arkivanov

fun bar(source: Observable<CharSequence>) {
}

fun foo(source: Observable<String>) {
    bar(source) // Compilation error
}

arkivanov

fun bar(source: Observable<out CharSequence>) {
    source.scan { a, b -> "$a,$b" } // Compilation error
}

fun foo(source: Observable<String>) {
    bar(source)
}

arkivanov

interface Observable<out T> {
}

arkivanov

fun bar(consumer: Consumer<String>) {
}

fun foo(consumer: Consumer<CharSequence>) {
    bar(consumer) // Compilation error
}

arkivanov

fun bar(consumer: Consumer<in String>) {
    if (consumer is Subject) {
        val value: String = consumer.value // Compilation error
    }
}

fun foo(consumer: Consumer<CharSequence>) {
    bar(consumer)
}

arkivanov

interface Consumer<in T> {
    fun accept(value: T)
}

arkivanov

fun foo() {
    val source: Observable<String> = observableOf("Hello")
    bar(source) // No error
}

fun bar(source: Observable<CharSequence>) {
}

arkivanov

public interface UserDataSource {
    Single<User> load();
}

arkivanov

// 1
observableOf<String>(null) // Compilation error

// 2
val o1: Observable<String?> = observableOf(null)
val o2: Observable<String> = o1 // Compilation error, types do not match

// 3
val o1: Observable<String?> = observableOf(null)
val o2: Observable<String> = o1.notNull() // No error, null values filtered out

arkivanov

class FragmentWithDialog(output: (Output) -> Unit) : Fragment() {

    sealed class Output {
        object DialogShown : Output()
    }
}

class FragmentWithInput(input: Observable<Input>) : Fragment() {

    sealed class Input {