David Portabella David Portabella - 2 months ago 36
Scala Question

scalaz, Disjunction.sequence returning a list of lefts

In scalaz 7.2.6, I want to implement

sequence
on
Disjunction
, such that if there is one or more lefts, it returns a list of those, instead of taking only the first one (as in
Disjunction.sequenceU
):

import scalaz._, Scalaz._

List(1.right, 2.right, 3.right).sequence
res1: \/-(List(1, 2, 3))

List(1.right, "error2".left, "error3".left).sequence
res2: -\/(List(error2, error3))


I've implemented it as follows and it works, but it looks ugly. Is there a
getRight
method (such as in scala
Either
class,
Right[String, Int](3).right.get)
? And how to improve this code?

implicit class RichSequence[L, R](val l: List[\/[L, R]]) {
def getLeft(v: \/[L, R]):L = v match { case -\/(x) => x }
def getRight(v: \/[L, R]):R = v match { case \/-(x) => x }

def sequence: \/[List[L], List[R]] =
if (l.forall(_.isRight)) {
l.map(e => getRight(e)).right
} else {
l.filter(_.isLeft).map(e => getLeft(e)).left
}
}

Answer

Playing around I've implemented a recursive function for that, but the best option would be to use separate:

implicit class RichSequence[L, R](val l: List[\/[L, R]]) {
  def sequence: \/[List[L], List[R]] = {
    def seqLoop(left: List[L], right: List[R], list: List[\/[L, R]]): \/[List[L], List[R]] =
      list match {
        case (h :: t) =>
          h match {
            case -\/(e) => seqLoop(left :+ e, right, t)
            case \/-(s) => seqLoop(left, right :+ s, t)
          }
        case Nil =>
          if(left.isEmpty) \/-(right)
          else -\/(left)
    }
    seqLoop(List(), List(), l)
  }


  def sequenceSeparate: \/[List[L], List[R]] = {
    val (left, right) = l.separate[\/[L, R], L, R]
    if(left.isEmpty) \/-(right)
    else -\/(left)
  }
}

The first one just collects results and at the end decide what to do with those, the second its basically the same with the exception that the recursive function is much simpler, I didn't think about performance here, I've used :+, if you care use prepend or some other collection.

You may also want to take a look at Validation and ValidationNEL which unlike Disjunction accumulate failures.

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