scala - Understanding flatMap declaration in List -

i looked @ list.flatmap declaration , kind of surprised this.

final override def flatmap[b, that](f: => gentraversableonce[b])                  (implicit bf: canbuildfrom[list[a], b, that]): 

where object list defines:

implicit def canbuildfrom[a]: canbuildfrom[coll, a, list[a]] =     reusablecbf.asinstanceof[genericcanbuildfrom[a]] 

so, if invoke flatmap on list list , don't see point in that type if deduced list[b] (because of implicit).

so, if invoke flatmap on list[a] list[a] , don't see point in that type if deduced list[b]

one thing you're missing flatmap isn't defined on list[+a]. inherited traversablelike, trait used of scalas collections. each of them can supply implicit canbuildfrom may overridden supply different resulting collection.

if want little taste of can custom canbuildfrom:

scala> :pa // entering paste mode (ctrl-d finish)  import scala.collection.generic.canbuildfrom import scala.collection.immutable._ import scala.collection.mutable import scala.{list, vector}  implicit val listtovectorcbf = new canbuildfrom[list[int], int, vector[int]] {   override def apply(from: list[int]): mutable.builder[int, vector[int]] = this.apply()   override def apply(): mutable.builder[int, vector[int]] = vector.newbuilder }  // exiting paste mode, interpreting.  scala> list(1,2,3).flatmap(list(_)) res6: vector[int] = vector(1, 2, 3)