Either[L,Either[L,R]]のように多段になる場合にRだけを取得する

gistfile1.scala

object Loan {

  def using[A <: { def close() }, B](resource: A)(func: A => B): Either[Exception, B] =
    try {
      Right(func(resource))
    } catch {
      case e: Exception => Left(e)
    } finally {
      if (resource != null) resource.close()
    }

}

def eith[R](arg: Either[Throwable, Any] ) : R = {
  arg match {
    case Left(e) => throw e
    case Right(o:Either[Throwable, Any]) => eith(o)
    case Right(o:R) => o
  }
}

val eithers = using(new ByteArrayOutputStream()) { out =>
  using(new DeflaterOutputStream(out, new Deflater())) { dos =>
    using(new OutputStreamWriter(dos, "UTF-8")) { osw =>
      osw.write(text)
      out
    }
  }
}

// Rだけが取れる
val result = eith[ByteArrayOutputStream](eithers)