Type Parameters and Generics

Generic Choice

Choose between two fields of the same generic type.

same-generic-type Both fields in `Choice[A]` use the same type parameter, so selecting either field produces the same result type. The example prints scalar properties of the selected value.

Generic Choice

preferFirst
GenericChoice.scala
Replay: real traced execution (multi-file project)
case class Choice[A](first: A, second: A)

object Main {
  def main(args: Array[String]): Unit = {
    val preferFirst = true
    val first = "fast"
    val second = "safe"
    val choice = Choice[String](first, second)
    val selected = if (preferFirst) choice.first else choice.second
    val length = selected.length

    println("selected=" + selected)
    println("length=" + length)
  }
}

case class Choice[A](first: A, second: A)

object Main {
  def main(args: Array[String]): Unit = {
    val preferFirst = false
    val first = "fast"
    val second = "safe"
    val choice = Choice[String](first, second)
    val selected = if (preferFirst) choice.first else choice.second
    val length = selected.length

    println("selected=" + selected)
    println("length=" + length)
  }
}

  1. preferFirst ← true, first ← fast, second ← safe, choice ← Choice(fast,safe)

    3object Main {4  def main(args: Array[String]): Unit = {5    val preferFirst→ true = true //@preferFirst=false6    val first→ fast = "fast"7    val second→ safe = "safe"8    val choice→ Choice(fast,safe) = Choice[String](firstfast, secondsafe)9    val selected→ fast = if (preferFirsttrue) choice.firstfast else choice.secondsafe10    val length→ 4 = selected.length→ 41112    println("selected=" + selectedfast)13    println("length=" + length4)14  }15}
    outputselected=fast
length=4

  1. preferFirst ← false, first ← fast, second ← safe, choice ← Choice(fast,safe)

    3object Main {4  def main(args: Array[String]): Unit = {5    val preferFirst→ false = false6    val first→ fast = "fast"7    val second→ safe = "safe"8    val choice→ Choice(fast,safe) = Choice[String](firstfast, secondsafe)9    val selected→ safe = if (preferFirstfalse) choice.firstfast else choice.secondsafe10    val length→ 4 = selected.length→ 41112    println("selected=" + selectedsafe)13    println("length=" + length4)14  }15}
    outputselected=safe
length=4