Different language, but I find this Kotlin RFC proposing union types has a nice canonical example (https://youtrack.jetbrains.com/projects/KT/issues/KT-68296/U...)

    inline fun <T> Sequence<T>.last(predicate: (T) -> Boolean): T {
        var last: T? = null
        var found = false
        for (element in this) {
            if (predicate(element)) {
                last = element
                found = true
            }
        }
        if (!found) throw NoSuchElementException("Sequence contains no element matching the predicate.")
        @Suppress("UNCHECKED_CAST")
        return last as T
    }