section7-2.hs

all' :: (a -> Bool) -> [a] -> Bool
all' _ [] = True
all' f (x:xs) = if f x then all' f xs else False

any' :: (a -> Bool) -> [a] -> Bool
any' _ [] = False
any' f (x:xs) = if f x then True else any' f xs

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

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