Type class in Kotlin (failed)

Sample.kt

import Option.None
import Option.Some

interface Type<out A, out B: Type<A, B>>

sealed class Option<out A>: Type<A, Option<A>> {
  object None: Option<Nothing>() {
    override fun toString(): String = "None"
  }

  data class Some<A>(val value: A): Option<A>()
}

interface Functor<A, B, F: Type<A, F>, G: Type<B, G>> {
  fun map(a: F, f: (A) -> B): G
}

fun <A, B> optionFunctor() = object: Functor<A, B, Option<A>, Option<B>> {
  override fun map(a: Option<A>, f: (A) -> B): Option<B> =
      when (a) {
        is Some<A> -> Some(f(a.value))
        is None -> None
      }
}

// 補助用
fun <A, B> Option<A>.map(f: (A) -> B): Option<B> = optionFunctor<A, B>().map(this, f)

fun main(args: Array<String>) {
  val a: Option<Int> = optionFunctor<Int, Int>().map(Some(5), { it * 2 })
  println(a) //=> Some(value=10)

  val b: Option<Int> = optionFunctor<Int, Int>().map(None, { it * 2 })
  println(b) //=> None

  val c: Option<Int> = Some("123") map { it.toInt() }
  println(c) //=> Some(value=123)
}