:one:: ```val serials = devices.map { it.motor.serial }``` :two:: ```val serials = devices.map(Devic...

# codereview

[hho]

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?

[Joel]

Don't forget about val serials = devices.map { it.motor }.map { it.serial }!

[Luke]

Keep in mind that, unless you are using Sequence, a list is instantiated at each .map call. So 1️⃣ is better imo

[Joel]

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.

[hho]

Thanks to all of you!