My friend's playing with Haskell, and asked me how I'd write a function to
take a list and return a list of the sums like so:
(f [1 2 4 8])
=> [1 3 7 15]
So I told him about Haskell's scanl function.
Is scanl available in Haskell?
These were my two versions of it and some simple benchmarks:
(defn scanl
([f seed coll]
(scanl f seed coll 1))
([f seed coll i]
(if (<= i (count coll))
(cons (reduce f seed (take i coll))
(scanl f seed coll (inc i))))))
(println (scanl + 0 [1 4 5 9]))
(defn scanl2 [f seed coll]
(loop [f f seed seed coll coll acc []]
(let [cnt (count acc)]
(if (< cnt (count coll))
(recur f seed coll (conj acc (reduce f seed (take (inc cnt) coll))))
acc))))
(println (scanl2 + 0 [1 4 5 9]))
(def first (with-out-str (time (scanl + 0 [1 4 5 9 6 7 8 9]))))
(def second (with-out-str (time (scanl2 + 0 [1 4 5 9 6 7 8 9]))))
(println first)
(println second)
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