<@U02BK8ABTQF> have you considered converting a `fun interface` to an arrow function? Given this exa...

# javascript

[Edoardo Luppi]

@Artem Kobzar have you considered converting a fun interface to an arrow function?
Given this example:

@JsExport
public fun interface ExampleFun {
  public fun doSomething(str: String)
}

@JsExport
public interface ExampleUsage {
  public fun example(fn: ExampleFun)
}

I can see the resulting TS declaration is:

export declare interface ExampleFun {
    doSomething(str: string): void;
    readonly __doNotUseOrImplementIt: {
        readonly "com.hcl.mainframe.zproto.ExampleFun": unique symbol;
    };
}
export declare interface ExampleUsage {
    example(fn: ExampleFun): void;
    readonly __doNotUseOrImplementIt: {
        readonly "com.hcl.mainframe.zproto.ExampleUsage": unique symbol;
    };
}

However it would be more meaningful to translate it to an arrow function.
It also allows better interoperability between Kotlin and Java, being that in Java a fun interface can be used as lambda.

[turansky]

However it would be more meaningful to translate it to an arrow function.

What if I have 2 fun interfaces and implement both in my class?
How it will work? 🙂

[Edoardo Luppi]

@turansky trying to visualize what you mean but I'm failing, could you post a very short example?

[turansky]

fun interface A {
    fun doA(): AAA
}

fun interface B {
    fun doB(): BBB
}

class C: A, B {
   // implementation
}

[Edoardo Luppi]

So by that you mean that with a method like

public interface ExampleUsage {
  public fun example(fn: A)
}

after transforming to an arrow function the code wouldn't work anymore.
Yeah that's true

[Edoardo Luppi]

What about limiting the use of fun interface for hierarchies in KotlinJS?
Or maybe an additional Js* annotation

[Artem Kobzar]

We could discuss it inside the team. Could I ask you to create a ticket in YouTrack with your arguments, why should it be a function type instead of the interface?

[Edoardo Luppi]

@Artem Kobzar sure. I'm going to discuss the POC multiplatform project internally tomorrow and I'll prepare a list of all the issues or usability problems I've found, so I'll open all the necessary tickets.