class foo{ enum current_operator;
init() { current_operator = ADD }
evalOp(int a, int b){
if (current_operator == ADD) return a + b;
else if (current_operator == MULTIPLY) return a * b;
}
setOp(enum op){ current_operator = op; }// pure impl
class bar{
evalOp(enum op, int a, int b){
if (op == ADD) return a + b;
else if (op == MULTIPLY) return a * b;
}In the first case, evalOp is not pure since one has no idea on what is the current_operator set as. In the second case it is pure since the operator is part of the input.
Purity refers to reproducibility, as in no matter what else is going on, that function will always behave the same. You can have pure functions in OOP based languages, but in practice devs just default to managing state and having weird behavior depending on outside state of a given function.
As a side note, and since you mention runtime performance cost: if you implement a dummy generic cache layer and you have pure functions you will have an easier time than having stateful impls. In this case, that cache layer must have the ability to introspect into foo, but in bar that cache layer just keeps track of input and that's it.
Pure impl v2:
class foo {
public:
//enum is a keyword, so let's call it _enum
const _enum current_operator;
foo(_enum co) : current_operator(co) { }
// ... evalOp() as in your stateful impl example ...
};
//Example use:
Reduce(foo{ADD}, 1, 2, 3, 4, 5, 6);
There are multiple definitions of idempotence, used even within computer science, and neither of you seem to be aware of the definition used by the other. The common mathematical definition of an idempotent function is that f(f(x)) = f(x) for all x. But in computer science there is another common definition which involves side effects not being repeated (that is, `f(); f();` is the same as `f();`).
where have you ever seen that definition ?!
https://en.wikipedia.org/wiki/Idempotence#Computer_science_m...
All pure functions are by definition idempotent, and this quality is often celebrated as yet another point of merit. What is the point of highlighting this quality if functional programmers do not expect to over-execute them?
> It's meant to give you a tool to better reason about the program logically. You don't have to worry about _when_ a pure function is called…
If you say you don’t have to worry about when a function is called, the implication is that you don’t have to worry about how many times that function has been called. Functional programmers tend to belittle this sort of reasoning, and yet this sort of reasoning continues to be necessary when evaluating long-running, interactive programs for correctness.
This downplaying of executional reasoning is precisely why even if using pure functions on their own doesn’t encourage overexecution, the systems which functional programmers build typically do. We see this most clearly in discussions of reactivity, the fetishization of spreadsheets as programming model. Discrete events are tranformed into discrete outputs in a many-to-many relationship and yet functional programmers continue to think it’s not important to reason about the code which executed in between, the when and how much, just because it’s a composition of pure functions.