Collectives™ on Stack Overflow

Find centralized, trusted content and collaborate around the technologies you use most.

Teams

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

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?
                Sure, but replacing Any::toString with T::toString won't compile either. I can pass a lambda, but is there a simper way?
– Dol.Gopil
                Sep 10, 2018 at 13:41
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) { 
    // ...
It would have compiled if do like this:
fun <T> doSomething(value: T, action: (value: Any) -> String = Any::toString) {
    //do something
fun <T : Any> doSomething(value: T, action: (t: T) -> String = Any::toString) {
    // ...
        
Thanks for contributing an answer to Stack Overflow!
Please be sure to answer the question. Provide details and share your research!
But avoid …
Asking for help, clarification, or responding to other answers.
Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.