Options and Error Handling

Result Labels

Carry success or error as a labeled result.

result-labels A result can be modeled as a success or error label with a value. This guards a risky step, like division, without an exception.

Result Labels

divisor
ResultLabels.scala
Replay: real traced execution (multi-file project)
object Main {
  def main(args: Array[String]): Unit = {
    val divisor = 4
    val ok = divisor != 0
    val value = if (ok) 100 / divisor else 0
    val outcome = if (ok) "ok:" + value else "error:divide-by-zero"

    println("ok=" + ok)
    println("outcome=" + outcome)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val divisor = 0
    val ok = divisor != 0
    val value = if (ok) 100 / divisor else 0
    val outcome = if (ok) "ok:" + value else "error:divide-by-zero"

    println("ok=" + ok)
    println("outcome=" + outcome)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val divisor = 5
    val ok = divisor != 0
    val value = if (ok) 100 / divisor else 0
    val outcome = if (ok) "ok:" + value else "error:divide-by-zero"

    println("ok=" + ok)
    println("outcome=" + outcome)
  }
}

  1. divisor ← 4, ok ← true, value ← 25, outcome ← ok:25

    1object Main {2  def main(args: Array[String]): Unit = {3    val divisor→ 4 = 4 //@divisor=0, 54    val ok→ true = divisor4 != 05    val value→ 25 = if (oktrue) 100 / divisor4 else 06    val outcome→ ok:25 = if (oktrue) "ok:" + value25 else "error:divide-by-zero"78    println("ok=" + oktrue)9    println("outcome=" + outcomeok:25)10  }11}
    outputok=true
outcome=ok:25

  1. divisor ← 0, ok ← false, value ← 0, outcome ← error:divide-by-zero

    1object Main {2  def main(args: Array[String]): Unit = {3    val divisor→ 0 = 04    val ok→ false = divisor0 != 05    val value→ 0 = if (okfalse) 100 / divisor0 else 06    val outcome→ error:divide-by-zero = if (okfalse) "ok:" + value0 else "error:divide-by-zero"78    println("ok=" + okfalse)9    println("outcome=" + outcomeerror:divide-by-zero)10  }11}
    outputok=false
outcome=error:divide-by-zero

  1. divisor ← 5, ok ← true, value ← 20, outcome ← ok:20

    1object Main {2  def main(args: Array[String]): Unit = {3    val divisor→ 5 = 54    val ok→ true = divisor5 != 05    val value→ 20 = if (oktrue) 100 / divisor5 else 06    val outcome→ ok:20 = if (oktrue) "ok:" + value20 else "error:divide-by-zero"78    println("ok=" + oktrue)9    println("outcome=" + outcomeok:20)10  }11}
    outputok=true
outcome=ok:20