• The ListMap type
• Indexing into instances

You can add indexing support to a user-defined type by implementing $get() and/or $set() methods. These methods let you index into instances of the type using instance[index] syntax, just like the builtin map and vec types.

As an example, let's build a simple ListMap type that maps keys to lists of values. If we try to look up a key that isn't in the map, we'll get back an empty list.

The ListMap type

Here's the full code for our ListMap:

class ListMap {
    var data;

    def $init() {
        self.data = {};
    }

    def $get(key) {
        if key in self.data {
            return self.data[key];
        }
        return [];
    }

    def $set(key, value) {
        if key in self.data {
            self.data[key]:append(value);
            return;
        }
        self.data[key] = [value];
    }
}

• Implementing the $get(key) method adds support for index-style read access to instances.

• Implementing the $set(key, value) method adds support for index-style write access to instances.

Indexing into instances

We can now read from and write to instances of our custom ListMap type using [], just like the builtin map and vec types.

If we try to look up a key that isn't in the map, we get back an empty vec, e.g.

var map = ListMap();

assert $is_vec(map["foo"]);
assert map["foo"]:is_empty();

If we write a new key to the map, the value is added to a new list, e.g.

map["foo"] = 123;

assert $is_vec(map["foo"]);
assert map["foo"]:count() == 1;
assert map["foo"][0] == 123;

If we write more values to the map using the same key, the values are appended to the list, e.g.

map["foo"] = 456;

assert $is_vec(map["foo"]);
assert map["foo"]:count() == 2;
assert map["foo"][0] == 123;
assert map["foo"][1] == 456;