Why does IntelliJ suggest to convert a call chain into a Sequence?

Question

Assume the following Kotlin example that maps the source set src to a destination set dst:

private val src: Set<String> = setOf("hello", "world")
private val dst: Set<Int> = src.map { it.length }.toSet()

This works fine. However, IntelliJ's code inspection suggests: Call chain on collection should be converted into 'Sequence':

Applying this suggestion results in

private val dst: Set<Int> = src.asSequence().map { it.length }.toSet()

What is the benefit of this?

Shirtwaist

Answer 1

In this case the suggestion is suboptimal. The correct way to rewrite this code (which also doesn't result in any IntelliJ warnings) is:

src.mapTo(hashSetOf()) { it.length }

This will avoid the creation of an intermediate list that will be subsequently converted to a set; the data will be added to the resulting set right away.

Handicapper

Thanks, I created an issue about this: youtrack.jetbrains.com/issue/KT-26926. – Nonresident

I haven't seen the usage of mapTo a lot. Is it somewhere mentioned as an idiom? Especially the shown use case if a great example – Intosh

Answer 2

Set.map returns a list, which you then immediately throw away after converting it to a set. The benefit of asSequence is that the sequence does the conversion, presumably without a temporary list.

Hokanson