rhai/doc/src/rust/override.md

Override a Built-in Function

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Any similarly-named function defined in a script overrides any built-in or registered native Rust function of the same name and number of parameters.

// Override the built-in function 'to_int'
fn to_int(num) {
    print("Ha! Gotcha! " + num);
}

let x = (123).to_int();

print(x);               // what happens?

A registered native Rust function, in turn, overrides any built-in function of the same name, number and types of parameters.