Scala Try in Depth

A Try value is a container that holds either a success or a failure instead of throwing. In depth, Try means chaining risky steps while failures stay inside the value.

Try turns a risky parse into a safe value

Parsing 42 succeeds, and parsing abc fails. GetOrElse gives 0 for the failure.

@main def main(): Unit =
  println(scala.util.Try("42".toInt).getOrElse(0))
  println(scala.util.Try("abc".toInt).getOrElse(0))

Map and recover handle success and failure

Tripling the parsed 10 gives 30. Recovering the bad parse with 0 gives 0.

@main def main(): Unit =
  val tripled = scala.util.Try("10".toInt).map(_ * 3)
  println(tripled.getOrElse(0))
  val fixed = scala.util.Try("abc".toInt).recover { case _: NumberFormatException => 0 }
  println(fixed.getOrElse(-1))

FlatMap chains risky steps safely

100 divided by 20 is 5. Division by zero fails inside the Try, so the fallback is negative 1 and the Option form is None.

@main def main(): Unit =
  val ok = scala.util.Try("20".toInt).flatMap(x => scala.util.Try(100 / x))
  println(ok.getOrElse(-1))
  val bad = scala.util.Try("0".toInt).flatMap(x => scala.util.Try(100 / x))
  println(bad.getOrElse(-1))
  println(bad.toOption)

Example

@main def main(): Unit =
  def parse(s: String): scala.util.Try[Int] = scala.util.Try(s.toInt)
  val total = parse("12").flatMap(a => parse("8").map(b => a + b))
  println(total.getOrElse(0))
  val badTotal = parse("12").flatMap(a => parse("xx").map(b => a + b))
  println(badTotal.getOrElse(0))
  println(badTotal.toOption)

Key points

Success: the Try side that holds a computed value.
Failure: the Try side that holds the failure instead of throwing it.
GetOrElse on Try: unwraps a success or supplies a fallback for a failure.
Map on Try: transforms a success and passes a failure through untouched.
Recover on Try: replaces a matching failure with a fallback value.

In this example, parse wraps each text number in a Try. FlatMap adds 12 and 8 to give 20. When the second parse fails, the whole chain fails, so the fallback is 0 and the Option form is None.

Note

a Try never throws for the failure it captured. The failure is data that code can inspect.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Scastie

@main def main(): Unit =
  def parse(s: String): scala.util.Try[Int] = scala.util.Try(s.toInt)
  val total = parse("12").flatMap(a => parse("8").map(b => a + b))
  println(total.getOrElse(0))
  val badTotal = parse("12").flatMap(a => parse("xx").map(b => a + b))
  println(badTotal.getOrElse(0))
  println(badTotal.toOption)

Exercise

What is scala.util.Try("7".toInt).map(_ * 2).getOrElse(0)?

7 0 14 49