MonadTransformerExamples.scala

 // ------------------------------------------------------
 // Combined List/Option/Either
 // ------------------------------------------------------
object MonadTransformers extends App {
  import scalaz._
  import scalaz.std.option._
  import scalaz.std.list._

  type OptionTList[α] = OptionT[List, α]

  val c1 = EitherT[OptionTList, String, String](OptionT[List, \/[String, String]](List(some(\/-("c1a")), some(\/-("c1b")))))

  val c2 = EitherT[OptionTList, String, String](OptionT[List, \/[String, String]](List(some(\/-("c2a")), some(\/-("c2b")))))
  val c3 = EitherT[OptionTList, String, String](OptionT(List(some(\/-("c3")), none, some(-\/("error")))))
  
  assert((c1 map (_ + "x")).run.run == List(some(\/-("c1ax")), some(\/-("c1bx"))))
  assert((c2 map (_ + "x")).run.run == List(some(\/-("c2ax")), some(\/-("c2bx"))))
  assert((c3 map (_ + "x")).run.run == List(some(\/-("c3x")), none, some(-\/("error"))))

  import scalaz.effect.IO
  import scalaz.syntax.monad._
  import scalaz.syntax.foldable._
  import scalaz.syntax.effect.id._

  val res = for {
    x ← c2
    y ← c3
  } yield "%s " format (x + y)

  val effect = res.traverse_(s => IO { print(s) }) >> IO { println }

  effect.unsafePerformIO // c2ac3 c2bc3
}