Abort

When you really need to stop unconditionally:

var theMeaningOfApp: Any = 42

if (theMeaningOfApp !is String) {
  kotlin.system.exitProcess(666)
}

Otherwise, rely on exceptions.

Not implemented code can be substituted by TODO() function that will raise a NotImplementedError if invoked.

fun logCrash(e: Throwable) {
  TODO()
}

fun showMessage(message: String) {
  TODO()
}

Depending on the seriousness, you can raise an Error, Exception, or a RuntimeException, as described in Throwing exceptions.

fun whatsTheMeaningOfApp(mode: String): String {
  val meaning = theMeaningOfApp as? String
  if (meaning != null) return meaning

  when (mode) {
    "curious" -> error("Can't understand it")
    "pondering" -> throw Exception("Don't know, but you can deal without it")
    else -> throw Error("Full breakdown")
  }
}

whatsTheMeaningOfApp("any") // Crash

Errors would indicate a fatal error and should abort your program, Exceptions are usually recoverable, so you can display a nice message to the user. But it’s all up to installed handlers ultimately. Both Error and Exception can be caught and silenced.

try {
  whatsTheMeaningOfApp("existential")
} catch (e: Throwable) {
  // Errors are not usually handled by convention, but can be caught. 
  // To log the issue, for example.
  logCrash(e)
  throw e
}

try {
  whatsTheMeaningOfApp("pondering")
} catch (e: Exception) {
  // Exceptions are more likely to be handled and suggest the possibility.
  showMessage("The app is fundamentally broken, sorry")
}