I agree with the sentiment that things should be made as simple as possible. The sine function really has no reason to be anything besides a function. I am not sure, though, that writing the simplest thing possible results in mostly pure functions. In my experience programming is mostly about managing a state. My programming jobs have generally been about tracking what the state of some other piece of hardware and/or software is. It seems hard to escape state in that case. In my spare time I have lately been writing a compiler like thing. That seems to be, among other things, about maintaining a stack of all the stuff that has been defined/declared previously. If so many things are about maintaining a state, how practical is this 'mostly functions' thing actually?