impossible: asString :: IO String -> String here is an article which in addition to this thread clarified it for me: https://wiki.haskell.org/All_About_Monads search for: 4.3 No way out The IO monad is a familiar example of a one-way monad in Haskell. Because you can't escape from the IO monad, it is impossible to write a function that does a computation in the IO monad but whose result type does not include the IO type constructor. This means that any function whose result type does not contain the IO type constructor is guaranteed not to use the IO monad. ... There is no way to get rid of the IO type constructor in the signature of any function that uses it, so the IO type constructor acts as a kind of tag that identifies all functions that do I/O. Furthermore, such functions are only useful within the IO monad. So a one-way monad effectively creates an isolated computational domain in which the rules of a pure functional language can be relaxed. Functional computations can move into the domain, but dangerous side-effects and non-referentially-transparent functions cannot escape from it.