Implementing List#flatMap

Question

Is there a better functional way to write flatMap?

def flatMap[A,B](list: List[A])(f: A => List[B]): List[B] =
    list.map(x => f(x)).flatten

Conceptually, I understand flatMap in terms of flatten.

Answer 1

An alternate approach:

def flatMap[A, B](list: List[A])(f: A => List[B]): List[B] =
  list.foldLeft(List[B]())(_ ++ f(_))

I don't know about “better”. (And if we start talking about efficient implementation, that's another can of worms...)