It's not a monad, but what is it?

这不是一个单子，但它是什么？

But it's just one monad.

但这只是一个单子。

As always, the IO monad is special and difficult to reason about.

一如既往，IO单子是特别困难的原因有关。

We haven't used the state monad, but there are lots of other useful monads.

我们没有使用过state monad，但是用过许多其他很有用的monads。

Is there a monad that doesn't have a corresponding monad transformer (except IO)?

是有一个单子，没有相应的单子转换器(除IO) ？