# User:WillNess

From HaskellWiki

I'm 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. `union`

and more at Prime numbers.