(defn keep
"Returns a lazy sequence of the non-nil results of (f item). Note,
this means false return values will be included. f must be free of
side-effects. Returns a transducer when no collection is provided."
{:added "1.2"
:static true}
([f]
(fn [rf]
(fn
([] (rf))
([result] (rf result))
([result input]
(let [v (f input)]
(if (nil? v)
result
(rf result v)))))))
([f coll]
(lazy-seq
(when-let [s (seq coll)]
(if (chunked-seq? s)
(let [c (chunk-first s)
size (count c)
b (chunk-buffer size)]
(dotimes [i size]
(let [x (f (.nth c i))]
(when-not (nil? x)
(chunk-append b x))))
(chunk-cons (chunk b) (keep f (chunk-rest s))))
(let [x (f (first s))]
(if (nil? x)
(keep f (rest s))
(cons x (keep f (rest s))))))))))