We already have IterableOps.groupMap and groupMapReduce methods, does it make sense to add a groupFlatMap method as well?

In the simplest case this could just be an alias for groupMapReduce(f)(g)(_ ++ _) but if implemented separately it can be more efficient:

• we can avoid an extra function call to the reducer "wrapper" function for each element
• we can use a mutable buffer internally to build up the list of values

Indeed groupMapReduce already states in its doc that it exists (solely?) because it's more efficient than rolling the behaviour by hand using groupBy(f).map(...). It also somehow "feels" like this is missing from the API since we generally expect map and flatMap to come as a pair.