Y U NO TRAMPOLINE

future.scala

import scalaz._
import scalaz.concurrent._
import Future._

object Main {

  def lamePrint(s: String): Future[Unit] = async { next =>
    println(s)
    next()
  }

  def loop(s: String): Future[Unit] = for (
    sync <- lamePrint(s);
    _    <- loop(s)
  ) yield ()


  final def main(args: Array[String]) {
    loop("boom").run
  }

}

io.scala

import scalaz._
import effect._
import IO._

object Main {

def loop(s: String): IO[Unit] = for(
    _ <- putStrLn(s);
    _ <- loop(s)
  ) yield ()

  final def main(args: Array[String]) {
    loop("boom").unsafePerformIO()
  }
  
}