haskell - double stream feed to prevent unneeded memoization?

Question

I'm new to Haskell and I'm trying to implement Euler's Sieve in stream processing style.

When I checked the Haskell Wiki page about prime numbers, I found some mysterious optimization technique for streams. In 3.8 Linear merging of that wiki:

primesLME = 2 : ([3,5..] `minus` joinL [[p*p, p*p+2*p..] | p <- primes']) 
  where
    primes' = 3 : ([5,7..] `minus` joinL [[p*p, p*p+2*p..] | p <- primes'])

joinL ((x:xs):t) = x : union xs (joinL t)

And it says

“<strong>The double primes feed is introduced here to prevent unneeded memoization and thus prevent memory leak, as per Melissa O'Neill's code.”</p>

How could this be? I can't figure out how it works.

Answer 1