Idea: lint when passing async functions into forEach

[greglittlefield-wf]

I recently ran into a few instances of code that used the following:

values.forEach((value) async {
  // ...
  doSomethingSync();
  await value.doSomethingAsync();
});

map.forEach((key, value) async {
  doSomethingSync();
  await value.doSomethingAsync();
});

The intended behavior in at least one of the cases was to wait for all of the awaits to be run before proceeding, which can be done via Future.forEach or a for loop with awaits.

Even if the intended behavior is what is actually happening, it might not be immediately obvious to someone unfamiliar with how async functions are executed in Dart 1.x.

The first example would be avoided by using the prefer_foreach rule (unless a tearoff were passed in instead), but the second would not. Still, for the first example, it might be good to have a more specific lint to catch this case.

Are Iterable.forEach/Map.forEach common-enough special cases where using async functions should result in a lint?

Another idea, though it would have far more impact and may be too noisy, would be to lint when an async function is being used where a void function is accepted.

CallbackSubscription registerVoidCallback(void callback()) { /*...*/ }

// Good
registerVoidCallback((() { /* ... */ });
// Would lint
registerVoidCallback((() async { /* ... */ });

[matanlurey]

Async functions will be executed synchronously in Dart 2, FWIW.

[greglittlefield-wf]

Good point! This lint would definitely have more impact in Dart 1.

[natebosch]

See also #1123

Should we merge these issues?

[jamesderlin]

I see this confusion all the time on StackOverflow. It would be nice if there were a warning about passing Future-returning callbacks to Iterable.forEach/Map.forEach.

[pq]

@jamesderlin: the next time you see this come up on SO, could you add a link? That'd be great for motivation. Thanks!

[jamesderlin]

Here are some of them:

• Async/await in List.forEach()
• [dart]flutter how to wait map.forEach to complete in Future function?
• Waiting async in Flutter
• list.forEach doesn't await properly in async method
• Strange async behaviour
• Flutter Await not waiting
• Flutter - Dart : wait a forEach ends
• My async call is returning before list is populated in forEach loop
• How to avoid using await key in dart Map by foreach function
• Flutter async/await doesn't work inside forEach
• Flutter await not waiting before continuing
• Why I get an empty list as result in an async Flutter function?
• wait foreach element of list
• Async not waiting for Future.then() - Dart
• unable to refresh state after forEach in flutter
• Method does not complete all instructions

[pq]

Amazing. Thank you!