You have some deeply flawed (or entirely absent) philosophical upbringing. There are no "foundations" to anything - all definitions are arbitrary.
To assert that a premise is fallacious mandates that you have some prior notion of "fallaciousness".
You don't have an objective criterion for asserting whether one definition is better than another because you don't have a notion of "betterness" - it's all conventional.
It's precisely because I am an over-zealous formalist is why I see Mathematics for what it is - grammar, syntax and semantics.
It's just language.
Re mutating getter:
Monads for example are a standard way of formalising state in a pure-function universe. You seem to have some fundamental misconception about what "mathematics" is, since you keep repeating something about purity. Mathematics is merely the rigorous study of formal systems, of which your mystical "mutating getter" is one such system.