I noticed an interesting coroutine issue when updating our a kotlinlang #coroutines

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I noticed an interesting coroutine issue when upda...

# coroutines

Robert Williams

05/15/2022, 11:04 AM

I noticed an interesting coroutine issue when updating our app kotlin version from 1.4 to 1.6

Robert Williams

05/15/2022, 11:04 AM

Here's a reproduction case: https://play.kotlinlang.org/#eyJ2ZXJzaW9uIjoiMS42LjIxIiwicGxhdGZvcm0iOiJqYXZhIiwiYXJncy[…]JhcHBlcihtYXBGdW5jdGlvbihpdCkpfSwgV3JhcHBlcihudWxsKSkifQ==

Robert Williams

05/15/2022, 11:05 AM

On 1.4 there's an error but we do get the correct output

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Wrapper(value=3)

Robert Williams

05/15/2022, 11:05 AM

On 1.6 it just fails with a different error

Robert Williams

05/15/2022, 11:05 AM

There's also a slightly less functional version

map2

which works correctly on all versions

Robert Williams

05/15/2022, 11:06 AM

And if you take out the

delay

it obviously works because it's not suspending

Robert Williams

05/15/2022, 11:08 AM

Not sure if this issue is in coroutines, compiler or it's just the code is wrong but maybe someone here knows more

Dan Fingal-Surma

05/16/2022, 7:19 AM

Doing some exploring. I started inlining code and this succeeds:

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inline fun <OldT, NewT> Wrapper<OldT>.map(mapFunction : (OldT) -> NewT) : Wrapper<NewT> = 
    value?.let { wrap(mapFunction(it)) } ?: Wrapper(null)

But this fails:

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inline fun <OldT, NewT> Wrapper<OldT>.map(mapFunction : (OldT) -> NewT) : Wrapper<NewT> = 
    value?.let(mapFunction.andThen(::wrap)) ?: Wrapper(null)

If I try to change

andThen

to not be an extension function, I get a compiler error:

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inline fun <OldT, NewT> Wrapper<OldT>.map(crossinline mapFunction : (OldT) -> NewT) : Wrapper<NewT> =
    value?.let(andThen(mapFunction, ::wrap)) ?: Wrapper(null)

inline fun <A, B, C> andThen(crossinline g: ((A) -> B), crossinline f: (B) -> C): (A) -> C {
    return { it: A -> f(g(it)) }
}

Suspension functions can be called only within coroutine body

I get the same error if I manually inline the map function:

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wrapped.value?.let(andThen({ oldVal ->
            delay(1000)
            oldVal.toString()
        }, ::wrap)) ?: Wrapper(null)

If I take your original code and annotate everything with

suspend

everywhere I can, it works: https://pl.kotl.in/zrgS-jyXi So it seems like something is getting mixed up in the loose way that inline allow accepting suspend funs. Note that the error is on the

from