Kotlin when guard conditions are Stable, not preview-only: introduced as a preview in Kotlin 2.1.0, they became Stable in Kotlin 2.2.0. To add an extra Boolean test to a subject-bearing branch, put if after its primary condition: is Animal.Cat if !animal.mouseHunter -> .... The branch runs only when both conditions match.
How to add a guard condition to a Kotlin when branch
A guard is a secondary Boolean condition attached to a primary condition in a subject-bearing when. It lets you keep the type or value check and its additional test at the same branch level instead of putting another conditional inside the branch body.
sealed interface Animal {
data class Cat(val mouseHunter: Boolean) : Animal { fun feedCat() {} }
data class Dog(val breed: String) : Animal { fun feedDog() {} }
}
fun feedAnimal(animal: Animal) {
when (animal) {
is Animal.Dog -> animal.feedDog()
is Animal.Cat if !animal.mouseHunter -> animal.feedCat()
else -> println("Unknown animal")
}
}Here, the cat branch requires both that animal is an Animal.Cat and that mouseHunter is false. Kotlin’s control-flow documentation describes guards as a way to make more complex branch control flow explicit and concise.
How Kotlin evaluates guards
Kotlin checks a branch’s primary condition first. If it does not match, it does not evaluate that branch’s guard. If the primary condition matches, Kotlin evaluates the guard; the body runs only if the guard is true. Branches retain when‘s ordered matching behavior, so an earlier matching branch can determine which body runs.
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A guard can contain Boolean logic, including && and ||, and parentheses can clarify how a compound expression is grouped. Guard conditions also support else if. A single when may mix guarded and unguarded branches.
Exhaustiveness and common limitations
Cover values that fail the guard
A guarded branch handles only values that satisfy both its primary condition and its guard. If a when is used as an expression, it must still be exhaustive: account for possibilities that match the primary condition but fail the guard, as well as other unmatched cases. Add appropriate branches or an else where needed. A when statement may have no matching branch and simply execute none of its branch bodies.
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Do not attach a guard to comma-separated conditions
A guard cannot be combined with multiple comma-separated conditions in one branch, such as 0, 1 -> .... Use separate branches when a guarded check is needed, or restructure the conditions without the comma-separated form.
Guard versus nested if
Neither style is universally better. A guard keeps the primary match and secondary test visible together, which can reduce nesting when a when has several cases. A nested if/else inside a branch can be clearer for a short binary decision or when it better matches the team’s established Kotlin style. Consider whether a reader can see the remaining branch cases and the expression’s exhaustive coverage easily in the chosen form.
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Version history and the -Xwhen-guards flag
Kotlin 2.1.0 introduced subject-bearing when guard conditions as a preview that required opt-in. Kotlin 2.2.0 promoted them to Stable; the current language feature index also lists them as Stable. The feature is therefore not preview-only in Kotlin 2.2.0 and later.
The compiler flag and Gradle configuration below are historical preview setup instructions for Kotlin 2.1.0, not the normal requirement for the Stable feature:
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kotlinc -Xwhen-guards main.ktkotlin {
compilerOptions {
freeCompilerArgs.add("-Xwhen-guards")
}
}The Kotlin 2.1.0 release notes document the preview and opt-in. Kotlin 2.2.0 release notes record the move to Stable. For a project on an older compiler, check its version-specific documentation rather than assuming the current status or configuration applies. IDE support and compatibility also depend on the Kotlin plugin and IDE versions; preview-era guidance is not a complete current compatibility matrix.
For broader context on Kotlin’s language compatibility approach, see the Kotlin evolution principles.
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