Are Kotlin functions really first class types?

Question

Does the fact that this doesn't compile mean that they're not quite first class types?

fun foo(s: String): Int = s.length
// This won't compile.
val bar = foo

Is there a way to do this without resorting to OO?

Answer 1

Does the fact that this doesn't compile mean that they're not quite first class types?

No, it doesn't.

In Kotlin, to reference a function or a property as a value, you need to use callable references, but it is just a syntax form for obtaining a value of a function type:

fun foo(s: String): Int = s.length

val bar = ::foo // `bar` now contains a value of a function type `(String) -> Int`

Once you get that value, you are not limited in how you work with it, which is what first-class functions are about.

Answer 2

You can use reference to the function :::

fun foo(s: String): Int = s.length

val bar = ::foo

And then invoke it:

bar("some string")