Force compilation error with sealed classes

Question

With sealed classes you can use exhaustive when expressions and omit the else clause when the expression returns a result:

sealed class SealedClass {   class First : SealedClass()   class Second : SealedClass() }  fun test(sealedClass: SealedClass) : String =     when (sealedClass) {       is SealedClass.First -> "First"       is SealedClass.Second -> "Second"     } 

Now if I were to add a Third to SealedClass, the compiler will complain that the when expression in test() is not exhaustive, and I need to add a clause for Third or else.

I am wondering however if this check can also be enforced when test() does not return anything:

fun test(sealedClass: SealedClass) {     when (sealedClass) {       is SealedClass.First -> doSomething()       is SealedClass.Second -> doSomethingElse()     } } 

This snippet does not break if Third is added. I can add a return statement before when, but this could easily be forgotten and may break if the return type of one of the clauses is not Unit.

How can I make sure I don't forget to add a branch to my when clauses?

Answer 1

The way to enforce exhaustive when is to make it an expression by using its value:

sealed class SealedClass {     class First : SealedClass()     class Second : SealedClass()     class Third : SealedClass() }  fun test(sealedClass: SealedClass) {     val x = when (sealedClass) {         is SealedClass.First -> doSomething()         is SealedClass.Second -> doSomethingElse()     }  // ERROR here      // or      when (sealedClass) {         is SealedClass.First -> doSomething()         is SealedClass.Second -> doSomethingElse()     }.let {}  // ERROR here } 

Answer 2

In inspiration by Voddan's answer, you can build a property called safe you can use:

val Any?.safe get() = Unit 

To use:

when (sealedClass) {     is SealedClass.First -> doSomething()     is SealedClass.Second -> doSomethingElse() }.safe 

I think it provides a clearer message than just appending .let{} or assigning the result to a value.

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There is an open issue on the Kotlin tracker which considers to support 'sealed whens'.