Notes on error handling and monads from London Scala Dojo on 19th February 2015

scala-dojo.scala

def userId(username: String): Option[Int] =
  Some(1)

def mostRecentOrderId(userId: Int): Option[Int] =
  Some(2)

def orderCost(orderId: Int): Option[Int] =
  Some(3)

def mostRecentOrderCost(username: String): Option[Int] =
  userId("Noel") match {
    case None => None
    case Some(userId) =>
      mostRecentOrderId(userId) match {
        case None => None
        case Some(orderId) =>
          orderCost(orderId) match {
            case None => None
            case Some(cost) => Some(cost)
          }
      }
  }

def flatMap(startValue: Option[Int], func: Int => Int): Option[Int] = {
  startValue match {
    case None => None
    case Some(intermediateValue) =>
      Some(func(intermediateValue))
  }
}

def flatMap(startValue: Option[Int], func: Int => Option[Int]): Option[Int] = {
  startValue match {
    case None => None
    case Some(intermediateValue) =>
      func(intermediateValue)
  }
}

def mostRecentOrderCost(username: String): Option[Int] =
  sequence(
    userId("Noel"),
    (userId: Int) => {
      sequence(
        mostRecentOrderId(userId),
        (orderId: Int) => {
          sequence(
            orderCost(orderId),
            (orderCost: Int) => {
              Some(orderCost)
            }
          )
        }
      )
    }
  )

def mostRecentOrderCost(username: String): Option[Int] =
  userId("Noel").flatMap { (uid: Int) =>
    mostRecentOrderId(uid).flatMap { (oid: Int) =>
      orderCost(oid).map { (cost: Int) =>
        cost
      }
    }
  }

def mostRecentOrderCost(username: String): Option[Int] =
  userId("Noel").flatMap(mostRecentOrderId).flatMap(orderCost)

def mostRecentOrderCost(username: String): Option[Int] =
  for {
    uid  <- userId("Noel")
    oid  <- mostRecentOrderId(uid)
    cost <- orderCost(oid)
  } yield cost

// Sealed traits

sealed trait Shape
final case class Circle(radius: Int) extends Shape
final case class Square(side: Int) extends Shape
final case class Rectangle(width: Int, height: Int) extends Shape

def readShapeFromFile: Shape = ???

readShapeFromFile match {
  case Circle(r)       => s"It's a circle of radius $r"
  case Square(s)       => s"It's a square of side $s"
  case Rectangle(w, h) => s"It's a rectangle of size $w by $h"
}

// Either

type Error = String

def userId(username: String): Either[Error, Int] =
  username match {
    case "Noel" => Right(1)
    case _      => Left("Bad username")
  }

def mostRecentOrderId(userId: Int): Either[Error, Int] =
  Right(2)

def orderCost(orderId: Int): Either[Error, Int] =
  Right(3)

def mostRecentOrderCost(username: String): Either[Error, Int] =
  for {
    uid  <- userId("Noel").right
    oid  <- mostRecentOrderId(uid).right
    cost <- orderCost(oid).right
  } yield cost

// \/

type Error = String

def userId(username: String): Error \/ Int =
  username match {
    case "Noel" => 1.right
    case _      => "Bad username".left
  }

def mostRecentOrderId(userId: Int): Error \/ Int =
  \/-(2)

def orderCost(orderId: Int): Error \/ Int =
  \/-(3)

def mostRecentOrderCost(username: String): Error \/ Int =
  for {
    uid  <- userId("Noel")
    oid  <- mostRecentOrderId(uid)
    cost <- orderCost(oid)
  } yield cost