++ on Grouped

[johnynek]

We could handle ++ on Grouped which could merge two Grouped[K, V] together. The trick is to handle cases where they got to be value type V from different starting (K, _) pipes. One trick would be to map the left to Left() and right to Right() before any value mapping. Then lift the value mappings into the Left or Right spaces, then you have Left[L] => Left[V] and Right[R] => Right[V] then you just do fold(identity)(identity) to unwrap both sides onto V, and you're still Grouped.

Alternatively, we could implement .either on Grouped[K, L] , Grouped[K, R] and then ++ can be implemented in terms of that for the case where L == R.

[johnynek]

Note, this could be used in writeIncremental on Grouped items.

[reconditesea]

Would love to have this.

Something I usually do is ++ two TypedPipe[K,V], do a group on the merged pipe and then reduce the Grouped. I think that's better than grouping them separately and joining two Grouped[K,V].
Wondering what will be the performance comparison of that with ++ on Grouped?