class Foo(val bars: List<String> = ArrayList())

class FooBar(val foo: Foo, val bar: String?)

fun main(args: Array<String>) {

    val foo1 = Foo(listOf("a", "b", "c"))
    val foo2 = Foo()
    val foo3 = Foo(listOf("d"))
    val foo4 = Foo()

    // How can I map above objects into a list of:

    // FooBar(foo1, "a")
    // FooBar(foo1, "b")
    // FooBar(foo1, "c")
    // FooBar(foo2, null)
    // FooBar(foo3, "d")
    // FooBar(foo4, null)
}