -> Promise<Unit>)

[Adam S]

like, if I had a library function that used a stdlib function with a lambda:

fun blah(data: List<String>) {
  data.map { d ->
    fs.foo(d) // async
  }
}

then this would be translated to

async function blah(data) {
  data.map { d ->
    await fs.foo(d)
  }
}

(or whatever the correct JS lambda signature equivalent is!)

[CLOVIS]

That's only possible if map is inline and all executions happen during the execution of the function. In all other cases, this code is wrong.

[CLOVIS]

Example:

val tasks = ArrayList<() -> Unit>()

// imagine this is in the standard library
fun registerForLater(block: () -> Unit) {
    tasks.add(block)
}

// this is also in the stdlib
fun executeAll() {
    tasks.forEach { it() }
}

// this is your function
@JsAsync
fun blah(data: List<String>) {
    data.forEach {
        registerForLater { fs.foo(it) } ← await ?
    }

    executeAll() // ??
}

[CLOVIS]

Try it with suspend , even if blah is suspend , you can't call suspend functions in the lambda of registerForLater , for exactly the same reason

[Adam S]

mmm okay, I think I get it

[Adam S]

so there are a few options: either the ‘add async’ compiler plugin throws an error if it can’t add async to a lambda, it’s just forbidden. Or it updates tasks and registerForLater to add async

[CLOVIS]

The problems are:
• first solution: you won't be able to call any code, because you'll hit functions like that everywhere in Kotlin
• second solution: it's contagious, so all usages of these functions must be converted as well, as well as their usages, etc

[Adam S]

yeah, it’s not perfect

[Adam S]

option 3: the ‘add async’ plugin adds async to everything

[CLOVIS]

But then, you can't call any Kotlin code from any non-async JS code

[CLOVIS]

And since the top-level code in JS can't be async…

[Adam S]

this is a test util, so it can return Promise<Unit>