1️⃣:

val serials = devices.map { it.motor.serial }

2️⃣:

val serials = devices.map(Device::motor).map(Motor::serial)

Is there any reason ever to go with 2️⃣?

Don't forget about

val serials = devices.map { it.motor }.map { it.serial }

!

Keep in mind that, unless you are using

Sequence

, a list is instantiated at each

.map

call. So 1️⃣ is better imo

In my experience, the functional purist would prefer a single transformation at each step (option 2, take your pick of syntax). However it does use more memory since you create another collection under the hood, so as you scale your code gravitates towards option 1. A good example would be a sum:

val totalMileage = devices.map { it.odometer }.sum() // O(n) memory

versus

val totalMileage = devices.fold(0) { acc, d -> acc + d.odometer } // O(1) memory

which is why

.sumBy

exists.

Thanks to all of you!