Реализовать drop используя fold

import Prelude hiding (drop,dropWhile)
 
drop :: Int -> [a] -> [a]
drop i = fst . foldl (\ (r,b) a -> if b>0 then (r,b-1) else (r++[a],0)) ([],i)
                                        
dropWhile :: (a -> Bool) -> [a] -> [a]
dropWhile p = fst . foldl (\ (r,b) a -> let b' = b && p a in 
                                        (if b' then r else r++[a],b')) ([],True)
 
main = do
    print $ drop 6 "Hello World!"
    print $ dropWhile (< 3) [1,2,3,4,5,1,2,3]

import Prelude hiding (drop,dropWhile)
 
drop :: Int -> [a] -> [a]
drop i l = fst $ foldr (\ a (r,b) -> if b>0 then (a:r,b-1) else (r,0)) ([],length l - i) l
                                        
dropWhile :: (a -> Bool) -> [a] -> [a]
dropWhile p l = drop (fst $ foldl (\ (i,b) a -> let b' = b && p a in
                                                (if b' then i+1 else i,b')) (0,True) l) l
 
main = do
    print $ drop 6 "Hello World!"
    print $ dropWhile (< 3) [1,2,3,4,5,1,2,3]

drop' :: Int -> [a] -> [a]
drop' n x = foldl (\ acc p -> if (p > n) then acc else tail acc) x [1..(length x)]
 
dropWhile' :: (a -> Bool) -> [a] -> [a]
dropWhile' p x = snd $ foldl f (True,[]) x
                 where f acc z = if (fst acc) then
                                    if (p z) then (True, []) else (False, (snd acc) ++ [z])
                                 else (False, (snd acc) ++ [z])

foldl'' :: (b -> a -> (b,Bool)) -> b -> t a -> b

import Prelude hiding (drop, dropWhile)
 
drop n xs = foldl tailor xs [1..n]
 
dropWhile f xs = foldl tailor xs $ truths f xs
 
tailor [] _ = []
tailor xs _ = tail xs
 
truths f [] = []
truths f (x:xs) = if f x then True : truths f xs else []
 
main = do
    let xs   = [1..10]
    let pd n = print $ drop n xs
    let pw f = print $ dropWhile f xs
    pd (-1)
    pd   0
    pd   1
    pd  20
    pw (<  0)
    pw (<  5)
    pw (< 15)