Fix Kotlin Sealed Class Not Exhaustive

In this tutorial, you'll learn about Fix Kotlin Sealed Class Not Exhaustive. We cover key concepts, practical examples, and best practices.

The Problem

When expression on a sealed class does not cover all subclasses, causing a compile error.

Quick Fix

Define sealed class

Wrong:

sealed class Result
class Success(val data: String) : Result()
class Error(val msg: String) : Result()

Output:

Compile error

Right:

sealed class Result
class Success(val data: String) : Result()
class Error(val msg: String) : Result()

Output:

Sealed hierarchy defined

Use exhaustive when

Wrong:

fun handle(result: Result) {
    when (result) {
        is Success -> println(result.data)
    }
} // Missing Error branch

Output:

Compile error

Right:

fun handle(result: Result) {
    when (result) {
        is Success -> println(result.data)
        is Error -> println(result.msg)
    }
} // No else needed

Output:

Exhaustive when

Add else branch for future

Wrong:

when (result) {
    is Success -> println(result.data)
    is Error -> println(result.msg)
    // No else - new subclass breaks this

Output:

Sealed class extended

Right:

when (result) {
    is Success -> println(result.data)
    is Error -> println(result.msg)
    else -> println("Unknown")
}

Output:

Future-safe

Prevention

• Define sealed class with limited subclasses
• Use exhaustive when expressions without else
• Add else branch for forward compatibility

Common Mistakes with sealed class

1. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors
2. Non-exhaustive pattern matches that compile with warnings then crash at runtime
3. Misunderstanding that String is [Char] with poor performance for large text operations

These mistakes appear frequently in real-world KOTLIN code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

This quick fix is part of the DodaTech Spring & JVM ecosystem series. Built by the developers of Doda Browser, DodaZIP, and Durga Antivirus Pro.