Scala3 boundary/break/label demo (uses Scala 3.4.1)

BreakDemo.scala

import scala.annotation.targetName
import scala.util.boundary
import scala.util.boundary.{Label, break}

// Works in Scala 3.4.1

def firstIndex[T](xs: List[T], p: T): Int = {
  boundary:
    for (x, i) <- xs.zipWithIndex do if (x == p) break(i)
    -1
}

object optional:
  inline def apply[T](inline body: Label[None.type] ?=> T): Option[T] = boundary(Some(body))

extension [T](r: Option[T])
  @targetName("?")
  inline def ?(using label: Label[None.type]): T = r.getOrElse(break(None))

def firstColumn[T](xss: List[List[T]]): Option[List[T]] =
  optional:
    xss.map(_.headOption.?)

@main
def main(): Unit = {
  val li = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

  println(firstIndex(li, 5))

  val listOfList = List(li, List(99, 100))
  println(firstColumn(listOfList))
  
  val listOfList2 = List(li, Nil)
  println(firstColumn(listOfList2))
}

How'd you use it to implement traverse on a list of Either? In other words, return the first left, or combine all the rights.

def firstLeftOrAllRights[A, B](eithers: List[Either[A, B]]): Either[A, List[B]] = {
eithers.partition(.isLeft) match {
case (lefts, rights) => lefts.headOption.map(Left()).getOrElse(Right(rights.map(_.right.get)))
}
}

or

def firstLeftOrAllRights[A, B](eithers: List[Either[A, B]]): Either[A, List[B]] = {
eithers.find(.isLeft) match {
case Some(Left(a)) => Left(a)
case _ => Right(eithers.filter(.isRight).map(_.right.get).toList)
}
}

I meant fail-fast, which is the whole point of break. I found a related discussion here.