async

import arrow.core.*
import arrow.fx.*
import java.lang.RuntimeException

typealias Callback = (List<String>?, Throwable?) -> Unit

class GithubId
object GithubService {
  fun getUsernames(callback: Callback) {
    //execute operation and call callback at some point in future
  }
}

fun main(args: Array<String>) {
  //sampleStart
  fun getUsernames(): IO<List<String>> =
    IO.async { cb: (Either<Throwable, List<String>>) -> Unit ->
      GithubService.getUsernames { names, throwable ->
        when {
          names != null -> cb(Right(names))
          throwable != null -> cb(Left(throwable))
          else -> cb(Left(RuntimeException("Null result and no exception")))
        }
      }
    }

  val result = getUsernames()
  //sampleEnd
  println(result.unsafeRunSync())
}