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dartdart-null-safety

A value of type 'Future<bool>' can't be returned from the function because it has a return type of 'Future<void>'

I have:

Future<bool> foo() async => true;

This is allowed:

Future<void> bar()  {
  return foo(); // Works
}

but this is not:

Future<void> baz() async {
  return foo(); // Error
}

In both bar and baz I'm returning a Future<bool>, but why first works but second fails?

Note: This question isn't about HOW to make it work but rather WHY one works and the other doesn't.

Solution

  • Dart has special rules for void returning functions: You are not allowed to return a value because it's declared as not returning any value, so actually trying to return a value is probably a mistake.

    The same happens without Future:

    void qux() {
  return true; // Error, can't return value from void function
}

    gives the error

    A value of type 'bool' can't be returned from the function 'qux' because it has a return type of 'void'.

    You are allowed to have return e; statements, but only if the type of e is void, Null or dynamic. (And you shouldn't even do that, those are just allowed in order to make existing code work.) (A => e body is always allowed, because people like to use it as a shorthand for just { e; }, mainly because the formatter keeps it on one line. I still recommend using { e; } as body of a void function.)

    Generalizing that to async functions with void future return types, you are not allowed to return an actual value from Future<void> ... async function. So, the only things you are allowed to return are void, dynamic, Null, Future<void>, Future<dynamic>, and Future<Null>. A Future<bool> is neither of those.

    What you should write instead is:

    Future<void> baz() async {
  await foo();
}