gistfile1.scala

import scalaz._, Scalaz._, effect._

type Env = Map[String,String]
type Error = String

case class AppStack[A](f: Env => IO[Validation[Error,A]])

object AppStack {
  implicit val appStackMonad = new Monad[AppStack] {
    def point[A](a: => A): AppStack[A] =
      AppStack(s => a.success[Error].point[IO])

    def bind[A, B](fa: AppStack[A])(f: A => AppStack[B]): AppStack[B] =
      AppStack { env =>
        ValidationT(fa.f(env)).flatMap { (a: A) =>
          ValidationT(f(a).f(env))
        }.run
      }
  }
}

// Exercise the AppStack

type StatusMessage = String

def foo: AppStack[String] =
  AppStack(e => e.get("foo").toSuccess("Error").point[IO])

def bar: String => AppStack[StatusMessage] = 
  s => s.point[AppStack]

def snaz: StatusMessage => Validation[Error,String] =
  sm => sm.success

def liftValidation[A](v: Validation[Error, A]): AppStack[A] =
  AppStack(e => v.point[IO])

def run =
  for {
    s <- foo
    m <- bar(s)
    z <- liftValidation(snaz(m))
  } yield z

def go1 = run.f(Map("foo" -> "fiz")).unsafePerformIO
def go2 = run.f(Map()).unsafePerformIO