Dependent Values

test.scala

package optionStuff

import scalaz._
import Scalaz._

object optionStuff extends App {
  case class Test(a: Option[String], b: Option[String])
  case object Error

  implicit val errorInstance = new Monoid[Error.type] {
    def append(f1: Error.type, f2: => Error.type): Error.type = Error
    def zero: Error.type = Error
  } 

  type S[A] = Error.type \/ Option[A]

  def createTest(a: S[String], b: S[String]): S[Test] = {
    val x = (a |@| b).tupled
    if (x.isLeft) Error.left else {
      x.flatMap {
        case (Some(x), y)    => Test(x.some, y).some.right
        case (None, None)    => Test(none, none).some.right
        case (None, Some(y)) => Error.left
        case _               => none.right
      }
    }
  }
  
  assert(createTest("ha".some.right, "ba".some.right) == \/-(Some(Test(Some("ha"),Some("ba")))))
  assert(createTest("ha".some.right, none.right) == \/-(Some(Test(Some("ha"),None))))
  assert(createTest(none.right, "ba".some.right) == -\/(Error))
  assert(createTest(none.right, none.right) == \/-(Some(Test(None,None))))
  assert(createTest(Error.left, "ba".some.right) == -\/(Error))
  println("done")
}