Categories for the Working Mathematician
《Categories for the Working Mathematician》 · 第 137 页 · Chapter VI, Section 1, Definition
A monad \(T=(T,\eta,\mu)\) in a category consists of an endofunctor \(T\) and natural transformations \(\eta:1\to T\), \(\mu:T^2\to T\) satisfying the associativity and unit laws.