How do I constrain a Kotlin extension function parameter to be the same as the extended type?

Question

I want to write an extension method on a generic type T, where the matched type constrains a method parameter.

I want this to compile:

\"Hello\".thin         


        

Answer 1

As mentioned by @Alexander Udalov it's not possible to do directly but there's a workaround where you define the extension method on another type like so:

data class Wrapper(val value: T)

val  T.ext: Wrapper get() = Wrapper(this)

fun  Wrapper.thing(p: T) {
    println("value = $value, param = $p")
}

With the above the following compiles:

"abc".ext.thing("A")

but the next fails

"abc".ext.thing(2)

with:

Kotlin: Type inference failed: Cannot infer type parameter T in fun  Wrapper.thing(p: T): Unit
None of the following substitutions
receiver: Wrapper  arguments: (String)
receiver: Wrapper  arguments: (Int)
can be applied to
receiver: Wrapper  arguments: (Int)

As suggested by @hotkey it seems that it should be possible to avoid the need for explicit Wrapper type with the following extension property:

val  T.thing: (T) -> Any? get() = { println("extension body") }

And then use it as "abc".thing("A") but it also fails. Surprisingly the following does compile "abc".thing.invoke("A")