Non-Existent Property Handling for Object Maps

Normally, when a property is accessed from an object map that does not exist, () is returned. Via Engine:: set_fail_on_invalid_map_property, it is possible to make this an error instead.

Other than that, it is possible to completely control this behavior via a special callback function registered into an Engine via on_map_missing_property.

Using this callback, for instance, it is simple to instruct Rhai to create a new property in the object map on the fly, possibly with a default value, when a non-existent property is accessed.

Function Signature

The function signature passed to Engine::on_map_missing_property takes the following form.

Fn(map: &mut Map, prop: &str, context: EvalContext) -> Result<Target, Box<EvalAltResult>>

where:

Return value

The return value is Result<Target, Box<EvalAltResult>>.

Target is an advanced type, available only under the internals feature, that represents a reference to a Dynamic value.

It can be used to point to a particular value within the object map.

Example

engine.on_map_missing_property(|map, prop, context| {
    match prop {
        "x" => {
            // The object-map can be modified in place
            map.insert("y".into(), (42_i64).into());

            // Return a mutable reference to an element
            let value_ref = map.get_mut("y").unwrap();
            Ok(value_ref.into())
        }
        "z" => {
            // Return a temporary value (not a reference)
            let value = Dynamic::from(100_i64);
            Ok(value.into())
        }
        // Return the standard property-not-found error
        _ => Err(EvalAltResult::ErrorPropertyNotFound(
                prop.to_string(), Position::NONE
             ).into()),
    }
});