"Pure" FP languages eliminate state. All programs contain an (implicit or explicit) model of the domain they are computing on. Many aspects of the world (domain

"Pure" FP languages eliminate state. All programs contain an (implicit or explicit) model of the domain they are computing on. Many aspects of the world (domains) that we need to model are stateful. Our programs, in modeling those domains, often must be stateful. This is why it's hard for programs to be entirely functional. On the other hand, state dramatically complicates reasoning about code, since outputs are no longer purely a function of inputs. Moreover, even with lexical scoping, it's not immediately clear what the state variables are a function of. As the scope of state variables increases, it becomes ever harder to determine what a state variable is a function of. This is why programs should be as functional as possible. Solution: a language like Clojure or Common Lisp, which naturally encourages a functional approach while allowing side effects when neeeded. Clojure has some advantages over CL in this regard, since it has several clever mechanisms such as agents and software transactional memory to control side effects.