avoid_futureor_void

Avoid using 'FutureOr' as the type of a result.

AVOID using FutureOr<void> as the type of a result. This type is problematic because it may appear to encode that a result is either a Future<void>, or the result should be discarded (when it is void). However, there is no safe way to detect whether we have one or the other case (because an expression of type void can evaluate to any object whatsoever, including a future of any type).

It is also conceptually unsound to have a type whose meaning is something like "ignore this object; also, take a look because it might be a future".

An exception is made for contravariant occurrences of the type FutureOr<void> (e.g., for the type of a formal parameter), and no warning is emitted for these occurrences. The reason for this exception is that the type does not describe a result, it describes a constraint on a value provided by others. Similarly, an exception is made for type alias declarations, because they may well be used in a contravariant position (e.g., as the type of a formal parameter). Hence, in type alias declarations, only the type parameter bounds are checked.

A replacement for the type FutureOr<void> which is often useful is Future<void>?. This type encodes that the result is either a Future<void> or it is null, and there is no ambiguity at run time since no object can have both types.

It may not always be possible to use the type Future<void>? as a replacement for the type FutureOr<void>, because the latter is a supertype of all types, and the former is not. In this case it may be a useful remedy to replace FutureOr<void> by the type void.

BAD:

FutureOr<void> m() {...}

GOOD:

Future<void>? m() {...}

This rule is experimental. It is being evaluated, and it may be changed or removed. Feedback on its behavior is welcome! The main issue is here: https://github.com/dart-lang/sdk/issues/59232.

To enable the avoid_futureor_void rule, add avoid_futureor_void under linter > rules in your analysis_options.yaml file:

linter:
  rules:
    - avoid_futureor_void

If you're instead using the YAML map syntax to configure linter rules, add avoid_futureor_void: true under linter > rules:

linter:
  rules:
    avoid_futureor_void: true