After thinking about it some more, I don't understand it either. I have
created a ticket for this:
https://www.assembla.com/spaces/clojure/tickets/457-lazy-recursive-definition-giving-incorrect-results.
As a workaround, eliminate the lazy intermediary "nums" sequence, e.g.
(def primes
(concat
[2]
(lazy-seq
(let [primes-from
(fn primes-from
[n]
(if (some #(zero? (rem n %))
(take-while #(<= (* % %) n) primes))
(recur (+ n 2))
(lazy-seq (cons n (primes-from (+ n 2))))))]
(primes-from 3)))))
Stu
> Thank you all for explaining this to me but I still don't understand
> clojures behavior in this case,
>
> Try running this code:
>
> (def nums (drop 2 (range)))
> (def primes (cons (first nums)
> (lazy-seq (->>
> (rest nums)
> (remove
> (fn [x]
> (let [dividors (take-while #(<= (* % %) x)
> primes)]
> (println (str "primes = " primes))
> (some #(= 0 (rem x %)) dividors))))))))
> (take 5 primes)
>
> It prints out:
> (primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> primes = (2)
> 2 3 5 7 9)
>
> How can (println primes) ever print out (2) ? That means that primes
> must have length 1 and that should be impossible no matter how chunked
> it is. It's hard to believe that that is intended behavior.
> I don't insist that a recursive lazy definition needs to work. It
> could also throw an error. But simply giving incorrect and
> unpredictable results can't be a good solution in my opinion.
>
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