Flatten is a steam-roller and should be used wisely: it recursively flatten a sequential collection. Most of the time this is not what you want. Most of the res

Flatten is a steam-roller and should be used wisely: it recursively flatten a sequential collection. Most of the time this is not what you want. Most of the rest of the time when you think that’s what you want it’s a code smell telling you that the shape of your items has gone out of control. And there’s the rare occurrence where you can really use it.