scala - 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(1, 2, 3, 4)

scala> res18.flatMap(identity)
res20: List[Int] = List(1, 2, 3, 4)

Is there any difference between these two functions? When is it appropriate to use one over the other? Are there any tradeoffs?

Answer 1

You can view flatMap(identity) as map(identity).flatten. (Of course it is not implemented that way, since it would take two iterations).

map(identity) gives you the same collection, so in the end it is the same as only flatten.

I would personally stick to flatten, since it is shorter/easier to understand and designed to exactly do this.