Dart: The argument type 'String' can't be assigned to the parameter type 'FutureOr<String>

I'm experimenting a bit with async methods but I'm having a hard time.

I would like to implement some code whose goal is to simply return a string after waiting 2 seconds.

This is my first attempt and related error

String setUserDecision() {
    return 'Yes'; 
}

Future<String> fetchUserDecision() async {
  return await Future.delayed(const Duration(seconds: 2), setUserDecision());
}

Future<void> main() async {
  print("Fetching user role...");
  String decision = await fetchUserDecision();
  print('The user decision is: $decision');
  print("Done");
}

Error: The argument type 'String' can't be assigned to the parameter type 'FutureOr<String> Function()?'.
  return await Future.delayed(const Duration(seconds: 2), setUserDecision());

I can't understand the error, I may think the String type is not right, but I don't understand how to change it. I also tried this solution, thinking it might make sense (maybe), but nothing.

Future<String> setUserDecision() async{
    return await 'Yes'; 
}

Future<String> fetchUserDecision() async {
  return await Future.delayed(const Duration(seconds: 2), setUserDecision());
}

Future<void> main() async {
  print("Fetching user role...");
  String decision = await fetchUserDecision();
  print('The user decision is: $decision');
  print("Done");
}

and also this

import 'dart:async';

String setUserDecision() {
    return  'Yes'; 
}

Future<String> fetchUserDecision() async {
  return await Future.delayed(const Duration(seconds: 2), setUserDecision() as FutureOr<String> Function()?);
}

Future<void> main() async {
  print("Fetching user role...");
  String decision = await fetchUserDecision();
  print('The user decision is: $decision');
  print("Done");
}

with this error

_CastError (type 'String' is not a subtype of type '(() => FutureOr<String>)?' in type cast)

The only solution that seems to work is this

Future<String> fetchUserDecision() async {
  return await Future.delayed(const Duration(seconds: 2), () {
    return 'Yes'; 
});
}

Future<void> main() async {
  print("Fetching user role...");
  String decision = await fetchUserDecision();
  print('The user decision is: $decision');
  print("Done");
}

but I would like to better understand the type of error and how to fix it.

Solution

Your problem is that you are calling your setUserDecision() function and takes the result of this and sends to Future.delayed which expect a function. Since the result of calling setUserDecision() is a String, you end up providing a String instead of a Function which makes Dart complain.

You should do this instead:

Future<String> fetchUserDecision() async {
  return await Future.delayed(const Duration(seconds: 2), setUserDecision);
}

Notice I have removed the () after setUserDecision. Now we provide the function as a function instead of calling it.