scala - Why can I not flatMap a List[Option] using underscore?

Question

Why can I write

val flat: List[Int] = List(Some(1), Some(2)).flatMap(i => i)

But not

val flat: List[Int] = List(Some(1), Some(2)).flatMap(_)

Answer 1

The underscore is a placeholder for a variable. It's not, in and of itself, a function. flatMap requires a function.

Also, in this case, there is a flatten method which is defined on List for which Option has within the Predef an implicit conversion such that what you've written can be condensed into just that call, List(Some(1)).flatten