unreachable.scala

// scalaVersion := "2.10.0-RC5"

// libraryDependencies += "org.scalaz" %% "scalaz-core" % "7.0.0-M6" cross CrossVersion.full
import scalaz.{OptionT,Writer} 
type Issue = String
type ListWriter[+A] = Writer[List[Issue],A]
type Box[+A] = OptionT[ListWriter,A]

object EmptyBox {
  def unapply[A](b: Box[A]): Option[List[Issue]] =
    b.run.value match { // value returns Id.Id[Option[A]], which I understand <:< Option[A]
      case Some(_) => None
      case None => Some(b.run.written)
    }
}

/*
 warning: unreachable code
             case None => Some(b.run.written)
                              ^
*/

object Box {
  def apply[A](a : Option[A], issues : List[Issue]): Box[A] = {
    val writer: ListWriter[Option[A]] = Writer(issues,a)
    OptionT(writer)// IDEA erroneously shows an error
  }

  def empty[A]: Box[A]                              = Box(Option.empty[A], Nil)
  def empty[A](issue: Issue): Box[A]                = Box(None, List(issue))
  def empty[A](issues: List[Issue]): Box[A]         = Box(None, issues)
  def apply[A](a: A): Box[A]                        = Box(Some(a), Nil)
  def apply[A](a: A, issue: Issue): Box[A]          = Box(Some(a), List(issue))
  def apply[A](a: A, issues: List[Issue]): Box[A]   = Box(Some(a), issues)
}

val box1 = Box.apply(3, "full")
val box2 = Box.empty[Int]("empty")

box1 match { case EmptyBox(s) => s; case _ => List("no match") } // List("no match")
box2 match { case EmptyBox(s) => s; case _ => List("no match") } // List("empty")
// so this seems proof that both paths are executing


box1.run.value match { case Some(x) => "match"; case None => "no match" }
/* warning: unreachable code
              box1.run.value match { case Some(x) => "match"; case None => "no match" }
                                                                           ^
res24: String = match
*/

box2.run.value match { case Some(x) => "match"; case None => "no match" }
/* warning: unreachable code
              box2.run.value match { case Some(x) => "match"; case None => "no match" }
                                                                           ^
res25: String = no match
*/

// No warnings, same output
(box1.run.value: Option[Int]) match { case Some(x) => "match"; case None => "no match" }
// res11: String = match

(box2.run.value: Option[Int]) match { case Some(x) => "match"; case None => "no match" }
// res12: String = no match