# User:WillNess

### From HaskellWiki

(Difference between revisions)

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<haskell> |
<haskell> |
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− | primes = 2 : g (fix g) -- double staged production idea due to M. O'Neill |
+ | -- inifinte folding idea due to Richard Bird |

− | where |
+ | -- double staged production idea due to Melissa O'Neill |

− | g xs = 3 : (gaps 5 $ foldi (\(q:qs) -> (q:) . union qs) |
+ | -- tree folding idea Dave Bayer / simplified formulation Will Ness |

− | [[p*p, p*p+2*p..] | p <- xs]) |
+ | primes = 2 : g (fix g) |

− | gaps k s@(x:xs) |
+ | where |

− | | k < x = k : gaps (k+2) s -- | k<=x = minus [k,k+2..] xs |
+ | g ps = 3 : gaps 5 (foldi (\(q:qs) -> (q:) . union qs) |

− | | True = gaps (k+2) xs -- inlined to avoid a space leak |
+ | [[p*p, p*p+2*p..] | p <- ps]) |

+ | gaps k s@(c:t) |
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+ | | k < c = k : gaps (k+2) s -- | k<=c = minus [k,k+2..] s |
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+ | | True = gaps (k+2) t -- fused to avoid a space leak |
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fix g = xs where xs = g xs -- global defn to avoid space leak |
fix g = xs where xs = g xs -- global defn to avoid space leak |
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</haskell> |
</haskell> |
||

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

## Revision as of 15:25, 7 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. More at Prime numbers.