MonadTransformerExamples.scala

 // ------------------------------------------------------
 // Combined List/Option/Either
 // ------------------------------------------------------
object MonadTransformers extends App {
  import scalaz._, Scalaz._
  import EitherT._

  type OptionTList[α] = ({ type λ[α] = OptionT[List, α] })#λ[α]

  val c1: EitherT[OptionTList, String, String] = eitherT[OptionTList, String, String](OptionT[List, \/[String, String]](List(some(\/-("c1a")), some(\/-("c1b")))))
  val c2: EitherT[OptionTList, String, String] = eitherT[OptionTList, String, String](OptionT[List, \/[String, String]](List(some(\/-("c2a")), some(\/-("c2b")))))
  val c3: EitherT[OptionTList, String, String] = eitherT[OptionTList, String, String](OptionT[List, \/[String, String]](List(some(\/-("c3")), none, some(-\/("error")))))

  // ∘ = map
  (c1 ∘ (_ + "x")).run.run assert_=== List(some(\/-("c1ax")), some(\/-("c1bx")))
  (c2 ∘ (_ + "x")).run.run assert_=== List(some(\/-("c2ax")), some(\/-("c2bx")))
  (c3 ∘ (_ + "x")).run.run assert_=== List(some(\/-("c3x")), none, some(-\/("error")))

  for {
    x ← c2
    y ← c3
  } yield println("%s " format (x + y)) // c2ac3 c2bc3
}