rhaj/rhai_engine/rhaibook/language/arrays-oob.md

Out-of-Bounds Index for Arrays

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This is an advanced feature that requires the [`internals`] feature to be enabled.

Normally, when an index is out-of-bounds for an [array], an error is raised.

It is possible to completely control this behavior via a special callback function registered into an [Engine] via on_invalid_array_index.

Using this callback, for instance, it is simple to instruct Rhai to extend the [array] to accommodate this new element, or to return a default value instead of raising an error.

Function Signature

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

Fn(array: &mut Array, index: i64, context: EvalContext) -> Result<Target, Box<EvalAltResult>>

where:

Parameter	Type	Description
array	[&mut Array][array]	the [array] being accessed
index	i64	index value
context	[EvalContext]	the current evaluation context

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 [array] or a new temporary value.

Example

engine.on_invalid_array_index(|arr, index, _| {
    match index {
        -100 => {
            // The array can be modified in place
            arr.push((42_i64).into());

            // Return a mutable reference to an element
            let value_ref = arr.last_mut().unwrap();
            Ok(value_ref.into())
        }
        100 => {
            // Return a temporary value (not a reference)
            let value = Dynamic::from(100_i64);
            Ok(value.into())
        }
        // Return the standard out-of-bounds error
        _ => Err(EvalAltResult::ErrorArrayBounds(
                arr.len(), index, Position::NONE
             ).into()),
    }
});