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Line 6: Line 6:
 
primes = 2 : g (fix g) -- double staged production idea due to M. O'Neill
 
primes = 2 : g (fix g) -- double staged production idea due to M. O'Neill
 
where
 
where
g xs = 3 : (gaps 5 $ foldi (\q:qs -> (q:) . union qs)
+
g xs = 3 : (gaps 5 $ foldi (\(q:qs)-> (q:) . union qs)
 
[[p*p, p*p+2*p..] | p <- xs])
 
[[p*p, p*p+2*p..] | p <- xs])
   

Revision as of 12:59, 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 (\(q:qs)-> (q:) . union qs)
                               [[p*p, p*p+2*p..] | p <- xs])
 
fix g = xs where xs = g xs
 
gaps k s@(x:xs) = if k < x      -- | k<=x = minus [k,k+2..] xs
        then k : gaps (k+2) s   -- inlined to avoid a space leak
        else     gaps (k+2) xs

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