Kotlin-native: Unit in Kotlin not converts to Void in Swift when generating iOS framework

[Albert-Gao]

I want to declare a function which returns nothing.
So I declared an interface in Kotlin side which includes the following function signature:

var errorCallback: (errMessage: String) -> Unit

when generating iOS framework, it will be converted to:

NativeLibsStdlibUnit*(^errorCallback)(NSString*);

@interface NativeLibsStdlibUnit : KotlinBase
+ (instancetype)unit NS_SWIFT_NAME(init());
@end;

Shouldn't it be Void in Swift?
I expected its counterpart in swift to be:

var successCallback: (_ result: String) -> Void

Is it a bug or I did something wrong?

[SvyatoslavScherbina]

This behaviour is intentional and related to some details of Kotlin generics such as covariance and type erasure.
It doesn't seem to impose any limitations on usage, does it?

[SvyatoslavScherbina]

When Unit is used as a return type of function (not a function type), it is translated to Swift Void (and Objective-C void). E.g.

fun foo(): Unit

is translated to

-(void)foo;

So the behaviour described above applies only to function types (i.e. types of lambdas).

[Albert-Gao]

@SvyatoslavScherbina Hi, thanks for the answer! I just got a little bit confused:

When Unit is used as a return type of function (not a function type), it is translated to Swift Void (and Objective-C void).

So, how to declare a lambda which returns nothing in kotlin native, and what's the counterpart in swift looks like?

[SvyatoslavScherbina]

If you need to implement your errorCallback in Swift, then you can assign a Swift lambda to it, and return NativeLibsStdlibUnit() from the lambda.

[Albert-Gao]

Which means I indeed need to follow the generated ObjC code which has the following signature:

var successCallback: (_ result: String) -> NativeLibsStdlibUnit()

A little bit weird but thanks!

[acecilia]

Hi :)
I just had the same issue. The following Kotlin function:

fun benchmark(callback: (String) -> Unit)

In Swift looks like:

func benchmark(callback: @escaping (String) -> KotlinUnit)

When the right thing would be:

func benchmark(callback: @escaping (String) -> Void)

Shouldn't this issue stay open?
Thanks!

[JakeWharton]

Same behavior as Java. Seems like it's working as intended.

[acecilia]

@JakeWharton For swift, the proper behavior would be to substitute Unit for Void, as it is already happening with functions and explained in https://kotlinlang.org/docs/reference/native/objc_interop.html

[JakeWharton]

The same is true for Java and yet as already stated there are reasons why that is not the case.

[acecilia]

Then seems to me that in both platforms the right thing to do is to substitute Unit for Void. As I understand, this is not happening due to implementation details related with "Kotlin generics such as covariance and type erasure".

That is why I am wondering if the issue should be reopened, so could be that a developer wants to look into it and fix it.

[SvyatoslavScherbina]

() -> Unit is subtype of () -> Any in Kotlin, so Swift counterparts should follow the same rule.
And () -> KotlinUnit is subtype of () -> AnyObject in Swift, while () -> Void is not.

This consideration is one of the reasons for mapping () -> Unit to () -> KotlinUnit.

[acecilia]

And why is Unit converted to Void in functions then? (According to https://kotlinlang.org/docs/reference/native/objc_interop.html)

[SvyatoslavScherbina]

Because this case is much more important, and consequences of translating Unit to Void are not that serious here.
Tradeoffs are everywhere.

[SvyatoslavScherbina]

Improved with #2968. Will likely be shipped within 1.3.40.