Custom Type Indexers

A custom type can also expose an indexer by registering an indexer function.

A custom type with an indexer function defined can use the bracket notation to get/set a property value at a particular index:

object [ index ]

object [ index ] = value ;

Like property getters/setters, indexers take a &mut reference to the first parameter.

They also take an additional parameter of any type that serves as the index within brackets.

Indexers are disabled when the no_index and no_object features are used together.

Cannot Override Arrays, BLOB’s, Object Maps, Strings and Integers

For efficiency reasons, indexers cannot be used to overload (i.e. override) built-in indexing operations for arrays, object maps, strings and integers (acting as bit-field operation).

The following types have built-in indexer implementations that are fast and efficient.

Examples

#[derive(Debug, Clone)]
struct TestStruct {
    fields: Vec<i64>
}

impl TestStruct {
    // Remember &mut must be used even for getters
    fn get_field(&mut self, index: String) -> i64 {
        self.fields[index.len()]
    }
    fn set_field(&mut self, index: String, value: i64) {
        self.fields[index.len()] = value
    }

    fn new() -> Self {
        Self { fields: vec![1, 2, 3, 4, 5] }
    }
}

let mut engine = Engine::new();

engine.register_type::<TestStruct>()
      .register_fn("new_ts", TestStruct::new)
      // Short-hand: .register_indexer_get_set(TestStruct::get_field, TestStruct::set_field);
      .register_indexer_get(TestStruct::get_field)
      .register_indexer_set(TestStruct::set_field);

let result = engine.eval::<i64>(
r#"
    let a = new_ts();
    a["xyz"] = 42;                  // these indexers use strings
    a["xyz"]                        // as the index type
"#)?;

println!("Answer: {}", result);     // prints 42

Convention for Negative Index

If the indexer takes a signed integer as an index (e.g. the standard INT type), care should be taken to handle negative values passed as the index.

It is a standard API convention for Rhai to assume that an index position counts backwards from the end if it is negative.

-1 as an index usually refers to the last item, -2 the second to last item, and so on.

Therefore, negative index values go from -1 (last item) to -length (first item).

A typical implementation for negative index values is:

// The following assumes:
//   'index' is 'INT', 'items: usize' is the number of elements
let actual_index = if index < 0 {
    index.checked_abs().map_or(0, |n| items - (n as usize).min(items))
} else {
    index as usize
};

The end of a data type can be interpreted creatively. For example, in an integer used as a bit-field, the start is the least-significant-bit (LSB) while the end is the most-significant-bit (MSB).

Convention for Range Index

It is very common for ranges to be used as indexer parameters via the types std::ops::Range<INT> (exclusive) and std::ops::RangeInclusive<INT> (inclusive).

One complication is that two versions of the same indexer must be defined to support exclusive and inclusive ranges respectively.

use std::ops::{Range, RangeInclusive};

let mut engine = Engine::new();

engine
    /// Version of indexer that accepts an exclusive range
    .register_indexer_get_set(
        |obj: &mut TestStruct, range: Range<i64>| -> bool { ... },
        |obj: &mut TestStruct, range: Range<i64>, value: bool| { ... },
    )
    /// Version of indexer that accepts an inclusive range
    .register_indexer_get_set(
        |obj: &mut TestStruct, range: RangeInclusive<i64>| -> bool { ... },
        |obj: &mut TestStruct, range: RangeInclusive<i64>, value: bool| { ... },
    );

engine.run(
"
    let obj = new_ts();

    let x = obj[0..12];             // use exclusive range

    obj[0..=11] = !x;               // use inclusive range
")?;

Indexer as Property Access Fallback

An indexer taking a string index is a special case – it acts as a fallback to property getters/setters.

During a property access, if the appropriate property getter/setter is not defined, an indexer is called and passed the string name of the property.

This is also extremely useful as a short-hand for indexers, when the string keys conform to property name syntax.

// Assume 'obj' has an indexer defined with string parameters...

// Let's create a new key...
obj.hello_world = 42;

// The above is equivalent to this:
obj["hello_world"] = 42;

// You can write this...
let x = obj["hello_world"];

// but it is easier with this...
let x = obj.hello_world;

Caveat – reverse is NOT true

The reverse, however, is not true – when an indexer fails or doesn’t exist, the corresponding property getter/setter, if any, is not called.

type MyType = HashMap<String, i64>;

let mut engine = Engine::new();

// Define custom type, property getter and string indexers
engine.register_type::<MyType>()
      .register_fn("new_ts", || {
          let mut obj = MyType::new();
          obj.insert("foo".to_string(), 1);
          obj.insert("bar".to_string(), 42);
          obj.insert("baz".to_string(), 123);
          obj
      })
      // Property 'hello'
      .register_get("hello", |obj: &mut MyType| obj.len() as i64)
      // Index getter/setter
      .register_indexer_get_result(|obj: &mut MyType, prop: &str|
          obj.get(index).cloned().ok_or_else(|| "not found".into())
      ).register_indexer_set(|obj: &mut MyType, prop: &str, value: i64|
          obj.insert(prop.to_string(), value)
      );

engine.run("let ts = new_ts(); print(ts.foo);");
//                                   ^^^^^^
//                 Calls ts["foo"] - getter for 'foo' does not exist

engine.run("let ts = new_ts(); print(ts.bar);");
//                                   ^^^^^^
//                 Calls ts["bar"] - getter for 'bar' does not exist

engine.run("let ts = new_ts(); ts.baz = 999;");
//                             ^^^^^^^^^^^^
//                 Calls ts["baz"] = 999 - setter for 'baz' does not exist

engine.run(r#"let ts = new_ts(); print(ts["hello"]);"#);
//                                     ^^^^^^^^^^^
//                 Error: Property getter for 'hello' not a fallback for indexer