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

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


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

reverseH :: [a] -> [a] -> [a]
reverseH [] ys = ys
reverseH (x:xs) ys = reverseH xs (x:ys)

reverse'' :: [a] -> [a]
reverse'' l = reverseH l []

-- Anonymous functions
-- '\' is for \lambda, coming from lambda calculus
-- \ x -> x + 1

-- \ x -> (\ y -> x + y)

-- \ x y -> x + y


-- Higher order functions

map' :: (a -> b) -> [a] -> [b]
map' f [] = []
map' f (x:xs) = f x : map' f xs

-- filter
filter' :: (a -> Bool) -> [a] -> [a]
filter' f [] = []
filter' f (x:xs) =
  if f x then x : filter' f xs
  else filter' f xs

zipWith' :: (a -> b -> c) -> [a] -> [b] -> [c]
zipWith' f [] y = []
zipWith' f (x:xs) [] = []
zipWith' f (x:xs) (y:ys) =
  f x y : zipWith' f xs ys