The interface with the outside world has always been one of the weakest points of functional languages. It is not easy to incorporate I/O without being allowed to do side-effects. Furthermore, functional languages allow redexes to be evaluated in any order while I/O generally has to be performed in a very specific order. In this paper we present a new solution for the I/O problem which we have incorporated in the language Concurrent Clean. Concurrent Clean offers a linear type system called Unique Types . It makes it possible to define functions with side-effects without violating the functional semantics. Now it is possible to change any object in the world in the way we wanted: e.g. arrays can be updated in-situ, arbitrary file manipulation is possible. We have used this powerful tool among others to create a library for window based I/O. Using an explicit environment passing scheme provides a high-level and elegant functional specification method for I/O, called Event...