# User:WillNess

### From HaskellWiki

(Difference between revisions)

Line 5: | Line 5: | ||

<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 |
+ | where |

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

− | -- inlined to avoid a space leak |
||

− | = if k < x |
||

− | then k : gaps (k+2) s |
||

− | else gaps (k+2) 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> |
||

## 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.