rhai/doc/src/language/print-debug.md

print and debug

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The print and debug functions default to printing to stdout, with debug using standard debug formatting.

print("hello");         // prints hello to stdout

print(1 + 2 + 3);       // prints 6 to stdout

print("hello" + 42);    // prints hello42 to stdout

debug("world!");        // prints "world!" to stdout using debug formatting

Override print and debug with Callback Functions

When embedding Rhai into an application, it is usually necessary to trap print and debug output (for logging into a tracking log, for example) with the Engine::on_print and Engine::on_debug methods:

// Any function or closure that takes an '&str' argument can be used to override
// 'print' and 'debug'
engine.on_print(|x| println!("hello: {}", x));
engine.on_debug(|x| println!("DEBUG: {}", x));

// Example: quick-'n-dirty logging
let logbook = Arc::new(RwLock::new(Vec::<String>::new()));

// Redirect print/debug output to 'log'
let log = logbook.clone();
engine.on_print(move |s| log.write().unwrap().push(format!("entry: {}", s)));

let log = logbook.clone();
engine.on_debug(move |s| log.write().unwrap().push(format!("DEBUG: {}", s)));

// Evaluate script
engine.eval::<()>(script)?;

// 'logbook' captures all the 'print' and 'debug' output
for entry in logbook.read().unwrap().iter() {
    println!("{}", entry);
}