So I've spent hours trying to work out exactly how this code produces prime numbers.

lazy val ps: Stream[Int] = 2 #:: Stream.from(3).filter(i =>
   ps.takeWhile{j => j * j <= i}.forall{ k => i % k > 0});

I've used a number of printlns etc, but nothings making it clearer.

This is what I think the code does:

/**
 * [2,3] 
 * 
 * takeWhile 2*2 <= 3 
 * takeWhile 2*2 <= 4 found match
 *      (4 % [2,3] > 1) return false.
 * takeWhile 2*2 <= 5 found match
 *      (5 % [2,3] > 1) return true 
 *          Add 5 to the list
 * takeWhile 2*2 <= 6 found match
 *      (6 % [2,3,5] > 1) return false
 * takeWhile 2*2 <= 7
 *      (7 % [2,3,5] > 1) return true
 *          Add 7 to the list
 */

But If I change j*j in the list to be 2*2 which I assumed would work exactly the same, it causes a stackoverflow error.

I'm obviously missing something fundamental here, and could really use someone explaining this to me like I was a five year old.

Any help would be greatly appreciated.

scala stream primes
Edit
Report