Peek is to get the reference of the first element. OnEach makes a lazy operation for every element on the sequence. I would say that onEach is more like map but lazy.

like forEach and also returns the iterable instance or like map, it is the same.

For me the key thing is that onEach applies lazily for sequences.

Map builds a new collection based on the function you provide it. At the end of a map call you end up with a potentially transformed collection instance.

It is not a map and it is not a forEach, that is why they created it.

> At the end of a map call you end up with a potentially transformed collection instance

The difference between onEach and map is that map returns a new collection that can have different type of elements. But onEach can modify the internal state of the objects when applied. Also onEach is lazy and map is not lazy in kotlin (as far as I know).

Why did I make the comparison with map and not with forEach? Because forEach is basically a translation of a for and map is a functional function and onEach it is also a functional function. You can agree or disagree with my way of trying to explain things, but being rude in your comments saying you are not right (when I know) and them providing another solution that you also know it isn't right, feels like a troll comment.

arrayOf("A","B","C").onEach{ print("onEach $it")}.forEach{ print("forEach $it")}

prints:

onEach A forEach A onEach B forEach B onEach C forEach C