Conditional UInt64 subtraction bug

[Isaac-DeFrain]

This took me so long to pin down!
There seems to be a bug when evaluating conditional UInt64 subtraction in SnarkyJS 0.9.8.

Occurrences
Ordinary conditional (throws an error when condition is false)

Circuit conditional (throws errors in both cases)

Converting the condition toBoolean() fixes the ordinary conditional

but not the Circuit conditional

[Isaac-DeFrain]

I realized that I wasn't using the latest version of node in those screenshots. I have confirmed these errors still occur with the latest node v20.1.0.

[mitschabaude]

Thanks for reporting @Isaac-DeFrain! This isn't a bug. UInt64.sub() asserts that the result doesn't underflow zero, and throws when it does. Since Circuit.if is a normal function call, the else case is evaluated, there's no way around that.

So, a safe subtraction of the smaller value from the larger one needs to be implemented differently. For example, you could first determine the min and max with 2 Circuit.ifs, and then do max.sub(min).

What this definitely highlights is that we should improve the error message!

[Isaac-DeFrain]

If there's a way to report the line in the smart contract method where the error occurs, that would be like a 10x improvement! When running tests with jest, you only get the line in the test where the error occurs.