I want to compute a lazy sequence of primes.
Here is the interface:
user=> (take 10 primes)
(2 3 5 7 11 13 17 19 23 29)
So far, so good.
However, when I take 500 primes, this results in a stack overflow.
core.clj: 133 clojure.core/seq
core.clj: 2595 clojure.core/filter/fn
LazySeq.java: 40 clojure.lang.LazySeq/sval
LazySeq.java: 49 clojure.lang.LazySeq/seq
RT.java: 484 clojure.lang.RT/seq
core.clj: 133 clojure.core/seq
core.clj: 2626 clojure.core/take/fn
LazySeq.java: 40 clojure.lang.LazySeq/sval
LazySeq.java: 49 clojure.lang.LazySeq/seq
Cons.java: 39 clojure.lang.Cons/next
LazySeq.java: 81 clojure.lang.LazySeq/next
RT.java: 598 clojure.lang.RT/next
core.clj: 64 clojure.core/next
core.clj: 2856 clojure.core/dorun
core.clj: 2871 clojure.core/doall
core.clj: 2910 clojure.core/partition/fn
LazySeq.java: 40 clojure.lang.LazySeq/sval
LazySeq.java: 49 clojure.lang.LazySeq/seq
RT.java: 484 clojure.lang.RT/seq
core.clj: 133 clojure.core/seq
core.clj: 2551 clojure.core/map/fn
LazySeq.java: 40 clojure.lang.LazySeq/sval
LazySeq.java: 49 clojure.lang.LazySeq/seq
RT.java: 484 clojure.lang.RT/seq
core.clj: 133 clojure.core/seq
core.clj: 3973 clojure.core/interleave/fn
LazySeq.java: 40 clojure.lang.LazySeq/sval
I'm wondering what is the problem here and, more generally, when working
with lazy sequences, how should I approach this class of error?
Here is the code.
(defn assoc-nth
"Returns a lazy seq of coll, replacing every nth element by val
Ex:
user=> (assoc-nth [3 4 5 6 7 8 9 10] 2 nil)
(3 nil 5 nil 7 nil 9 nil)
"
[coll n val]
(apply concat
(interleave
(map #(take (dec n) %) (partition n coll)) (repeat [val]))))
(defn sieve
"Returns a lazy seq of primes by Eratosthenes' method
Ex:
user=> (take 4 (sieve (iterate inc 2)))
(2 3 5 7)
user=> (take 10 (sieve (iterate inc 2)))
(2 3 5 7 11 13 17 19 23 29)
"
[s]
(lazy-seq
(if (seq s)
(cons (first s) (sieve
(drop-while nil? (assoc-nth (rest s) (first s)
nil))))
[])))
(def primes
"Returns a lazy seq of primes
Ex:
user=> (take 10 primes)
(2 3 5 7 11 13 17 19 23 29)
"
(concat [2] (sieve (filter odd? (iterate inc 3)))))
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