Debugging Guide

Miden contracts don't support traditional debugging tools like console.log or print statements. Instead, you can use assert_eq statements to check values during execution.

Using assert_eq

The assert_eq function compares two Felt values and fails if they differ:

use miden::*;

// Check if a value equals an expected value
assert_eq(actual_value, expected_value);

assert_eq is a function, not a macro. Use assert_eq(a, b) without the exclamation mark.

Debugging with Cycle Counts

When your code fails, the error output includes a cycle count indicating where execution stopped. You can use this to narrow down problems:

1. Note the cycle count when your code fails
2. Place an assert_eq before the code you suspect is failing
3. Run again and check the result:
   • If the assertion fails at an earlier cycle count: the value you're checking is wrong
   • If the assertion passes and fails at the same cycle count: the value is correct, the problem is elsewhere

Example

pub fn withdraw(&mut self, depositor: AccountId, amount: Felt) {
    let balance = self.get_balance(depositor);

    // Debug: Check if balance is what you expect
    assert_eq(balance, felt!(1000));

    // If the above passes, the problem is below this line
    // If it fails, the balance isn't what you expected

    let new_balance = balance - amount;
    self.balances.set(key, new_balance);
}

By moving the assert_eq statement around, you can isolate which value is incorrect.

Limitations

• No console.log or print debugging in contract code
• assert_eq only works with Felt values
• This is currently the primary debugging technique available