-- 目前有问题，因为我不熟悉 Haskell 这点 guard 语法模式匹配问题
fold'' :: (ac -> a -> ac) -> ac -> [a] -> ac
fold'' f v l           
   | let [] = l = v
   | let (x : xs) = l = f (fold'' f v xs) x

-- 这点和 map、Functor、Monad、monoid 什么的理论无关
fold' :: (ac -> a -> ac) -> ac -> [a] -> ac
fold' f v [] = v
fold' f v (x:xs) = f (fold' f v xs) x

sumOf = fold' (+) 0
main = print $ sumOf [0, 1, 3, 4]

fold' :: (ac -> a -> ac) -> ac -> [a] -> ac
fold' f v [] = v
fold' f v (x:xs) = f (fold' f v xs) x

sumOf = fold' (+) 0
main = print $ sumOf [0, 1, 3, 4]