ADT로 모델링하기

대수적 데이터 타입

case class Point(x: Int, y: Int) // product type

object Main:
  def main(args: Array[String]): Unit =
    println(Point(1, 2))

곱 타입

case class User(name: String, age: Int)

object Main:
  def main(args: Array[String]): Unit =
    val u = User("Ada", 36)
    println(u.name)
    println(u.age)

합 타입

enum PaymentMethod:
  case Cash
  case Card(number: String)
  case Crypto(wallet: String)

object Main:
  def main(args: Array[String]): Unit =
    val p: PaymentMethod = PaymentMethod.Card("1234")
    println(p)

표현할 수 없는 잘못된 상태 만들기

enum Connection:
  case Disconnected
  case Connected(sessionId: String)

object Main:
  def main(args: Array[String]): Unit =
    // No way to have a sessionId while Disconnected
    val c: Connection = Connection.Connected("abc")
    println(c)

나쁜 설계와 좋은 설계

enum Door:
  case Open
  case Closed
  case Locked(key: String)

object Main:
  def main(args: Array[String]): Unit =
    val d: Door = Door.Locked("k1")
    println(d)

곱 타입과 합 타입 결합

case class Address(city: String, zip: String)

enum Contact:
  case Email(value: String)
  case Postal(address: Address)

object Main:
  def main(args: Array[String]): Unit =
    val c: Contact = Contact.Postal(Address("Paris", "75001"))
    println(c)

ADT의 패턴 일치

enum Shape:
  case Circle(r: Double)
  case Rect(w: Double, h: Double)

object Main:
  def area(s: Shape): Double = s match
    case Shape.Circle(r)  => 3.14159 * r * r
    case Shape.Rect(w, h) => w * h

  def main(args: Array[String]): Unit =
    println(area(Shape.Rect(2, 3)))

재귀적 ADT

enum Expr:
  case Num(value: Int)
  case Add(left: Expr, right: Expr)

object Main:
  def eval(e: Expr): Int = e match
    case Expr.Num(v)    => v
    case Expr.Add(l, r) => eval(l) + eval(r)

  def main(args: Array[String]): Unit =
    val e = Expr.Add(Expr.Num(2), Expr.Num(3))
    println(eval(e))

Option으로 선택적 값 표현

case class Profile(name: String, nickname: Option[String])

object Main:
  def main(args: Array[String]): Unit =
    val p = Profile("Grace", None)
    println(p.nickname.getOrElse("(none)"))

수량을 정확하게 모델링하기

case class Quantity(value: Int)
case class Price(cents: Long)
case class LineItem(qty: Quantity, price: Price)

object Main:
  def main(args: Array[String]): Unit =
    val item = LineItem(Quantity(3), Price(500))
    println(item)

ADT가 중요한 이유

enum OrderStatus:
  case Pending
  case Shipped(tracking: String)
  case Delivered(at: Long)

object Main:
  def main(args: Array[String]): Unit =
    val s: OrderStatus = OrderStatus.Shipped("TRK1")
    println(s)

빠른 확인

복습

enum Event:
  case Created(id: Int)
  case Deleted(id: Int, reason: String)

object Main:
  def describe(e: Event): String = e match
    case Event.Created(id)         => s"created $id"
    case Event.Deleted(id, reason) => s"deleted $id: $reason"

  def main(args: Array[String]): Unit =
    println(describe(Event.Deleted(1, "spam")))