Best way to turn a Lists of Eithers into an Either of Lists?

scala either
Eric Bowman - abstracto - picture Eric Bowman - abstracto - · Jun 27, 2011 · Viewed 13.4k times · Source

I have some code like the below, where I have a list of Eithers, and I want to turn it into an Either of Lists ... in particular (in this case), if there are any Lefts in the list, then I return a Left of the list of them, otherwise I return a Right of the list of the rights.

val maybe: List[Either[String, Int]] = getMaybe
val (strings, ints) = maybe.partition(_.isLeft)
strings.map(_.left.get) match {
  case Nil => Right(ints.map(_.right.get))
  case stringList => Left(stringList)
}

Calling get always makes me feel like I must be missing something.

Is there a more idiomatic way to do this?

Answer

Viktor Klang picture Viktor Klang · Jun 27, 2011 
data.partition(_.isLeft) match {                            
  case (Nil,  ints) => Right(for(Right(i) <- ints) yield i)        
  case (strings, _) => Left(for(Left(s) <- strings) yield s)
}

For one pass:

data.partition(_.isLeft) match {                            
  case (Nil,  ints) => Right(for(Right(i) <- ints.view) yield i)        
  case (strings, _) => Left(for(Left(s) <- strings.view) yield s)
}

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