Of course, most of useful software involves input and output and state. Functional programming doesn't preclude that. There are compilers, package managers, documentation systems, etc, written in Haskell. They all use files and databases. Of course, your programs have to be designed differently.

For reference, here's is a talk by Don Stewart et. al about the design of XMonad:

http://www.scribd.com/doc/19503176/The-Design-and-Implementa...

I guess if one can write a window manager in Haskell, one can also write a todo list.