Mathematical purity is indeed what allows you to treat your logic like bricks and building blocks. My point is that "a monad is a monoid in the category of endofunctors" is not the only "pure math" out there, and also that your domain model is likely the most impure part of your whole program. Functional programming is awesome! But mostly ignores the existence of essential mutable state, and embracing functional programming is only a small part of the "mathematical purity" struggle that is indeed the only way to build truly reusable building blocks. If you're spending lots of time building clean, pure, mathematical data manipulation logic, but the data you're manipulating is "Dog : Animal" and "Cat : Animal", you're in a garbage in/garbage out situation. Worry about the mathematical purity of your data model itself. It will marry and dance and harmonize with the purity of your functional logic!

> Mutating a variable is less resource intensive then generating a new variable.

Not always.