Clojure - tail recursive sieve of Eratosthenes

Question

I have this implementation of the sieve of Eratosthenes in Clojure:

(defn sieve [n]
  (loop [last-tried 2 sift (range 2 (inc n))]
    (if
      (or (nil? las         


        

Answer 1

Problem: filter does lazy evaluation, so each new level of filtering hangs around on the call stack.

Fix: Change (filter ...) to (doall (filter ...)).

See the explanation here.

Answer 2

I second Michal Marczyk's comment about checking out cgrande's beautiful incremental SoE. I did some really primitive benchmarks and put them up at http://clojure.roboloco.net/?p=100, for those curious about lazy prime generator performance.

Answer 3

If you look at the backtrace

(try
 (sieve 200000)
 (catch java.lang.StackOverflowError e
  (.printStackTrace e)))

it looks like this:

...
at clojure.lang.LazySeq.sval(LazySeq.java:42)
at clojure.lang.LazySeq.seq(LazySeq.java:56)
at clojure.lang.RT.seq(RT.java:440)
at clojure.core$seq__4176.invoke(core.clj:103)
at clojure.core$filter__5033$fn__5035.invoke(core.clj:1751)
at clojure.lang.LazySeq.sval(LazySeq.java:42)
at clojure.lang.LazySeq.seq(LazySeq.java:56)
...

It's too many filters that's causing the overflow, not the loop.

Unfortunately, I don't see an obvious solution for this.