Rabbit Hole: found String, required java.lang.String

Question

Hit a really strange issue today while trying to add an implicit method to Either.

implicit class EitherProvidesRollback[String,B](e: Either[String,B]) {
  def rollback(
    ss: Option[Session], overrideMsg: Option[String]): Either[String,B] = {

    e.fold(
      msg=> {
        ss.map(_.rollback)
        // found String required java.lang.String, FTW
        // Left(i18n(overrideMsg.getOrElse(msg)))

        // behold, the horrible hack
        Left(i18n(overrideMsg.getOrElse(msg).toString).asInstanceOf[String])
      },
      Right(_)
    )
  }
}

the i18n method takes a String, which AFAICT is exactly what it's getting. The workaround, as per this thread, is to T <: String in the implicit class' type signature. It appears Predef might be at play here.

Is there a way to get this working without horrendous runtime casting while at the same time preserving the type signature as exactly String?

Thanks

Answer 1

There are two types called String in your code. One is java.lang.String, the other is the type parameter String that EitherProvidesRollback takes. I'm guessing using String as a type parameter to EitherProvidesRollback is your problem. It should only require B as its type parameter.