SumsVsUnions.kt

// sum type (typical Kotlin implementation)
sealed class Expr
data class Var(val name: String): Expr()
data class Constant(val value: Int): Expr()
object Empty: Expr()

// union type (hypothetical)
data class Var(val name: String)
data class Constant(val value: Int)
object Empty
typealias Expr = Var | Constant | Empty

// should work for both
fun choice(e: Expr) = when(e) {
    Empty -> ...
    is Var -> ...
    is Constant -> ...
    // no else needed here
}