Is there a way while exporting suspend functions to JS, to automatically convert them to Promise?

# javascript

[spierce7]

Is there a way while exporting suspend functions to JS, to automatically convert them to Promise?

[Big Chungus]

Nope,you need manual conversions to and from Promises

[turansky]

Or compiler plugin

[spierce7]

and such a compiler plugin doesn't exist, right?

[Dylan Sale]

Doh, I just hit this roadblock too. Seems like a pretty massive oversight and makes writing a kotlin multiplatform -> kotlin/js library incredibly tedious.

[Big Chungus]

If there's actually a demand for it, writing a ksp processor to generate such bindings would not be that hard

[Edoardo Luppi]

At this point I'll add myself to the list of the ones wanting auto-generation of bindings.
But beware I'm not talking of additional "static" functions, the suspending function should just wrap the coroutine internally

[Edoardo Luppi]

Example:

suspend fun example(...): Int { ... }

should become:

fun example(...): Promise<Int> =
   GlobalScope.promise { ... }

[Edoardo Luppi]

The same could also be valid for the JVM target, just using CompletableFuture<T>

[Big Chungus]

Main difference is that it's unsafe to use GlobalScope on jvm due to multithreading

[Edoardo Luppi]

That's true, maybe there is a neat way to infer the coroutine scope we want to use for the transformed method.
But anyway, a starting point done in the JS target would already boost productivity and adoption imo

[Big Chungus]

I'll play with ksp tonight to see how easy this might be

[Edoardo Luppi]

@Big Chungus thanks!
I suppose then that KSP is actually able to transform the source or IR? (asking as I'm ignorant on KSP or compiler plugins)

[Big Chungus]

No, ksp can only generate new code. For existing code transformation you need compiler plugin.

[Big Chungus]

But for this case, you should not modify the existing code just so js could use it (as it will make it unusable kotlin-side)

[Big Chungus]

I'm thinking some @JsPromiseExport annotation on suspend functions that would generate @JsExport non suspend function wrapping everything in promises

[Edoardo Luppi]

So the generated exported function would still be part of the entity containing the original function, or were you thinking of generating a sort of extension functions?

[Edoardo Luppi]

Asking because on the second case, it makes it weird using it on the JS/TS side.

[Edoardo Luppi]

Also just remembered one thing related to what @spierce7 said

while exporting suspend functions to JS

You can't really export suspending functions in JS ([WRONG_EXPORTED_DECLARATION] Declaration of such kind (suspend function) can't be exported to JS), so you're forced to create a wrapper anyway

[Big Chungus]

Best explained by example...
Given your code as

@JsPromiseExport
suspend fun doShit(diarrhea: Boolean): Int { ... }

KSP would generate

@JsExport
@JsName("doShit")
fun _doShit(diarrhea: Boolean): Promise<Int> = GlobalScope.promise{doShit(diarrhea)}

Then from js you only see the generated version with original name

let shitSize = await doShit(true);

[Edoardo Luppi]

@Big Chungus would that be the same if doShit is inside a class?
E.g., to obtain

let shitSize = await myInstance.doShit(true);

[Big Chungus]

Yep

[Edoardo Luppi]

Ok that's basically what I was trying to understand. Thanks!

[Big Chungus]

Well not exactly since you cannot modify class with ksp. It will likely be an extension fun so that from js you have await doShit(instance, true);

[Edoardo Luppi]

😞 sad. That's why I was saying it would be weird to use it that way

[Big Chungus]

But that's not KSP limitation as even with compiler plugin this would need to assume that entire class is @JsExport annotated which is very unlikely to be the case in most scenarious.

[Edoardo Luppi]

I think the "assumption" is delegated to the user, in the sense that if I place a @JsPromiseExport in a class that's not `@JsExport`ed, it's my fault I make something crash

[Big Chungus]

But I'll see if there are any workarounds for class case once I get to my IDE. I'll keep you posted - sounds like a fun side project

[Edoardo Luppi]

Thanks! It's indeed strange no one ever attempted at it. Maybe KotlinJS is not used enough in an interoperable context

[Dylan Sale]

Hey guys, I just found this plugin which basically does this. No idea if it’s decent though. https://github.com/ForteScarlet/kotlin-suspend-transform-compiler-plugin

[Edoardo Luppi]

@Dylan Sale pretty cool. Seems like the JS side is stuck on an issue tho

[Edoardo Luppi]

See https://youtrack.jetbrains.com/issue/KT-53993