On 8/9/07, Chad Scherrer
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
Finally, even if no one else is using it, it would be good to settle
on reasonable names for things more easily. Is there a better name for this function? Is there a reason not to call it "extract"?
Other possible names which occur to me include select, slice, mask. I think I like 'select' best myself, but 'extract' works too. Amusingly, extract is intimately related to function composition. Suppose we have listify :: (Int -> Int) -> [Int] listify = flip map [0..] Then if f, g :: Int -> Int, and f is monotonically increasing, we have the identity (listify f) `extract` (listify g) = listify (g . f) This randomly occurred to me as I was falling asleep last night and I thought I would share. =) -Brent