# User:WillNess

### From HaskellWiki

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g xs = 3 : gaps 5 (foldi (\(c:cs) -> (c:) . union cs) [] | g xs = 3 : gaps 5 (foldi (\(c:cs) -> (c:) . union cs) [] | ||

[[x*x, x*x+2*x..] | x <- xs]) | [[x*x, x*x+2*x..] | x <- xs]) | ||

− | + | gaps k s@(c:t) | |

− | + | | k < c = k : gaps (k+2) s -- == minus [k,k+2..] (c:t), k<=c, | |

− | + | | True = gaps (k+2) t -- fused to avoid a space leak | |

− | fix g = xs where xs = g xs | + | fix g = xs where xs = g xs -- global defn to avoid space leak |

</haskell> | </haskell> | ||

## Revision as of 06:57, 26 October 2011

I'm 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 xs = 3 : gaps 5 (foldi (\(c:cs) -> (c:) . union cs) [] [[x*x, x*x+2*x..] | x <- xs]) gaps k s@(c:t) | k < c = k : gaps (k+2) s -- == minus [k,k+2..] (c:t), k<=c, | 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.

The math formula for Sieve of Eratosthenes,

where

- . . . or, :) :) .

Trial division sieve:

If you're put off by self-referentiality, just replace or on the right-hand side of equations with .