maximum' :: (Ord a) => [a] -> a
maximum' [] = error "empty list"
maximum' [x] = x
maximum' (x:xs) = max x (maximum' xs)

replicate' :: Int -> a -> [a]
replicate' n x
    | n <= 0 = []
    | otherwise = x : replicate' (n-1) x

take' :: (Num i, Ord i) => i -> [a] -> [a]
take' n _
    | n <= 0 = []
take' _ [] = []
take' n (x:xs) = x : take' (n-1) xs

reverse' :: [a] -> [a]
reverse' [] = []
reverse' (x:xs) = reverse' xs ++ [x]

repeat' :: a -> [a]
repeat' x = x : repeat' x
-- take 5 (repeat' 3)

zip' :: [a] -> [b] -> [(a,b)]
zip' _ [] = []
zip' [] _ = []
zip' (x:xs) (y:ys) = (x,y) : zip' xs ys

elem' :: (Eq a) => a -> [a] -> Bool
elem' a [] = False
elem' a (x:xs)
    | a == x = True
    | otherwise = elem' a xs

quicksort :: (Ord a) => [a] -> [a]
quicksort [] = []
quicksort (x:xs) =
    let smallerOrEqual = [a | a <- xs, a <= x]
        larger = [a | a <- xs, a > x]
    in quicksort smallerOrEqual ++ [x] ++ quicksort larger

quicksort "the quick brown fox jumps over the lazy dog"
-- "        abcdeeefghhijklmnoooopqrrsttuuvwxyz"