Well, clearly in a language that doesn't make a distinction between functions that have side effects and functions that don't you'll just have to pinky swear yo

Well, clearly in a language that doesn't make a distinction between functions that have side effects and functions that don't you'll just have to pinky swear you won't put any in your functions, and maybe try to wrap the side effects the built-in functions do have (like errno in C) with your own functions. But that's not the point; a functional style can be achieved in any language as long as it has functions.