The separation of pure and impure code in Haskell isn't something inherent to monads but is instead a side-effect (hah) of the typing rules of the language. It's easy to see how this separation need not hold when using monads (look at any monad library in virtually any other language), but more importantly there are ways to accomplish this separation without the use of monads (an effect-tracking type system is being developed along side OCaml's algebraic effects/multicore project).