Enums and sealed result types

A mark can be pending or recorded, and only a recorded state should carry a score. Enums describe fixed named alternatives, while a sealed hierarchy can model alternatives with different data. Keeping the states explicit avoids unrelated flags that can disagree.

A sealed interface restricts its direct implementers under Kotlin’s rules and supports exhaustive when handling of known alternatives. A data object names a singleton state; a data class can carry case-specific values. An enum is useful when each constant fits the same declared type structure.

Connect this to when expressions and matching and smart casts and early validation.

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

sealed interface Result
data object Pending : Result
data class Recorded(val score: Int) : Result
fun describe(result: Result): String = when (result) {
  Pending -> "Awaiting result"
  is Recorded -> "Score: ${result.score}"
}
fun main() {
  println(describe(Pending))
  println(describe(Recorded(80)))
}

Expected output

Awaiting result
Score: 80

Follow the execution

Pending needs no fabricated numeric score. The Recorded branch narrows the type and reads its score. Both alternatives are handled, so the function returns a message for each possible demonstrated state.

Try this in the Kotlin compiler by pasting the complete program.

Your task: What should you do after adding another permitted result state?

Kotlin/JVM · complete program

sealed interface Result
data object Pending : Result
data class Recorded(val score: Int) : Result
fun describe(result: Result): String = when (result) {
  Pending -> "Awaiting result"
  is Recorded -> "Score: ${result.score}"
}
fun main() {
  println(describe(Pending))
  println(describe(Recorded(80)))
}

Worked answer

Update the exhaustive handling where necessary so the new state has an intentional meaning instead of being silently ignored.

Use enum entries for simple fixed labels, and sealed alternatives for richer state models. Type structure still does not validate a Recorded score’s range unless its construction or operations enforce that policy.

Update the exhaustive handling where necessary so the new state has an intentional meaning instead of being silently ignored. Use enum entries for simple fixed labels, and sealed alternatives for richer state models.

Further reading: official Kotlin documentation for this topic.