Wyczerpujące dopasowywanie

Czym jest wyczerpujące dopasowywanie?

Kompletne dopasowanie

sealed trait Color
case object Red extends Color
case object Green extends Color
case object Blue extends Color
object Main {
  def name(c: Color): String = c match {
    case Red   => "red"
    case Green => "green"
    case Blue  => "blue"
  }
  def main(args: Array[String]): Unit = {
    println(name(Blue))
  }
}

Kompilator czuwa

Późniejsze dodanie wariantu

Dlaczego unikać wildcard?

sealed trait Status
case object Active extends Status
case object Inactive extends Status
object Main {
  def label(s: Status): String = s match {
    case Active   => "on"
    case Inactive => "off"
  }
  def main(args: Array[String]): Unit = {
    println(label(Active))
  }
}

Kompletność wariantów z danymi

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

Łączenie z guardami

sealed trait Temp
case class Celsius(v: Int) extends Temp
object Main {
  def feel(t: Temp): String = t match {
    case Celsius(v) if v >= 30 => "hot"
    case Celsius(v)            => "not hot"
  }
  def main(args: Array[String]): Unit = {
    println(feel(Celsius(35)))
    println(feel(Celsius(10)))
  }
}

Kompletność z Option

object Main {
  def show(o: Option[Int]): String = o match {
    case Some(v) => s"value $v"
    case None    => "nothing"
  }
  def main(args: Array[String]): Unit = {
    println(show(Some(7)))
    println(show(None))
  }
}

Zagnieżdżona kompletność

sealed trait Tree
case object Leaf extends Tree
case class Node(v: Int, l: Tree, r: Tree) extends Tree
object Main {
  def depth(t: Tree): Int = t match {
    case Leaf          => 0
    case Node(_, l, r) => 1 + math.max(depth(l), depth(r))
  }
  def main(args: Array[String]): Unit = {
    println(depth(Node(1, Leaf, Node(2, Leaf, Leaf))))
  }
}

Dlaczego to ma znaczenie

Połączenie wszystkiego

sealed trait Command
case object Start extends Command
case object Stop extends Command
case class SetSpeed(v: Int) extends Command
object Main {
  def run(c: Command): String = c match {
    case Start       => "starting"
    case Stop        => "stopping"
    case SetSpeed(v) => s"speed $v"
  }
  def main(args: Array[String]): Unit = {
    List(Start, SetSpeed(5), Stop).foreach(c => println(run(c)))
  }
}

Szybkie sprawdzenie

Podsumowanie