# User:WillNess

### From HaskellWiki

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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 | where | ||

− | g xs = 3 : (gaps 5 $ foldi (\(q:qs)-> (q:) . union qs) | + | g xs = 3 : (gaps 5 $ foldi (\(q:qs) -> (q:) . union qs) |

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

+ | gaps k s@(x:xs) | ||

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

+ | | True = gaps (k+2) xs -- inlined 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> | ||

<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 13:38, 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]) gaps k s@(x:xs) | k < x = k : gaps (k+2) s -- | k<=x = minus [k,k+2..] xs | True = gaps (k+2) xs -- inlined to avoid a space leak fix g = xs where xs = g xs -- global defn to avoid space leak

`foldi`

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