Kotlin expandable null checking

Question

I have a method as follows:

fun <T1, T2, T3> ifCoordsNotNull(v1: T1?, v2: T2?, v3: T3?, allNotNull: (T1, T2, T3) -> (Unit)) {
    if (v1 != null && v2 != null && v3 != null) {
        allNotNull(v1, v2, v3)
    }
}

I know this is a way to check if multiple values are null; however, it seems to be not too efficient for my needs unless I make several related methods.

Is there a way I could put any/all variables I need to check for null into a list/set, and then recover the results (if any) in a callback with a single method?

To be more specific, I can use the current method like this:

ifCoordsNotNull(loc["x"], loc["y"], loc["z"]) { x, y, z ->

But would prefer, if possible, to create a more ergonomic method that can work in many situations like this:

ifNotNull(linkedSetOf(loc["x"], loc["y"], loc["z"])) { x, y, z ->

If there's anything similar, or a better practice for my needs, please do let me know. Thank you.

score -1 · Answer 1 · answered Dec 02 '18 at 10:58

-1

You can make use of vararg keyword and filterNotNull, for example:

fun ifNotNull(vararg input: Any?, function: (s: List<Any>) -> Unit) {
    val list = input.filterNotNull().toList()

    if (list.size != input.size) {
        throw RuntimeException()
    }

    function(list)
}

Results would be:

ifNotNull(1, 2, 3) {
    println(it)
} // [1, 2, 3]

ifNotNull("a", 1, null) {
    println(it)
} // Throws

answered Dec 02 '18 at 10:58

Alexey Soshin

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The problem is that once `function` argument needs to access the input with the correct type, you need to write something like `ifNotNull(1, 2, 3) { list -> val x = list[0] as Int; ... }`. Yuck. It can be at least improved for the case where all arguments are the same type. – Alexey Romanov Dec 02 '18 at 12:59
Yep. But since example uses three different types, I decided to forgo generics here. – Alexey Soshin Dec 02 '18 at 13:20

score -1 · Answer 2 · answered Dec 02 '18 at 11:16

-1

What you'd need to do it properly is generics with variable number of type parameters, which Kotlin doesn't have. Actually the only language I know of which does have something like this is C++.

Otherwise you'll either:

lose types and/or number of parameters as in Alexey Soshin's answer
have to introduce classes for different numbers of parameters which ends up as more work than declaring a method for each number of parameters.

answered Dec 02 '18 at 11:16

Alexey Romanov

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If the requirement is that *all* parameters of *any* type ar not null - why would your point 1 be a problem? – David Soroko Dec 02 '18 at 12:03
@DavidSoroko Please see my comment to Alexey's answer. – Alexey Romanov Dec 02 '18 at 13:00
Right, this is about the actual processing and not the input validation, I agree. – David Soroko Dec 02 '18 at 13:10

David Soroko · Answer 3 · 2018-12-02T12:07:46.727

Some variant on the following perhaps:

fun check(vars: Array<Any?>): Boolean = vars.filterNotNull().size == vars.size


fun <T1, T2, T3> ifCoordsNotNull(v1: T1?, v2: T2?, v3: T3?, allNotNull: (T1, T2, T3) -> (Unit)) {
    if (check(arrayOf(v1, v2, v3))) {
        allNotNull(v1!!, v2!!, v3!!)
    }
}

You may be able to get rid of !! using Contracts (https://kotlinlang.org/docs/reference/whatsnew13.html#contracts)

Kotlin expandable null checking

3 Answers3