我正在撰写有关折叠的博客文章/论文。大多数情况下，我坚持fold : (a -> b -> b) -> b -> [a] -> b签名，但折叠左边也可以。
我需要的是尽可能多的例子，其中fold用于实现众所周知的常用位，例如：
sum = foldr (+) 0
product = foldr (*) 1
any = foldr (||) False
all = foldr (&&) True
(++ ys) = foldr (:) ys
length = foldr (\_ n -> n + 1) 0
reverse = foldr (\x xs -> xs ++ [x]) []
map f = foldr (\x xs -> f x : xs) []
filter p = foldr (\x xs -> if p x then x : xs else xs) []
concat = foldr (++) []
compose = foldr (.) id
上面的所有示例都是在Haskell中编写和测试的，但我可以接受任何语言，包括OO-one。
提前致谢。
更新：
foldl f a xs = foldr (\x k a -> k (f a x)) id xs a
factorial n = foldr (*) 1 [1..n]