Clojure doesn't do TCO with recursive functions--you have to use 
`loop`/`recur`[1]. This is one of the (thankfully few) "gotchas" in 
Clojure, at least/especially for people coming from other functional 
languages (or so I assume--coming from Python, I hadn't heard that much 
about TCO in the first place). There's a pretty good chance whichever intro 
book you're working through will talk about this in a chapter or so :)


[1] http://clojure.org/functional_programming#Functional 
Programming--Recursive Looping


On Thursday, November 5, 2015 at 10:01:44 AM UTC-5, Matthew Ulrich wrote:
>
> Thanks, Sergio - this is a good way to construct the sieve.
>
> It took me a little while to convince myself that it's the same, but it's 
> definitely a good solution.
>
> In general, I guess it's not possible to generate an infinite sequence 
> that utilizes the tail-call optimization.... it makes sense as there is no 
> "tail".
>
> Have a good day,
>
> Matty
>
> On Wednesday, November 4, 2015 at 10:07:46 AM UTC-5, Sergio Rupena wrote:
>>
>> Hi
>>
>> You can try the following
>>
>> (defn dividers   [primes n]
>>   (take-while #(<= (* % %) n) primes))
>>
>> (defn prime? [primes n]
>>   (every? #(pos? (rem n %))
>>           (dividers primes n)))
>>
>> (defn range-peek [coll]
>>   (iterate inc (-> coll peek inc)))
>>
>> (defn sieve
>>   ([] (cons 2 (lazy-seq (sieve [2]))))
>>   ([primes]
>>    (let [p (->> primes
>>                 range-peek
>>                 (filter (partial prime? primes))
>>                 first)]
>>      (cons p (lazy-seq (sieve (conj primes p)))))))
>>
>> (last (take 10000 (sieve)))
>>
>> This version keeps the visited primes in a vector so it will grow in 
>> memory but won’t overflow otherwise.
>>
>> Sergio
>>
>> On 04 Nov 2015, at 15:44, Matthew Ulrich <matty...@gmail.com> wrote:
>>
>> All - 
>>
>> I'm trying to generate a lazy sequence of primes using Erastosthenes 
>> Sieve (from Abelson & Sussman) and am encountering some issues with lazy 
>> sequence.
>>
>> Here's what I have:
>> ---
>>
>> (defn divisible?
>>   [input numerator]
>>   (= 0 (mod input numerator)))
>>
>> (defn sieve
>>   [stream]
>>   (lazy-seq
>>     (cons
>>       (first stream)
>>       (sieve (filter #(not (divisible? % (first stream))) (rest 
>> stream))))))
>>
>> (def primes (sieve (iterate inc 2)))
>>
>> ---
>>
>> This is fine now when I:
>>
>> => (take 5 (drop 1000 primes))
>> (7927 7933 7937 7949 7951)
>>
>>
>> But when I:
>>
>> => (take 5 (drop 4000 primes))
>> StackOverflowError    clojure.lang.LazySeq.sval  (LazySeq.java:40)
>>
>>
>> --
>>
>> I understand how the StackOverflow occurs with the filter on the 
>> recursively generated sequence... however I can't think of a way to make 
>> this work..
>>
>> I'm pretty new to clojure so I'm not very familiar with all of the 
>> language features; is there some tool to help me realize this or should I 
>> just go about the implementation in a different way?
>>
>> Thanks - have a great morning!
>>
>> Matty
>>
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