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.