How to use mandatory field in dart args package - dart

Question

I am trying to add a Mandatory field option for the argument in my dart script. I am using addOption() as visible in the code below. and to run the code in the terminal.
$dart args_demo.dart -h
If I run the code without mandatory: true in the method addOption(), I get no error and a help text is shown, everything works fine.
But, when I add this part mandatory: true in the method addOption(), and run the script with above command, I get this error massage
Unhandled exception:
FormatException: Option name is mandatory.
#0 Parser.parse.<anonymous closure> (package:args/src/parser.dart:101:9)
#1 _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:397:8)
#2 MapView.forEach (dart:collection/maps.dart:342:10)
#3 Parser.parse (package:args/src/parser.dart:95:22)
#4 ArgParser.parse (package:args/src/arg_parser.dart:335:42)
#5 main (file:///home/penguine/Desktop/codeTesting/argPackage/args_demo.dart:12:26)
#6 _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:281:32)
#7 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)
Please explain how can I use the mandatory field without getting error.
thanks in advance.
import 'dart:io';
import 'package:args/args.dart';
void main(List<String> args){
final parser = ArgParser()
..addSeparator('===options and flags and help===')
..addOption('name', abbr: 'n', help: 'Provide your name', mandatory: true)
..addFlag('help', abbr: 'h', help: 'Provide usage instruction', negatable: false)
..addOption('paas', help: 'what is your favorite PaaS', allowed: ['AWS', 'GCP', 'Heroku', 'MSA'], defaultsTo: 'GCP');
final results = parser.parse(args);
//print(results);
if (results.wasParsed('help')){
print(parser.usage);
exit(0);
}
if (results.wasParsed('name')) {
print('=>> ${results['name']}');
}
print('PAAS =>> ${results['paas']}');
}

Answer 1

Note that there is nothing special about the -h/--help option itself. It is a boolean flag like any other. Consequently, since you've made -n/--name mandatory, it is exactly that: it is always required, even if you're just trying to print usage text.
The way that -h/--help is handled provides its own clue how you can make an option conditionally required: check ArgResults.wasParsed() and explicitly fail if it's not provided:
final results = parser.parse(args);
if (results.wasParsed('help')){
print(parser.usage);
exit(0);
}
if (!results.wasParsed('name')) {
stderr.writeln('--name is required.');
exit(1);
}
I agree that this behavior makes mandatory: true not very useful. It's a relatively recent addition to package:args, and I've filed https://github.com/dart-lang/args/issues/194.