What do you guys think?

# stdlib

[Alpesh Vas]

What do you guys think?

[Xavier F. Gouchet]

Can you share a sample of what your extension function does?

[karelpeeters]

Or maybe even the function itself?

[Alpesh Vas]

Sure

[Alpesh Vas]

Consider this code snippet

data class Customer(
        val id: Int,
        val name: String,
        val phoneNumber: String
)

val customerIds = listOf(1, 2, 3, 4, 5)

val customers = listOf(
        Customer(id = 0, name = "Alpesh", phoneNumber = "019"),
        Customer(id = 2, name = "Vas", phoneNumber = "245"),
        Customer(id = 3, name = "Alp3", phoneNumber = "678"),
        Customer(id = 6, name = "Alp4", phoneNumber = "901"))

Let say, now I want to find intersection of customers array and customerIds and return list of Customer after intersection.

[Alpesh Vas]

Now, kotlin has this intersect function

/**
 * Returns a set containing all elements that are contained by both this set and the specified collection.
 * 
 * The returned set preserves the element iteration order of the original collection.
 */
public infix fun <T> Iterable<T>.intersect(other: Iterable<T>): Set<T> {
    val set = this.toMutableSet()
    set.retainAll(other)
    return set
}

We can't use this function as types of both iterables are different in our case. We need some transform function here.

[Alpesh Vas]

So I've created this *customized extension function *

fun <T, A> Iterable<A>.intersect(other: Iterable<T>, valueTransform: (A) -> T ): List<A> {
    val set = this.map(valueTransform).toMutableSet()
    set.retainAll(other)
    return this.filter {valueTransform(it) in set}
}

[Alpesh Vas]

And I can achieve what I wanted to achieve by
val customerAfterInterSection = customers.intersect(customerIds){it.id}

[Alpesh Vas]

So I think we can add this ^^ function in kotlin's stdlib for collections.

[Alpesh Vas]

Let me know if you need more examples.

[Xavier F. Gouchet]

Can't you just do

val customerAfterIntersect = customers.filter { it.id in customerIds }

[Alpesh Vas]

We can but, it.id in customerIds is not efficient operation here right?

[Alpesh Vas]

Because customerIds is list, you'll have to convert it to set separately.

[Xavier F. Gouchet]

I'm not sure how big the difference in performance is between list and set

[Xavier F. Gouchet]

But you can still just create your set by hand and have a two kinds solution

[Alpesh Vas]

True, but that extension function is doing it for you. You won't need to create auxiliary set every time.

[Alpesh Vas]

By the way look up in list is O(n) whilst set does it in O(1)

[Alpesh Vas]

@Xavier F. Gouchet, @karelpeeters any suggestions on this? Is this worthy of to be a part of stdlib functions?

[nkiesel]

what about val customerAfterIntersect = customers.filter { it.id in customerIds.toSet() }

[gildor]

I personally don't think that it make sense to include it to stdlib, pretty specific use case for me, like what if I want to have different mapping for each collection

[gildor]

But be free to create an issue on Kotlin issue tracker of you think it's common case that may be helpful

[Alpesh Vas]

https://kotlinlang.slack.com/archives/C0B8Q383C/p1573200411032900?thread_ts=1573153561.031000&cid=C0B8Q383C
This is just an example, you can always create a new function with two different transform functions as arguments.

[gildor]

Yes, that is my point. If you suggest something for stdlib it should be some complete solution, that covers some range of cases. If function has too narrow use case it just shouldn't be a part stdlib

[Derek Peirce]

I second that this function is too specific for stdlib. Even this implementation is calling valueTransform on each element twice, which is fine if it is cheap, but problematic if it is expensive and has side-effects.

[Fudge]

what about val customerAfterIntersect = customers.filter { it.id in customerIds.toSet() }

toSet needs to go outside the loop or it will spend o(n) time making the set every loop

[Alpesh Vas]

Yup ^