print and debug

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

let x = 42;

print(`hello${x}`);     // 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:


#![allow(unused)]
fn main() {
// Any function or closure that takes an '&str' argument can be used to override 'print'.
engine.on_print(|x| println!("hello: {}", x));

// Any function or closure that takes a '&str' and a 'Position' argument can be used to
// override 'debug'.
engine.on_debug(|x, src, pos| println!("DEBUG of {} at {:?}: {}", src.unwrap_or("unknown"), pos, 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, src, pos| log.write().unwrap().push(
                        format!("DEBUG of {} at {:?}: {}", src.unwrap_or("unknown"), pos, s)
               ));

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

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

on_debug Callback Signature

The function signature passed to Engine::on_debug takes the following form:

Fn(text: &str, source: Option<&str>, pos: Position) + 'static

where:

ParameterTypeDescription
text&strtext to display
sourceOption<&str>source of the current evaluation, if any
posPositionposition (line number and character offset) of the debug call

The source of a script evaluation is any text string provided to an AST via the AST::set_source method.

If a module is loaded via an import statement, then the source of functions defined within the module will be the module’s path.