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Ethereum: Why is my phantom variable broken when it has an axiom?

As any developer familiar with the Ethereum ecosystem knows, variables can sometimes behave in unexpected ways when used as phantom variables. In this article, we’ll explore why your listingUpdatesCount variable may be broken (or “broken”) even though it should have a value of 0.

What is a phantom variable?

In the context of Ethereum programming, a phantom variable is a variable that exists in memory but does not affect the actual state of the blockchain. These variables are often used as a placeholder or to provide temporary values ​​​​while a more permanent solution is implemented.

The init_state axiom

When you use a phantom variable within a smart contract, it is essential to ensure that its value complies with the init_state axiom. This axiom states that any initial state of a variable must have a corresponding initial value for that variable when using a phantom variable.

In your case, you have the following line:

ghost mathint listingUpdatesCount {
init_state axiom listingUpdatesCount == 0;
}

However, as we discussed above, this will not work correctly due to the use of a non-numeric type (listingUpdatesCount) instead of an initial value.

The problem with non-numeric types

In Solidity, types can be numeric (such as uint or int) or non-numeric. When you try to assign a non-numeric value to a phantom variable, Ethereum will treat it as if the value were a numeric type. As a result, any subsequent operations performed on the phantom variable may have unpredictable behavior.

The consequences of altered values

When your listingUpdatesCount phantom variable is attacked, you may encounter unexpected behavior in your contract, including:

• Unwanted state changes: Changes to the value of your phantom variable can cause unintended state changes within your contract.

• Logical errors

  : Without a proper understanding of how your contract behaves with non-numeric values, you may result in logical errors or bugs that are difficult to track down.

Suggested solutions

To resolve this issue, you need to ensure that your listingUpdatesCount phantom variable is replaced with an actual numeric value. Here’s what you can do:

• Replace the phantom variable: Update your contract to use a numeric type for your phantom variable.

uint listingUpdatesCount {
init_state axiom listingUpdatesCount == 0;
}

• Use a numeric initialization approach: Instead of using init_state, consider initializing your phantom variable directly with a value, like this:

uint listingUpdatesCount = 0; // or any other valid numeric initial value
// More on that later...
listingUpdatesCount = 1; // or any other desired value

By following these steps, you should be able to resolve the havoc’d values ​​​issue and ensure that your phantom variable behaves as expected.

Let me know if you have any further questions about this article!