Rob Rix’s RXCollections offers:
Folds, maps, filters, and detects for Cocoa collections (including your own), with as little chaff as possible.
The interesting part is that:
Maps and filters take a second argument, a collection into which to place their results. This can be a set or an array currently, and if you pass nil,
RXFilterwill build a collection of the same type as the collection being mapped or filtered.
And they can work with custom collection types that conform to the proper protocol.