scala: what is the most elegant way to convert a list of Tries to a Try of list?

Question

Suppose I have a List of tries in scala, for example: List(Try(a), Try(b), Try(c)). If want to write a code snippets that returns Success(List(a, b, c)) if all tries are successes, and returns a Failure if one of the tries is a failure.

The only way I found to do it is:

private def convertArrayOfSuccessesToSuccessOfByteArray(
      tryArrayForOutput: Array[Try[Byte]]
  ): Success[Array[Byte]] = {
    val outputArray = ArrayBuffer[Byte]()
    tryArrayForOutput.foreach(tryElem => {
      val Success(elem) = tryElem
      outputArray.append(elem)
    })
    Success(outputArray.toArray)
  }

As you can see, it's pretty cumbersome and not so "functional".

score 1 · Answer 1 · answered Dec 09 '21 at 10:01

1

Just get each value and wrap in a Try to catch the first error, if any

Try(tryArrayForOutput.map(_.get))

answered Dec 09 '21 at 10:01

Tim

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1

It can definitely work, but in many places calling "get" on a Try object is considered a bad practice. – CrazySynthax Dec 09 '21 at 10:10

score 1 · Answer 2 · answered Dec 09 '21 at 10:10

@Tim's answer is good in this case.

One more general functional way would be to fold and flatMap:

private def convertArrayOfSuccessesToSuccessOfByteArray(tryArrayForOutput: Seq[Try[Byte]]): Try[Seq[Byte]] = {
    tryArrayForOutput.foldLeft(Try(Seq[Byte]())) { (acc: Try[Seq[Byte]], t: Try[Byte]) =>
      acc.flatMap { seq: Seq[Byte] =>
        t.map((b: Byte) => seq :+ b)
      }
    }
  }

This can be generalised and is typically a method provided by libraries like Cats.

scala: what is the most elegant way to convert a list of Tries to a Try of list?

2 Answers2