Hi, I have a problem with the usage of the `Lists.filterNotNull() extension. The function is descri...

# compiler

[huehnerlady]

Hi,

I have a problem with the usage of the `Lists.filterNotNull() extension.
The function is described as follows:

public fun <T : Any> Iterable<T?>.filterNotNull(): List<T> {
    return filterNotNullTo(ArrayList<T>())
}

So I expect I can use it with a non-nullable list with nullable items, and I get a non-nullable list with just non-nullable items back.

But when I use this method like so:

fun List<String?>?.extract(): List<String> =
    this?.filterNotNull().map {
      "foo"
    }.toList()

I get a compiler error as shown in my screenshot.
Why is this the case? I would have expected that the output is a non-nullable list, at least that is what I read out of the signature?

I am using kotlin 1.3.72

[bezrukov]

So I expect I can use it with a non-nullable list with nullable items

this is correct

But the issue is that you wrote an extension for nullable list (List<String?>*?*)

[dmitriy.novozhilov]

Call with safe operator ?. returns nullable object
Correct code for your case should look smth like this:

fun List<String?>?.extract(): List<String> =
    this?.filterNotNull()?.map { "foo" } ?: emptyList()

[huehnerlady]

I thought the ? returns null if it is null and otherwise continues with a smartcasted non-nullable type?

[huehnerlady]

as I use this?.filterNotNull() it should be smartcasted to a non-nullable object when calling the filter function I thought?

[dmitriy.novozhilov]

No, there is no such smartcasts. If they will exist then there will be ambiguities for functions with nullable receivers
Consider such case:

fun Int?.foo(): String = "hello"
fun Int.foo(): String = "world"

fun test(s: String?) {
    val message = s?.length.foo()
    println(message)
    // what should be printed?
    // "hello", "world" or "null"?
    // currently it is unequivocally prints "hello"
}

[huehnerlady]

ok, I missunderstood then, many thanks for your help 🙂