Shouldn't ``` public operator fun <T> Iterable<T>.minus(element: T): List<T> ``` b...

# stdlib

[mg6maciej]

Shouldn't

public operator fun <T> Iterable<T>.minus(element: T): List<T>

be defined with element as nullable?

public operator fun <T> Iterable<T>.minus(element: T?): List<T>

I.e. if you call listOf<String>(...) - null as String? it would just return the same List<String> and not List<String?> as it currently does.

[mg6maciej]

Anyone has any input on this idea? I think such a change could be implemented without breaking any code using minus currently. It could even be changed to element: Any?, but I'm not sure this would be very useful.

[snrostov]

what if T nullable itself?

[mg6maciej]

There would be no problem. It would work exactly the same.

[mg6maciej]

T is subclass of T?

[snrostov]

listOf(1, null, 2) - null this would work?

[mg6maciej]

listOf(1, null, 2) - null as Int?

[snrostov]

For now, operator works like this:

println(listOf(1, null, 2) - null as Int?)
outputs: 1, 2

if you change operator to something like this:

public operator fun <T> Iterable<T>.minus(element: T?): List<T> {
    if (element == null) {
        return this.toList()
    }
...

then

println(listOf(1, null, 2) - null as Int?)
output: 1, null, 2

or I'm missed something?

[mg6maciej]

val list1: List<Int?> = arrayListOf(1, null).minusNullable(null)
        println("list1: $list1")
        val list2: List<Int> = arrayListOf(1, 2).minusNullable(null)
        println("list2: $list2")

compiles and prints:

list1: [1]
list2: [1, 2]

where minusNullable is a slightly changed minus operator (essentially argument type changed to T?):

fun <T> Iterable<T>.minusNullable(element: T?): List<T> {
    val result = ArrayList<T>()
    var removed = false
    return this.filterTo(result) { if (!removed && it == element) { removed = true; false } else true }
}

[mg6maciej]

@snrostov if (element == null) { return this.toList() } is wrong. If it's null, you still want to remove it if it exists in the list.

[mg6maciej]

Good thing about changing argument type to T? is that it still returns List<T> even if you pass something that is potentially null.

[snrostov]

I see now, thanks for explanation

[mg6maciej]

And as T? is supertype of T (as well as Any and Any? are), no code existing currently would break.

[ilya.gorbunov]

This has to be investigated, whether it's possible to add something like this: https://github.com/JetBrains/kotlin/blob/1.1.0/libraries/stdlib/src/kotlin/collections/MutableCollections.kt#L15
so that type of element to be removed could be any direct supertype of element type of the collection, and not only T?.

[ilya.gorbunov]

@mg6maciej Please open an issue

[mg6maciej]

Thanks. I will.

[mg6maciej]

Although I'm mostly interested in a change to T?, but Any? could possibly also be used instead.

[mg6maciej]

@ilya.gorbunov The more I use Kotlin in a "immutable only" way, the more I think there is a need to a good immutable collections library 😉

[mg6maciej]

So I wasn't really considering change to MutableCollection::remove, just to Iterable::minus (with element, as well as other arguments).

[mg6maciej]

So I wanted to make this change in Kotlin's stdlib and see how it works, but I failed to run all tests. Is there a "tutorial" what needs to be done to run stdlib's test suite? I finished pretty early at setting JDK_16, 17 and 18. @ilya.gorbunov

[mg6maciej]

@ilya.gorbunov It's not only for direct supertype, unless direct means something else.

open class A
open class B : A()
class C : B()

fun usageTest() {
    val list = mutableListOf<C>()
    list.remove(null as Any?)
    list.remove(Any())
    list.remove(A())
    list.remove(B())
    list.remove(null as C?)
    list.remove(C())
}

All compile.

[mg6maciej]

Interesting tho that it is implemented for mutable collections like I would like it to behave for "immutable" ones.

[mg6maciej]

https://youtrack.jetbrains.com/issue/KT-17569

[ilya.gorbunov]

It's not only for direct supertype, unless direct means something else.

Yeah, turns out I've actually meant something else 😆
I mean you shouldn't be able to minus a String from a List<Int> even if you can minus Any from that list. That's how MutableCollection.remove extension works.
Although now you can, so it might be a source breaking change.

[ilya.gorbunov]

So I wanted to make this change in Kotlin's stdlib and see how it works, but I failed to run all tests. Is there a "tutorial" what needs to be done to run stdlib's test suite? I finished pretty early at setting JDK_16, 17 and 18.

Yes, take a look at this readme: https://github.com/JetBrains/kotlin/tree/master/libraries
If you have any problems with building and running tests, welcome to share them in #kontributors

[mg6maciej]

"Although now you can, so it might be a source breaking change." Oh, true. It returns List<Any> in that situation. That's a shame.