"This guide explored the theoretical backgrounds of Haskell’s IO monad. On the way we have seen functors, natural transformations, and finally monads. All these purely theoretic concepts appeared to have a quite practical correspondence in the Haskell programming language, as emphasized by the according examples. It should be clear now, how the IO monad is used to pass around the state of the world, without allowing the programmer to access it “too much”.
"This guide explored the theoretical backgrounds of Haskell’s IO monad. On the way we have seen functors, natural transformations, and finally monads. All these purely theoretic concepts appeared to have a quite practical correspondence in the Haskell programming language, as emphasized by the according examples. It should be clear now, how the IO monad is used to pass around the state of the world, without allowing the programmer to access it “too much”.
http://stefan-klinger.de/files/monadGuide.pdf