Is there any difference between flatten and flatMap(identity)?

Question

scala> List(List(1), List(2), List(3), List(4))
res18: List[List[Int]] = List(List(1), List(2), List(3), List(4))

scala> res18.flatten
res19: List[Int] = List         


        

Answer 1

Conceptually, there is no difference. Practically, flatten is more efficient, and conveys a clearer intent.

Generally, you don't use identity directly. It's more there for situations like it getting passed in as a parameter, or being set as a default. It's possible for the compiler to optimize it out, but you're risking a superfluous function call for every element.

You would use flatMap when you need to do a map (with a function other than identity) immediately followed by a flatten.