Not sure what the objection to the keep function is, other than maybe the performance optimisations that chunked sequences allow. Taking away the chunked consid

Not sure what the objection to the keep function is, other than maybe the performance optimisations that chunked sequences allow. Taking away the chunked consideration gives something like: (let [x (f (first s))] (if (nil? x) (keep f (rest s)) (cons x (keep f (rest s))))) Which looks almost identical (with differences, as you seem to be giving filter rather than keep, which are different functions). The library function COULD be defined like this, it just wouldn't be as fast.