Hi,
Am 22.08.2011 um 16:32 schrieb Asim Jalis:
> Is there a way to rewrite mapcat (or apply concat) so that they don't
> evaluate the incoming seq unnecessarily?
user=> (defn f [[x]] (println "Computing x:" x) [(inc x)])
#'user/f
user=> (defn lazy-mapcat
[f coll]
(lazy-seq
(when-let [s (seq coll)]
(concat (f (first s)) (lazy-mapcat f (rest s))))))
#'user/lazy-mapcat
user=> (->> [0] (iterate f) (take 0))
()
user=> (->> [0] (iterate f) (apply concat) (take 0))
Computing x: 0
Computing x: 1
Computing x: 2
()
user=> (->> [0] (iterate f) (mapcat identity) (take 0))
Computing x: 0
Computing x: 1
Computing x: 2
()
user=> (->> [0] (iterate f) (lazy-mapcat identity) (take 0))
()
Sincerely
Meikel
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