Misconception

mistakes — cgrand, 8 July 2008 @ 10 h 01 min

I thought that there was some kind of doall in apply to force evaluation of the arguments seq but it’s not the case when the applied function is variadic.

lazy butlast

utilities — cgrand, 1 July 2008 @ 9 h 55 min

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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