Алгебраические типы данных

Что такое ADT

Типы-суммы

sealed trait Light
case object Red extends Light
case object Yellow extends Light
case object Green extends Light
object Main {
  def main(args: Array[String]): Unit = {
    val l: Light = Yellow
    println(l)
  }
}

Типы-произведения

case class Point(x: Int, y: Int)
object Main {
  def main(args: Array[String]): Unit = {
    val p = Point(3, 4)
    println(s"${p.x} and ${p.y}")
  }
}

Сочетание типов-сумм и типов-произведений

sealed trait Shape
case class Circle(radius: Double) extends Shape
case class Rectangle(w: Double, h: Double) extends Shape
object Main {
  def main(args: Array[String]): Unit = {
    val s: Shape = Rectangle(3, 4)
    println(s)
  }
}

Работа с ADT

sealed trait Shape
case class Circle(radius: Double) extends Shape
case class Rectangle(w: Double, h: Double) extends Shape
object Main {
  def area(s: Shape): Double = s match {
    case Circle(r)       => 3.14159 * r * r
    case Rectangle(w, h) => w * h
  }
  def main(args: Array[String]): Unit = {
    println(area(Circle(2)))
    println(area(Rectangle(3, 4)))
  }
}

Моделирование предметной области

sealed trait PaymentMethod
case class CreditCard(number: String) extends PaymentMethod
case class BankTransfer(iban: String) extends PaymentMethod
case object Cash extends PaymentMethod
object Main {
  def main(args: Array[String]): Unit = {
    val m: PaymentMethod = Cash
    println(m)
  }
}

Рекурсивные ADT

sealed trait Tree
case object Leaf extends Tree
case class Node(value: Int, left: Tree, right: Tree) extends Tree
object Main {
  def main(args: Array[String]): Unit = {
    val t: Tree = Node(1, Leaf, Node(2, Leaf, Leaf))
    println(t)
  }
}

Свёртка рекурсивного ADT

sealed trait Tree
case object Leaf extends Tree
case class Node(value: Int, left: Tree, right: Tree) extends Tree
object Main {
  def sum(t: Tree): Int = t match {
    case Leaf            => 0
    case Node(v, l, r)   => v + sum(l) + sum(r)
  }
  def main(args: Array[String]): Unit = {
    val t = Node(1, Node(2, Leaf, Leaf), Node(3, Leaf, Leaf))
    println(sum(t))
  }
}

Зачем нужны ADT

ADT и наследование

Объединяем всё вместе

sealed trait Json
case class JNum(n: Double) extends Json
case class JStr(s: String) extends Json
case object JNull extends Json
object Main {
  def render(j: Json): String = j match {
    case JNum(n) => n.toString
    case JStr(s) => "\"" + s + "\""
    case JNull   => "null"
  }
  def main(args: Array[String]): Unit = {
    List(JNum(3.0), JStr("hi"), JNull).foreach(j => println(render(j)))
  }
}

Быстрая проверка

Итоги