如何在Scala中轻松定义更复杂的PartialFunction？

import scala.PartialFunction

val toUpper: PartialFunction[String, String] = {
  case str if str.nonEmpty => str.toUpperCase
}

val result = toUpper.applyOrElse("hello", (_: String) => "N/A")
println(result) // 输出 "HELLO"

val toLower: PartialFunction[String, String] = {
  case str if str.nonEmpty => str.toLowerCase
}

val combinedFunction = toUpper.orElse(toLower)

val result2 = combinedFunction.applyOrElse("WORLD", (_: String) => "N/A")
println(result2) // 输出 "world"