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<haskell>
 
<haskell>
 
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
 +
  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
 
fix g = xs where xs = g xs
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
 
 
</haskell>
 
</haskell>
  
 
<code>foldi</code> is on [[Fold#Tree-like_folds|Tree-like folds]]. More at [[Prime numbers]].
 
<code>foldi</code> is on [[Fold#Tree-like_folds|Tree-like folds]]. More at [[Prime numbers]].

Revision as of 07:53, 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.