On Sun, Sep 11, 2011 at 8:28 AM, Stuart Halloway
<[email protected]> wrote:
> cycle actually calls lazy-seq. A quick way to check such things at the REPL
> is with source:
> user=> (source cycle)
> (defn cycle
> "Returns a lazy (infinite!) sequence of repetitions of the items in coll."
> {:added "1.0"
> :static true}
> [coll] (lazy-seq
> (when-let [s (seq coll)]
> (concat s (cycle s)))))
> <snip>
Is there any reason not to define it as
(defn cycle
"Returns a lazy (infinite!) sequence of repetitions of the items in coll."
{:added "1.0"
:static true}
[coll]
(let [l (lazy-seq
(when-let [s (seq coll)]
(concat s l))))
This will have the effect that holding any element of the cycle holds
the head; however, even the original holds the head of s as long as
the cycle is referenced, so that at worst doubles memory use for
finite s, and for finite walks along the cycle with infinite s, as s
(or the first N elements of s) get held onto in both cases, but an
equal number of cycle elements get held onto with the self-referencing
cycle instead of potentially only one at a time.
> Now I'll test it with 9876543210, a number which "ev?" was able to
> handle:
>
> user=> (time (mod3 9876543210))
> "Elapsed time: 37759.615 msecs"
> 1
> user=> (mod 987654321 3)
> 0
>
> Whoa! The computation finished in reasonable time, but with the WRONG
> answer! How did that happen?
> Did I find a bug?
>
> No, there is simply a typo in your input arg.
True, but both numbers are congruent to 0 mod 3, so where did the 1
come from in (time (mod3 9876543210))?
--
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