On 8/9/07, Chad Scherrer <[EMAIL PROTECTED]> 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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