On 8/9/07, Chad Scherrer <chad.scherrer@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
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