If the first parameter of a function is of type rhai::NativeCallContext, then it is treated specially by the Engine.

NativeCallContext is a type that encapsulates the current native call context and exposes the following:

engine()&Enginethe current Engine, with all configurations and settings.
This is sometimes useful for calling a script-defined function within the same evaluation context using Engine::call_fn, or calling a function pointer.
fn_name()&strname of the function called (useful when the same Rust function is mapped to multiple Rhai-callable function names)
source()Option<&str>reference to the current source, if any
iter_imports()impl Iterator<Item = (&str, &Module)>iterator of the current stack of modules imported via import statements
imports()&Importsreference to the current stack of modules imported via import statements; requires the internals feature
iter_namespaces()impl Iterator<Item = &Module>iterator of the namespaces (as modules) containing all script-defined functions
namespaces()&[&Module]reference to the namespaces (as modules) containing all script-defined functions; requires the internals feature
call_fn_dynamic_raw()Result<Dynamic, Box<EvalAltResult>>call a native Rust function with the supplied arguments; this is an advanced method

Implement Safety Checks with NativeCallContext

The native call context is useful for protecting a function from malicious scripts.

fn main() {
use rhai::{Array, NativeCallContext, EvalAltResult, Position};

// This function builds an array of arbitrary size, but is protected
// against attacks by first checking with the allowed limit set
// into the 'Engine'.
pub fn grow(context: NativeCallContext, size: i64)
                            -> Result<Array, Box<EvalAltResult>>
    // Make sure the function does not generate a
    // data structure larger than the allowed limit
    // for the Engine!
    if size as usize > context.engine().max_array_size()
        return EvalAltResult::ErrorDataTooLarge(
            "Size to grow".to_string(),
            size as usize,

    let array = Array::new();

    for x in 0..size {


Implement a Callback

The native call context can be used to call a function pointer or closure that has been passed as a parameter to the function, thereby implementing a callback:

fn main() {
use rhai::{Dynamic, FnPtr, NativeCallContext, EvalAltResult};

pub fn greet(context: NativeCallContext, callback: FnPtr)
                            -> Result<String, Box<EvalAltResult>>
    // Call the callback closure with the current context
    // to obtain the name to greet!
    let name = callback.call_dynamic(&context, None, [])?;
    Ok(format!("hello, {}!", name))