I used to think that Monad was some alternative realm in Haskell, a realm where purity didn't exist and everything was written in an alternative, built-in language (do-notation). That's because people tend to refer to using Monads as "working in the X-Monad", as if it is some... place. But then it turned out... Oh, so it's basically just a signature/interface/type class.
True! But one neat thing is that, if you have a black-box abstract data type then you can force people to use the provided Monad interface if they want to do stuff with it. The IO monad does exactly this to create its "alternative realm"; there is no way to get "out" of IO once you get in , the only way to run an IO computation is to have it be directly or indirectly called by "main" and the only primitive way to compose IO computations is with the monadic combinators.
