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<code>foldi</code> is on [[Fold#Tree-like_folds|Tree-like folds]]. More at [[Prime numbers]].
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<code>foldi</code> is on [[Fold#Tree-like_folds|Tree-like folds]]. More at [[Prime numbers#Sieve_of_Eratosthenes|Prime numbers]].

Revision as of 08:07, 7 August 2011

I am a newbie, interested in Haskell.

I like this:

primes = 2 : g (fix g) -- double staged production idea due to M. O'Neill
 where
  g xs = 3 : (gaps 5 $ foldi (\x:xs -> (x:) . union xs)
                             [[p*p, p*p+2*p..] | p <- xs])
  gaps k s@(x:xs) -- | k<=x = minus [k,k+2..] xs
                  -- inlined to avoid a space leak
       = if k < x 
           then k : gaps (k+2) s 
           else     gaps (k+2) xs 
 
fix g = xs where xs = g xs

foldi is on Tree-like folds. More at Prime numbers.