Sealed and result
- Kotlin course
- Intermediate
- Lesson 28/116
- 5 min
Learn a Result-like pattern with sealed classes.
The Result pattern
To carry an operation outcome, you can build a Result pattern with a sealed class. This pattern expresses success and failure as distinct types.
Example: sealed class Result; data class Success(val value: String) : Result(); data class Failure(val reason: String) : Result(). A function returns Result; the caller handles both cases with when. This is common in APIs that prefer carrying the outcome explicitly over throwing exceptions. The error case becomes data; control flow stays linear.
Success and Failure
In the Result pattern, Success carries the successful value and Failure carries the failure reason. Both are data classes; each has its own field.
Example: fun handle(r: Result): String = when (r) { is Success -> "ok: ${r.value}"; is Failure -> "err: ${r.reason}" }. In the is Success branch, r.value is accessed via smart cast; in the is Failure branch, r.reason is. This handles success and failure in the same function with a single return type. It is clearer and less error-prone than exception handling.
Practice The Result sealed class and the handle function
Define a sealed class named Result. Add two data class subtypes: Success(val value: String) and Failure(val reason: String), both inheriting from Result(). Write a function handle(r: Result): String whose body uses when (r) to return "ok: ${r.value}" for Success and "err: ${r.reason}" for Failure. In main create a Success("200") instance and print the result of handle.
Starter code
// Define the Result sealed class
// data class Success and Failure subtypes
// handle function: branch with when
// instantiate Success in main, print handleSample solution
sealed class Result
data class Success(val value: String) : Result()
data class Failure(val reason: String) : Result()
fun handle(r: Result): String {
return when (r) {
is Success -> "ok: ${r.value}"
is Failure -> "err: ${r.reason}"
}
}
fun main() {
val r: Result = Success("200")
println(handle(r))
}Expected output
ok: 200
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 <code>sealed class Result</code> and <code>data class Success(val value: String) : Result()</code> are defined. If a function returns <code>Result</code>, what does it do?
- It carries a success or failure outcome Correct answer
- It always returns Success
- It throws a runtime error
- It only returns a String
Explanation: Result carries a success or failure outcome.
2 Match the Result pattern
| Term | Meaning |
|---|---|
sealed class Result |
Closed base class |
data class Success |
Success state |
data class Failure |
Failure state |
when (r) |
Branch by result |
3 Complete the Result subtype definition
data class Failure(val reason: String) : ____()data class Failure(val reason: String) : Result()Explanation: Failure inherits from the Result sealed class.