Object Oriented Programming - Side Effect Free Programming

I was thinking of an example of how to kind of visualize declarative programming and like you said you are using your functions. Most of the time functional programming languages have this function which is called map and map the name it's already there. I just kind of think of that like oh that's making a declaration and we do that a lot in object oriented programming but probably we can't count on the what happens inside the method that where we get that value to assign to be completely side effect free so that brings up the subject of side effect free behavior as well.