Scala: ZIO 2: Getting the actual exception when using ZIO.attempt

By Alvin Alexander. Last updated: May 10, 2024

As a brief Scala ZIO 2 note, if you are using ZIO.attempt and want to show the actual exception you are working with (instead of just Throwable), use refineToOrDie, as shown in this example:

// this came from ZIOnomicon or the official ZIO docs
val printLine2: IO[IOException, String] =
  ZIO.attempt(scala.io.StdIn.readLine())
     .refineToOrDie[IOException]

and this example:

def parseInput(s: String): ZIO[Any, NumberFormatException, Int] =
  ZIO.attempt(s.toInt)                      // ZIO[Any, Throwable, Int]
     .refineToOrDie[NumberFormatException]  // ZIO[Any, NumberFormatException, Int]

As I show in the comments in the second example, the error value in a plain ZIO.attempt will be Throwable:

  ZIO.attempt(s.toInt)                      // ZIO[Any, Throwable, Int]

but with refineToOrDie, you can refine that to what it actually is, a NumberFormatException in this case:

  ZIO.attempt(s.toInt)                      // ZIO[Any, Throwable, Int]
     .refineToOrDie[NumberFormatException]  // ZIO[Any, NumberFormatException, Int]

scala

exception

ioexception

numberformatexception

throwable

Scala: ZIO 2: Getting the actual exception when using ZIO.attempt

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