Hi,
On 5 Apr., 01:27, Ken Wesson <[email protected]> wrote:
> Oh, and it also doesn't hold onto the head:
Your version of partition-by is basically equivalent to:
(defn partition-by
[f coll]
(lazy-seq
(when-let [s (seq coll)]
(let [fst (first s)
value (f fst)
[group tail] (split-with #(-> % f (= value)) (rest s))]
(cons (cons fst group) (partition-by f tail))))))
It traverses the input sequence twice: once for the take-while, once
for the drop-while. And it does "kind of" hold onto the head. It
doesn't really hold the head, but it still holds references to the
items of the first group, because the (drop-while first ..) is not
executed immediately. Only when the rest is actually realised with
seq, the references to the first group are dropped. So it is a weaker
type of holding the head. That's what I meant with "you would hold
unto the head *in some way*." With some numbers the effect is maybe
not visible, but holding some huge data structures without any notice
why, is maybe a problem.
Sincerely
Meikel
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