String array access in typedef (Haxe)

package ;

abstract StringKey({}) from {} {
    @:extern @:arrayAccess public inline function arrayAccess(key:String):Dynamic {
        return Reflect.field(this, key);
    }
    @:extern @:arrayAccess public inline function arrayWrite<T>(key:String, value:T):T {
        Reflect.setField(this, key, value);
        return value;
    }
}

typedef Some = {
    sk: StringKey,
}

class Main {

    static function main() {
        var a:Some = {
            sk: {
                a: 4,
                b: 3
            }
        }
        trace(a);
        trace(a.sk["a"]);
    }

}