什么是单子？ - What is a monad?

问题：

Having briefly looked at Haskell recently, what would be a brief, succinct, practical explanation as to what a monad essentially is? 最近对Haskell进行了简要介绍以后，关于monad本质上是什么的简要而简洁的实际解释是什么？

I have found most explanations I've come across to be fairly inaccessible and lacking in practical detail. 我发现我遇到的大多数解释都是至关难以理解的，而且缺少实际细节。

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解决方案：

参考一： https://stackoom.com/question/BhF/什么是单子
参考二： https://oldbug.net/q/BhF/What-is-a-monad