# User:WillNess

### From HaskellWiki

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− | <code>foldi</code> is on [[Fold#Tree-like_folds|Tree-like folds]] page. | + | <code>foldi</code> is on [[Fold#Tree-like_folds|Tree-like folds]] page. <code>union</code> and more at [[Prime numbers#Sieve_of_Eratosthenes|Prime numbers]]. |

## Revision as of 07:47, 8 August 2011

I am a newbie, interested in Haskell.

I like *this*:

-- inifinte folding idea due to Richard Bird -- double staged production idea due to Melissa O'Neill -- tree folding idea Dave Bayer / simplified formulation Will Ness primes = 2 : g (fix g) where g ps = 3 : gaps 5 (foldi (\(q:qs) -> (q:) . union qs) [[p*p, p*p+2*p..] | p <- ps]) gaps k s@(c:t) | k < c = k : gaps (k+2) s -- | k<=c = minus [k,k+2..] s | True = gaps (k+2) t -- fused to avoid a space leak fix g = xs where xs = g xs -- global defn to avoid space leak

`foldi`

is on Tree-like folds page. `union`

and more at Prime numbers.