Return type of void async function in Dart

Question

In Dart, if you have an async function that doesn't return anything, should it return Future<void> or simply void? Both seem to work, but why?

void foo() async {
  print('foo');
}

Future<void> bar() async {
  print('bar');
}

void main() async {
  await foo();
  await bar();
  print('baz');
}

Compiles with no errors or warnings and prints

foo
bar
baz

Answer 1

In your code, both functions foo() and bar() are not returning anything so any function return type is valid for both functions eg:

void/T/Future<T>... foo() async {
  print('foo');
}

Future<void>/T/Future<T>... bar() async {
  print('bar');
}

and here,

await foo();
await bar();

await just waits till the execution of these async functions is complete. As there is no return type, await has no purpose here(redundant) so this is and should not be a compilation error.

---

The difference is that Future<void> gives you information of the execution of the function like when the execution is complete and it also allows you to specify what to do when the function is executed by using bar().then(...) and bar().whenComplete(...).

While foo() function return void and void does not hold as such any info like Future<void>. If you try to await bar() it will convert that Future object from Future<void> to void.

---

Future is just a container, with await returns the values once the async "tasks" are completed. Without await, Future objects give you information about the execution of the function from which they are returning.