Why is `join` also known as `mu`?

Question

I noticed in the Idris documentation that join is also known as flatten and mu.

Idris> :doc join
Prelude.Monad.join : Monad m => m (m a) -> m a
    Also called flatten or mu

    The function is Total

IIRC, mu (or μ) is used as a binder for recursive data types. I've not seen mu in this context before. What's the background on that?

Answer 1

答案非常简短：类别理论。

中长答案： 如果你看一下维基百科上的formal definition of a Monad（对不起，因为没有LaTeX支持，所以不要复制它），你会看到＆＃34; mu＆＃34;和＆＃34; eta＆＃34;用作两个＆＃34; natural transformations＆＃34;，其中mu是从T2到T的那个（如果你看一下类型签名是有意义的，它从（m (m a)到{ {1}}）。

深入回答：Monads Made Difficult（参见＆＃34;自然变换＆＃34;）