The difference between lambda and KFunction in Kotlin

Question

The following does not compile:

fun<T> doSomething(value: T, action: (value: T) -> String = Any::toString){
  //do something
}

The error is:

Kotlin: Type mismatch: inferred type is KFunction1<Any, String> but (T) -> String was expected

Making it work is easy:

fun<T> doSomething(value: T, action: (t: T) -> String = {t -> t.toString()}) = action(value)

However, this leaves me wondering: what is the difference between lambdas and KFunctions? Why do we need both?

Also is there a simpler way to provide Any::toString as the default action?

Any: causing problem

IntelliJ Amiya
– IntelliJ Amiya

2018-09-10 13:26:12 +00:00
Commented Sep 10, 2018 at 13:26 — IntelliJ Amiya
– IntelliJ Amiya, Commented Sep 10, 2018 at 13:26

Tim · Accepted Answer · 2018-09-10 13:32:03Z

4

The reason why the code does not compile has nothing to do with the difference between lambdas and KFunctions. It doesn't compile because the parameter needs to be a function of type (T) -> String, and Any::toString is a function of type (Any) -> String.

edited Sep 10, 2018 at 13:32

Tim

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answered Sep 10, 2018 at 13:25

yole

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2 Comments

Dol.Gopil Over a year ago

Sure, but replacing Any::toString with T::toString won't compile either. I can pass a lambda, but is there a simper way?

yole Over a year ago

I don't think there is.

Shivam Bhusri · Accepted Answer · 2020-02-02 05:52:37Z

2

It would have compiled if do like this:

fun <T> doSomething(value: T, action: (value: Any) -> String = Any::toString) {
    //do something
}

or

fun <T : Any> doSomething(value: T, action: (t: T) -> String = Any::toString) {
    // ...
}

answered Feb 2, 2020 at 5:52

Shivam Bhusri

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Comments

David Soroko · Accepted Answer · 2018-09-10 14:35:25Z

1

When you obtain any function (lambda or otherwise) reference with :: you are using reflection. KFunction is Kotlin's way to to wrap around reflected functions.

As to making Any::toString work - there is a way but you may not like it:

fun <T> doSomething(value: T, action: (t: T) -> String = Any::toString as (T) -> String) { 
    // ...
}

answered Sep 10, 2018 at 14:35

David Soroko

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