Proposal to add groupFlatMap for every groupMap
def groupFlatMap[K, B](key: A => K)(f: A => IterableOnce[B]): immutable.Map[K, CC[B]]
is to
def groupMap[K, B](key: A => K)(f: A => B): immutable.Map[K, CC[B]]
as
def flatMap[B](f: A => IterableOnce[B]): CC[B]
is to
def map[B](f: A => B): CC[B]
Would groupFlatMapReduce also be useful?
…
After further thought, maybe its not obvious what would happen if IterableOnce is empty.
Can you provide some examples of when these would be useful?
This can be (mostly) implemented in terms of groupMapReduce:
def groupFlatMap[A, K, B](as: Iterable[A])(key: A => K)(f: A => IterableOnce[B]): Map[K, Iterable[B]] =
as.groupMapReduce(key)(a => View.from(f(a)))(_ ++ _)
It is not (yet?) possible to supply a Factory for building groups (here, I use View but it would be nice to build anything and in an efficient way), so the return type is Map[K, Iterable[B]] instead of something more specific.
Note that, as you noted, an important difference with the classic groupBy operations, is that with groupFlatMap groups can be empty.