[Haskell-cafe] where to put handy functions?

[Brent Yorgey]

On 8/9/07, Chad Scherrer <chad.scherrer at gmail.com> wrote:
>
>
> extract :: [Int] -> [a] -> [a]
> extract = f 0
>     where
>     f _ _ [] = []
>     f _ [] _ = []
>     f k nss@(n:ns) (x:xs) = if n == k then x:f (k+1) ns xs
>                             else f (k+1) nss xs
>
> This behaves roughly as
> extract ns xs == map (xs !!) ns
>
> except that it's a lot more efficient, and it still works if ns or xs
> (but not both) are infinite. Oh, and "ns" are required to be ordered
> and non-negative.


Nifty function there. =)  And for the record, it works just fine if both
lists are infinite -- it just produces an infinite output list, but it's
lazy so no problem:

*Main> take 10 $ extract [1,3..] [2..]
[3,5,7,9,11,13,15,17,19,21]


-Brent
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