# Track Progress and Force-Termination

It is impossible to know when, or even whether, a script run will end (a.k.a. the Halting Problem).

When dealing with third-party untrusted scripts that may be malicious, to track evaluation progress and to force-terminate a script prematurely (for any reason), provide a closure to the Engine via the Engine::on_progress method:


#![allow(unused)]
fn main() {
let mut engine = Engine::new();

engine.on_progress(|count| {    // parameter is number of operations already performed
if count % 1000 == 0 {
println!("{}", count);  // print out a progress log every 1,000 operations
}
None                        // return 'None' to continue running the script
// return 'Some(token)' to immediately terminate the script
});
}


The closure passed to Engine::on_progress will be called once for every operation. Return Some(token) to terminate the script immediately, with the provided value (any Dynamic) acting as a termination token.

## Termination Token

The Dynamic value returned by the closure for Engine::on_progress is a termination token. A script that is manually terminated returns with Err(EvalAltResult::ErrorTerminated) wrapping this value.

The termination token is commonly used to provide information on the reason or source behind the termination decision.

If the termination token is not needed, simply return Some(Dynamic::UNIT) to terminate the script run with () as the token.

## Operations Count vs. Progress Percentage

Notice that the operations count value passed into the closure does not indicate the percentage of work already done by the script (and thus it is not real progress tracking), because it is impossible to determine how long a script may run.

It is possible, however, to calculate this percentage based on an estimated total number of operations for a typical run.