Flatten Scala Try

Question

Is there a simple way to flatten a collection of try\'s to give either a success of the try values, or just the failure? For example:

def map(l:List[Int]) =          


        

Answer 1

Maybe not as simple as you hoped for, but this works:

def flatten[T](xs: Seq[Try[T]]): Try[Seq[T]] = {
  val (ss: Seq[Success[T]]@unchecked, fs: Seq[Failure[T]]@unchecked) =
    xs.partition(_.isSuccess)

  if (fs.isEmpty) Success(ss map (_.get))
  else Failure[Seq[T]](fs(0).exception) // Only keep the first failure
}

val xs = List(1,2,3,4,5,6)
val ys = List(1,2,3,5,6)

println(flatten(map(xs))) // Failure(java.lang.Exception: failed)
println(flatten(map(ys))) // Success(List(1, 2, 3, 5, 6))

Note that the use of partition is not as type safe as it gets, as witnessed by the @unchecked annotations. In that respect, a foldLeft that accumulates two sequences Seq[Success[T]] and Seq[Failure[T]] would be better.

If you wanted to keep all failures, you can use this:

def flatten2[T](xs: Seq[Try[T]]): Either[Seq[T], Seq[Throwable]] = {
  val (ss: Seq[Success[T]]@unchecked, fs: Seq[Failure[T]]@unchecked) =
    xs.partition(_.isSuccess)

  if (fs.isEmpty) Left(ss map (_.get))
  else Right(fs map (_.exception))
}

val zs = List(1,4,2,3,4,5,6)

println(flatten2(map(xs))) // Right(List(java.lang.Exception: failed))
println(flatten2(map(ys))) // Left(List(1, 2, 3, 5, 6))
println(flatten2(map(zs))) // Right(List(java.lang.Exception: failed, 
                           //            java.lang.Exception: failed))