折叠和折叠进一步的解释和例子 [英] foldr and foldl further explanations and examples

问题描述

我查看了不同的页面和一般折叠以及其他一些内容，并且它们很好地解释了它。

I've looked at different folds and folding in general as well as a few others and they explain it fairly well.

在这种情况下，lambda的工作方式仍然存在问题。

I'm still having trouble on how a lambda would work in this case.

foldr (\y ys -> ys ++ [y]) [] [1,2,3]

有人可以一步一步地尝试向我解释这一点。

Could someone go through that step by step and try to explain that to me.

此外， foldl 也能如何工作。

And also how would foldl work as well.

推荐答案

foldr f z []     = z
foldr f z (x:xs) = x `f` foldr f z xs

And

k y ys = ys ++ [y]

：

Let's unpack:

foldr k [] [1,2,3]
= k 1 (foldr k [] [2,3]
= k 1 (k 2 (foldr k [] [3]))
= k 1 (k 2 (k 3 (foldr k [] [])))
= (k 2 (k 3 (foldr k [] []))) ++ [1]
= ((k 3 (foldr k [] [])) ++ [2]) ++ [1]
= (((foldr k [] []) ++ [3]) ++ [2]) ++ [1]
= ((([]) ++ [3]) ++ [2]) ++ [1]
= (([3]) ++ [2]) ++ [1]
= ([3,2]) ++ [1]
= [3,2,1]

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