Adding orElse function to firstWhere method

I am trying to add the onElse function to the itterator.firstWhere method but I cannot get the syntax right.

I have tried something like

List<String> myList = 

String result = myList.firstWhere((o) => o.startsWith('foo'), (o) => null);

But the compiler has an error of

> 1 positional arguments expected, but 2 found

I am sure it is a simple syntax problem, but it has me stumped

In case someone came here thanks to google, searching about how to return null if firstWhere found nothing, when your app is Null Safe, use the new method of package:collection called firstWhereOrNull.

import 'package:collection/collection.dart'; // You have to add this manually, for some reason it cannot be added automatically

// somewhere...
MyStuff? stuff = someStuffs.firstWhereOrNull((element) => element.id == 'Cat');

About the method: https://pub.dev/documentation/collection/latest/collection/IterableExtension/firstWhereOrNull.html

'orElse' is a named optional argument.

void main() {
  checkOrElse(['bar', 'bla']);
  checkOrElse(['bar', 'bla', 'foo']);
}

void checkOrElse(List<String> values) {
  String result = values.firstWhere((o) => o.startsWith('foo'), orElse: () => '');

  if (result != '') {
    print('found: $result');
  } else {
    print('nothing found');
  }
}

void main() {
  List<String> myList = ['oof'];

  String result = myList.firstWhere((element) => 
element.startsWith('foo'),
      orElse: () => 'Found nothing');
  print(result);
}

myObject = myList.firstWhere
       ((element) => element.myIdentifier == someIdentifier, orElse: ()=> null);