Simple EitherT demo

gistfile1.scala

package demo

import scalaz._
import scalaz.syntax._
import scalaz.effect.IO
import scalaz.effect.IO._
import scalaz.syntax.std.either._

object Demo {
  type M[A] = EitherT[IO, Throwable, A]

  def op1: M[String] = EitherT(IO(\/-("foo")))
  //def op2(s: String): M[Int] = EitherT(IO(\/-(s.length)))
  def op2(s: String): M[Int] = EitherT(IO(-\/(new Throwable)))
  def op3(i: Int): M[Unit] = EitherT(IO(\/-(println(i))))

  def main(args: Array[String]): Unit = {
    val effect: M[Unit] =
      for {
        res1 <- op1
        res2 <- op2(res1)
        res3 <- op3(res2)
      } yield res3

    val res: Throwable \/ Unit =
      effect.run.unsafePerformIO
    println(s"res: $res")
  }
}