Running Clarity Smart Contracts u0 u1 u2 function

Question

If I run the following:

(define-data-var even-values uint u0)

(define-public (count-even (number uint)) (begin ;; increment the "event-values" variable by one. (var-set even-values (+ (var-get even-values) u1))

            ;; check if the input number is even (number mod 2 equals 0).
            (if (is-eq (mod number u2) u0)
                    (ok "the number is even")
                    (err "the number is uneven")
            )
    )

)

;; Call count-even two times. (print (count-even u4)) (print (count-even u7))

;; Will this return u1 or u2? (print (var-get even-values))

Why did it return u1 and not u2, even though the count-even function is called twice?

I tried running this in different ways, but it seems to always give u1 and not u2

Answer 1

The reason this is happening is because of the way Clarity functions behave on error. Since Clarity is a smart contract programming language, it's important that functions behave atomically, or as one unit. If we run into an error, we want to make sure that nothing that took place in the function occurs on chain.

Let's say we are running a function that updates the balance of somebody's fungible token, but something in the function fails, we need to make sure that balance wasn't updated.

Because of this, if an error is returned in Clarity, the entire function will revert. So in this case, any time you are returning the err when a number is uneven, the previous action of setting the number variable will be reverted.