withFilter instead of filter

Question

Is it always more performant to use withFilter instead of filter, when afterwards applying functions like map, flatmap etc.?

Why are only map, flatmap and foreach supported? (Expected functions like forall/exists as well)

Answer 1

From the Scala docs:

Note: the difference between c filter p and c withFilter p is that the former creates a new collection, whereas the latter only restricts the domain of subsequent map, flatMap, foreach, and withFilter operations.

So filter will take the original collection and produce a new collection, but withFilter will non-strictly (i.e. lazily) pass unfiltered values through to later map/flatMap/withFilter calls, saving a second pass through the (filtered) collection. Hence it will be more efficient when passing through to these subsequent method calls.

In fact, withFilter is specifically designed for working with chains of these methods, which is what a for comprehension is de-sugared into. No other methods (such as forall/exists) are required for this, so they have not been added to the FilterMonadic return type of withFilter.