gistfile1.scala

  def extractEither[
    Key, TLeft, TRight, M[_] <: TraversableOnce[_]
  ]
  (monad: M[(Key, Either[TLeft, TRight])])
  (implicit cbf: CanBuildFrom[
    M[(Key, Either[TLeft, TRight])],
    (Key, TRight),
    M[(Key, TRight)]
  ]): Either[TLeft, M[(Key, TRight)]] = {
    val builder = cbf(monad)
    builder.sizeHint(monad.size)
    monad.foreach { x =>
      //            ^___ missing parameter type - why?
      val (key, either) = x.asInstanceOf[(Key, Either[TLeft, TRight])]
      either.fold(
        leftVal => return Left(leftVal),
        rightVal => builder += ((key, rightVal))
      )
    }
    Right(builder.result())
  }