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Tuesday, July 1, 2008

lazy butlast

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Named but-last to avoid name clash with butlast which is a low-level function (it is used in the definition of defn).
(defn but-last
"Return a lazy sequence of all but the n last items in coll."
([coll] (but-last coll 1))
([coll n]
((fn this [s os]
(if os
(lazy-cons (first s) (this (rest s) (rest os))))) (seq coll) (drop n coll))))

Update: Rich Hickey improved this function and added it to boot.clj:
(defn drop-last
"Return a lazy seq of all but the last n (default 1) items in coll"
([s] (drop-last 1 s))
([n s] (map (fn [x _] x) (seq s) (drop n s))))
and shared with us, mere mortals, a piece of wisdom:
I constantly have to remind myself to try to use a higher-order fn, because lazy-cons is so easy
From now on, I'll try to question each lazy-cons.

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