I think it's a good post as well, but filterMap is a contrived example. It comes out of the box: flatMap can be used as a filterMap, because Option[T] is an acc

I think it's a good post as well, but filterMap is a contrived example. It comes out of the box: flatMap can be used as a filterMap, because Option[T] is an acceptable substitute (through implicit convernsions) for Iterable[T]. (An option is a collection type; just one with 0 or 1 elements.) The existing collections functions in Scala take you very, very far. If you need to write your own primitives for performance reasons, it can get tricky, but that's really uncommon. Odersky's book (chapter 25) explains how to do that. Collections libraries are hard in Scala because of what you get. Once you define a few simple functions and possibly implicit conversions, you get all 50+ sequence methods "for free", and if you do it right, your map type functions will return collections of the same type (runtime and static) as the original-- without explicit typing. You get a lot of leverage, but you have to work a little bit for it.