async<T> static method

Future<Option<T>> async<T>(_OptionAsyncEarlyReturnFunction<T> fn)

Creates a context for async early return, similar to "Do notation". Works like the Rust "?" operator, which is a "Early Return Operator". Here "$" is used as the "Early Return Key". when "$" is used on a type _None, immediately the context that "$" belongs to is returned with None(). e.g.

FutureOption<int> intNone() async => const None();

FutureOption<int> earlyReturn(int val) => Option.async(($) async {
 int x = await intNone()[$]; // returns [None] immediately
 return Some(x + 3);
});
expect(await earlyReturn(2), const None());

This should be used at the top level of a function as above. Passing "$" to any other functions, nesting, or attempting to bring "$" out of the original scope should be avoided.

Implementation

@pragma("vm:prefer-inline")
static Future<Option<T>> async<T>(
    _OptionAsyncEarlyReturnFunction<T> fn) async {
  try {
    return await fn(const _OptionEarlyReturnKey._());
  } on _OptionEarlyReturnNotification catch (_) {
    return Future.value(None);
  }
}