Standard Library Utilities

Either Fallback

Represent success or a named error without throwing.

either `Either[String, Int]` stores either an error label on the left or a useful value on the right. `getOrElse` supplies a fallback when the value is missing.

Either Fallback

ok
EitherFallback.scala
Replay: real traced execution (multi-file project)
object Main {
  def main(args: Array[String]): Unit = {
    val ok = true
    val result: Either[String, Int] = if (ok) Right(42) else Left("missing")
    val value = result.getOrElse(0)
    val status = if (result.isRight) "ok" else "fallback"

    println("status=" + status)
    println("value=" + value)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val ok = false
    val result: Either[String, Int] = if (ok) Right(42) else Left("missing")
    val value = result.getOrElse(0)
    val status = if (result.isRight) "ok" else "fallback"

    println("status=" + status)
    println("value=" + value)
  }
}

  1. ok ← true, result ← Right(42), value ← 42, status ← ok

    1object Main {2  def main(args: Array[String]): Unit = {3    val ok→ true = true //@ok=false4    val result→ Right(42): Either[String, Int] = if (oktrue) Right(42) else Left("missing")5    val value→ 42 = resultRight(42).getOrElse(0)6    val status→ ok = if (result.isRighttrue) "ok" else "fallback"78    println("status=" + statusok)9    println("value=" + value42)10  }11}
    outputstatus=ok
value=42

  1. ok ← false, result ← Left(missing), value ← 0, status ← fallback

    1object Main {2  def main(args: Array[String]): Unit = {3    val ok→ false = false4    val result→ Left(missing): Either[String, Int] = if (okfalse) Right(42) else Left("missing")5    val value→ 0 = resultLeft(missing).getOrElse(0)6    val status→ fallback = if (result.isRightfalse) "ok" else "fallback"78    println("status=" + statusfallback)9    println("value=" + value0)10  }11}
    outputstatus=fallback
value=0