Kotlin Exhaustive when

The when keyword is a powerful control flow construct in Kotlin that allows for very concise branching code. If a when block has all cases covered it is called "exhaustive", i.e. it has a branch for each possible scenario. Depending on the usage of the when block, the compiler will impose different restrictions on whether or not then when block must be exhausted.

Cases #

A when block must be exhaustive if it is used as an "expression" (i.e. returns a value) but is not required to be exhaustive when used as a "statement"

val myInt = Random.nextInt()

// Expression
val result = when(myInt) {
    1 -> ...
    else -> ...
}

// Statement
when(myInt) {
    1 -> ...
}

A when block can be exhaustive whether it has a "subject" (i.e. a value passed to the when block - when(mySubject)) or not, but the type of the subject affects the compiler requirements.

The compiler requires the when block to be exhaustive if the subject is a restrictive type, which are Sealed classes, Enum classes, and Booleans. This is enforced for both expressions and statements.

val myBool = Random.nextBoolean()
when(myBool) {
    true -> ...
    else -> ...
}

If the subject of the when block is not a restrive type and is not used as an expression, it is not required to be exhaustive.

val myInt = Random.nextInt()
when(myInt) {
    1 -> ...
    2 -> ...
}

If a when block has an else branch specified, it will always be exhaustive since that branch will be called if no other branch applies.

val myInt = Random.nextInt()
when(myInt) {
   1 -> ...
   else -> ...
}

If you want to explore on your own, check out this gist I created that shows a fuller version of the examples above. Until next time, thanks!

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