# User:WillNess

### From HaskellWiki

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− | I am a newbie, interested in Haskell. |
+ | A perpetual Haskell newbie. I like ''[http://ideone.com/qpnqe this one-liner]'': |

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− | I like ''[http://ideone.com/qpnqe this]'': |
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<haskell> |
<haskell> |
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− | primes = 2 : g (fix g) -- double staged production idea due to M. O'Neill |
+ | -- infinite 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 / improved tree structure |

− | [[p*p, p*p+2*p..] | p <- xs]) |
+ | -- Heinrich Apfelmus / simplified formulation Will Ness |

+ | primes = 2 : _Y ((3:) . gaps 5 |
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+ | . foldi (\(x:xs) -> (x:) . union xs) [] |
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+ | . map (\p-> [p*p, p*p+2*p..])) |
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− | fix g = xs where xs = g xs |
+ | _Y g = g (_Y g) -- multistage production |

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

− | then k : gaps (k+2) s -- inlined to avoid a space leak |
+ | | k < c = k : gaps (k+2) s -- fused for better performance |

− | else gaps (k+2) xs |
+ | | otherwise = gaps (k+2) t -- k==c |

</haskell> |
</haskell> |
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− | <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. <code>union</code> and more at [[Prime numbers#Sieve_of_Eratosthenes|Prime numbers]]. |

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+ | The constructive definition of primes is the Sieve of Eratosthenes: |
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+ | |||

+ | ::::<math>\textstyle\mathbb{S} = \mathbb{N}_{2} \setminus \bigcup_{p\in \mathbb{S}} \{p\,q:q \in \mathbb{N}_{p}\}</math> |
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+ | using standard definition |
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+ | ::::<math>\textstyle\mathbb{N}_{k} = \{ n \in \mathbb{N} : n \geq k \}</math>   . . . or,  <math>\textstyle\mathbb{N}_{k} = \{k\} \bigcup \mathbb{N}_{k+1}</math>   :) :) . |
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+ | Trial division sieve is: |
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+ | |||

+ | ::::<math>\textstyle\mathbb{T} = \{n \in \mathbb{N}_{2}: (\forall p \in \mathbb{T})(2\leq p\leq \sqrt{n}\, \Rightarrow \neg{(p \mid n)})\}</math> |
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+ | |||

+ | If you're put off by self-referentiality, just replace <math>\mathbb{S}</math> or <math>\mathbb{T}</math> on the right-hand side of equations with <math>\mathbb{N}_{2}</math>, but even ancient Greeks knew better. |

## Latest revision as of 09:30, 6 August 2013

A perpetual Haskell newbie. I like *this one-liner*:

-- infinite folding idea due to Richard Bird -- double staged production idea due to Melissa O'Neill -- tree folding idea Dave Bayer / improved tree structure -- Heinrich Apfelmus / simplified formulation Will Ness primes = 2 : _Y ((3:) . gaps 5 . foldi (\(x:xs) -> (x:) . union xs) [] . map (\p-> [p*p, p*p+2*p..])) _Y g = g (_Y g) -- multistage production gaps k s@(c:t) -- == minus [k,k+2..] (c:t), k<=c, | k < c = k : gaps (k+2) s -- fused for better performance | otherwise = gaps (k+2) t -- k==c

`foldi`

is on Tree-like folds page. `union`

and more at Prime numbers.

The constructive definition of primes is the Sieve of Eratosthenes:

using standard definition

- . . . or, :) :) .

Trial division sieve is:

If you're put off by self-referentiality, just replace or on the right-hand side of equations with , but even ancient Greeks knew better.