Why does arrow have an unzip method that, even tho...

# arrow

[Alexander]

Why does arrow have an unzip method that, even though this is already part of the standard library?

Standard library method:

public fun <T, R> Iterable<Pair<T, R>>.unzip(): Pair<List<T>, List<R>> {
    val expectedSize = collectionSizeOrDefault(10)
    val listT = ArrayList<T>(expectedSize)
    val listR = ArrayList<R>(expectedSize)
    for (pair in this) {
        listT.add(pair.first)
        listR.add(pair.second)
    }
    return listT to listR
}

Arrow method:

public fun <A, B> Iterable<Pair<A, B>>.unzip(): Pair<List<A>, List<B>> =
  fold(emptyList<A>() to emptyList()) { (l, r), x ->
    l + x.first to r + x.second
  }

To my eyes, this looks worse than what's already built-in because it's using immutable lists in every step, which means it will be creating two new lists for each iteration? While the built-in uses mutable lists internally, and just appends to those lists in each step of the itteration.

[Alejandro Serrano.Mena]

one possible cause is that Kotlin had introduced the function after Arrow already had it

[simon.vergauwen]

Yes, I think this is the case and this went by unnoticed so far but it's the perfect time to deprecate and get rid of it when we release Arrow 2.0 soon ☺️

[simon.vergauwen]

Thank you for reporting @Alexander. In general we also want to step away from immutable/read-only lists, and use mutable list instead like Kotlin Std does.

[Alejandro Serrano.Mena]

@Alexander feel free to send a PR with the deprecation, or creating an issue

[Alexander]

Will do!