# User:WillNess

### From HaskellWiki

(Difference between revisions)

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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 (\q:qs -> (q:) . union qs) | |

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

− | + | ||

− | + | ||

− | + | ||

− | + | ||

− | + | ||

fix g = xs where xs = g xs | fix g = xs where xs = g xs | ||

+ | |||

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

+ | then k : gaps (k+2) s -- inlined to avoid a space leak | ||

+ | else gaps (k+2) xs | ||

</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]]. More at [[Prime numbers#Sieve_of_Eratosthenes|Prime numbers]]. |

## Revision as of 12:58, 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 (\q:qs -> (q:) . union qs) [[p*p, p*p+2*p..] | p <- xs]) fix g = xs where xs = g xs gaps k s@(x:xs) = if k < x -- | k<=x = minus [k,k+2..] xs then k : gaps (k+2) s -- inlined to avoid a space leak else gaps (k+2) xs

`foldi`

is on Tree-like folds. More at Prime numbers.