Union types

Language Design
70

On your example, you need to have a function that treat the declared result :

fun getResult() : List<String>|Throwable = TODO()
fun handleResult(foo: List<String>|Throwable) = TODO() 
handleResult(getResult())

until you check the type

fun getResult() : List<String>|Throwable = TODO()

fun handleResult(foo: List<String>) = TODO() 
fun handleResult(foo: Throwable) = TODO()    

val x = getResult()
if (x !is Throwable) {
    handleResult(x)  // call handleResult(foo: List<String>)
}

But the strength of union type adopt by typescript, ceylon and the next scala version, is for types (like examples show in precedent comments)

// Let be properties type be a map of key string and value (not simple object)
val propreties : Map<String, String|LocalDate|Number>

Others examples with json type.