什麼是單子？ - What is a monad?

問題：

Having briefly looked at Haskell recently, what would be a brief, succinct, practical explanation as to what a monad essentially is? 最近對Haskell進行了簡要介紹以後，關於monad本質上是什麼的簡要而簡潔的實際解釋是什麼？

I have found most explanations I've come across to be fairly inaccessible and lacking in practical detail. 我發現我遇到的大多數解釋都是至關難以理解的，而且缺少實際細節。

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解決方案：

參考一： https://stackoom.com/question/BhF/什麼是單子
參考二： https://oldbug.net/q/BhF/What-is-a-monad