Kotlin Extension Methods

05 June 2018

Kotlin is a great language, and one of its killer features is extensions.

The documentation stresses that extensions are resolved statically. I recently came across an interesting situation that appears to be a side effect of this fact.

Here’s my contrived, StackOverflow-appropriate example:

// extremely useful extension method
fun Int.nop() = this

fun main(args: Array<String>) {
    val x = 4

    println(x.nop())
}

I define an extension method on Int that simply returns the Int itself, and print the result of calling that method. As you’d expect, the output is:

4

However, something interesting happens when trying to use a reference to this extremely useful extension method:

// extremely useful extension method
fun Int.nop() = this

fun main(args: Array<String>) {
    val x = 4
    val nopRef = Int::nop
    println(x.nopRef())
}

This causes a compiler error of the form:

Extensions.kt:11:15: error: unresolved reference: nopRef
    println(x.nopRef())
              ^

After a bout of head-to-desk interaction and referencing the aforementioned documentation concerning static resolution, I found that using the following variation compiled just fine and produced the same 4 we saw earlier.

// extremely useful extension method
fun Int.nop() = this

fun main(args: Array<String>) {
    val x = 4
    val nopRef = Int::nop

    // comiler error
    // println(x.nopRef())

    // works
    println(nopRef(x))
}

The extension method seems to lose its extension-ness™, but otherwise (sunglasses) functions the same. I’m still not sure what exactly causes this behavior, or if it’s something that can be fixed by the Kotlin developers.

Fin.