What exactly are extension expressions, do you mean extension methods? If so, Swift does support them. I actually miss interface extensions in Kotlin, but thats a java cross compatibility issue.
inline fun <T, R> with(receiver: T, f: T.() -> R): R
with(list) {
add(item)
add(item)
}
Another thing that's bit me in butt recently is that `Protocol`s aren't concrete types, unlike an `interface` in Java/Kotlin.