# Functional interface invoke

**URL:** <https://discuss.kotlinlang.org/t/functional-interface-invoke/9717>\
**Category:** Language Design\
**Created:** [October 7, 2018, 11:49am UTC](https://discuss.kotlinlang.org/t/functional-interface-invoke/9717 "2018-10-07T11:49:04Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![tieskedh](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/tieskedh/32/2816_2.png) [@tieskedh](https://discuss.kotlinlang.org/u/tieskedh)\
**Post date:** [October 7, 2018, 11:49am UTC](https://discuss.kotlinlang.org/t/functional-interface-invoke/9717/1 "2018-10-07T11:49:04Z")

</div>

Maybe it’s an idea to add invoke-functions to functional-interfaces?  
The way it should act is like generated extension-functons.

---

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**Author:** ![Wasabi375](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/wasabi375/32/4741_2.png) [@Wasabi375](https://discuss.kotlinlang.org/u/Wasabi375)\
**Post date:** [October 7, 2018, 12:55pm UTC](https://discuss.kotlinlang.org/t/functional-interface-invoke/9717/2 "2018-10-07T12:55:20Z")

</div>

I think there is some undocumented magic going on with function types or at least I can’t find the right documentation.

```kotlin
fun foo() {}
val a = ::foo // a is of type KFunction0<Unit>

```

The strange thing is that based on the documentation I can find, there is no class `KFunction0`. Also Intellij does not let me jump to it’s declaration. I guess it has something to do with the special function type syntax `() -> Unit`. The only somewhat related class I could find is [`Function0<T>`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.jvm.functions/-function0/index.html) but this only exists on the JVM and I have no idea what it is used for.

```kotlin
fun foo() {}
val a: KFunction0<Unit> = ::foo // valid although KFunction0 does not seem to exist
val b: Function0<Unit> = ::foo // valid as well, I guess KFunction0 extends Function0 which declares `invoke`
val c: () -> Unit = ::foo // as far as I can tell this is a special syntax for KFunction0

```

Another strange fact is that looking at the docs for Kotlin 1.3 they have `KFunction0`, etc but only for Kotlin native? The native version of `KFunction0` does not have the `invoke` operator though.  
So I guess what I’m trying to say is that the documentation of how this works is lacking in multiple regards.

Anyways, why exactly do you need the functional-interfaces to have `invoke` declared? I mean you can still call it regardless.

---

<div class="post-metadata">

**Author:** ![tieskedh](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/tieskedh/32/2816_2.png) [@tieskedh](https://discuss.kotlinlang.org/u/tieskedh)\
**Post date:** [October 7, 2018, 5:37pm UTC](https://discuss.kotlinlang.org/t/functional-interface-invoke/9717/3 "2018-10-07T17:37:52Z")

</div>

> <https://github.com/JetBrains/kotlin/blob/master/spec-docs/function-types.md>

The reason I wanted to have the invoke-function with functional interfaces is because that is the way java defines lambda’s and lambda’s should be invokable, or is that to bluntly?

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<div class="post-metadata">

**Author:** ![Wasabi375](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/wasabi375/32/4741_2.png) [@Wasabi375](https://discuss.kotlinlang.org/u/Wasabi375)\
**Post date:** [October 7, 2018, 5:55pm UTC](https://discuss.kotlinlang.org/t/functional-interface-invoke/9717/4 "2018-10-07T17:55:25Z")

</div>

Which functional interfaces exactly are you talking about. The way I understand it `() -> Unit` or any other functional interface is represented by some synthetic class `KFunction0<T>` which implements [`Function0<T>`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.jvm.functions/-function0/index.html) which declares `invoke`.  
So I guess the functional interfaces have `invoke` already.

Maybe you can give an example of what you are trying to do (I’m not sure if I understand your problem yet).  
As far as I know there should be no problem combining kotlin lambdas with java and vice versa.

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<div class="post-metadata">

**Author:** ![tieskedh](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/tieskedh/32/2816_2.png) [@tieskedh](https://discuss.kotlinlang.org/u/tieskedh)\
**Post date:** [October 7, 2018, 6:00pm UTC](https://discuss.kotlinlang.org/t/functional-interface-invoke/9717/5 "2018-10-07T18:00:07Z")

</div>

I put the link in because of your comments about the functions.  
I was talking about java’s interfaces, annotated with @FunctionalInterface

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<div class="post-metadata">

**Author:** ![Wasabi375](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/wasabi375/32/4741_2.png) [@Wasabi375](https://discuss.kotlinlang.org/u/Wasabi375)\
**Post date:** [October 7, 2018, 6:16pm UTC](https://discuss.kotlinlang.org/t/functional-interface-invoke/9717/6 "2018-10-07T18:16:32Z")

</div>

Ok, now I get you I think. For everyone else here is a [link to the interface](https://docs.oracle.com/javase/10/docs/api/java/lang/FunctionalInterface.html) 😉

So you want to be able to do something like this

```kotlin
// java
interface Foo extends FunctionalInterface {
    void foo();
}

// Kotlin
val foo: Foo = getFooFromJava() 
foo()

```

I agree this should be possible. Another solution to the problem would be something like this is reinterpreting `Foo` as `() -> Unit` in a similar way that kotlin maps other java types to their corresponding kotlin types. I think I prefer this as it would limit the number of functional interfaces we have to deal with in Kotlin.  
That way you would have

```kotlin
val foo: () -> Unit = getFooFromJava()

```