Coincidencia exhaustiva

¿Qué es el pattern matching exhaustivo?

Un match completo

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))
  }
}

El compilador le respalda

Añadir una variante más adelante

Por qué evitar un comodín

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))
  }
}

Exhaustividad con datos

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)))
  }
}

Combinar con guards

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)))
  }
}

Exhaustividad con 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))
  }
}

Exhaustividad anidada

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))))
  }
}

Por qué es importante

Unirlo todo

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)))
  }
}

Comprobación rápida

Resumen