Elegant error handling in Dart like Scala's `Try` - scala

Question

A data class in Dart:
import 'package:validate/validate.dart';
class AuthUser {
final String email, token, username, bio, image;
AuthUser(this.email, this.token, this.username, this.bio, this.image) {
Validate.isEmail(this.email);
}
#override
String toString() {
return 'AuthUser{email: $email, token: $token, username: $username, bio: $bio, image: $image}';
}
}
where Validate.isEmail will throws an Error when failed to match:
static void matchesPattern(String input, RegExp pattern,[String message = DEFAULT_MATCHES_PATTERN_EX]) {
if (pattern.hasMatch(input) == false) {
throw new ArgumentError(message);
}
}
static void isEmail(String input,[String message = DEFAULT_MATCHES_PATTERN_EX]) {
matchesPattern(input,new RegExp(PATTERN_EMAIL),message);
}
Now I want to use an elegant way to new this class.
When using Scala, I can use Try(new AuthUser(...)) and patten-matching it.
And in Dart, first I tried RxDart,
void main() {
testWidgets('Counter increments smoke test', (WidgetTester tester) async {
Observable.just(AuthUser("email", "token", "username", "bio", "img"))
.doOnError((e, s) => print("oh no"))
.listen((e) => print(e.toString()));
});
}
Not work, the test failed for the error(which means RxDart doesn't catch errors at all!!!)
And I want to try Future, failed also.
And I want to use dartz, but I am worried because there is just one maintainer...
Any advice?

Answer 1

If you are OK with using Future what's wrong with this advice: Using Future.sync() to wrap your code? The code will look like this:
void main() {
var f = Future.sync(() {AuthUser("email", "token", "username", "bio", "img"); });
f.then((v) => print("Value: " + v.toString())).catchError((e) => print("Failure: " +e.toString()));
}
The main trick is that Future.sync effectively enables lazy evaluation of the parameter but you have to pass your parameter wrapped in a function. This is actually the same trick Scala compiler does for Try (i.e. for call-by-name parameters) but takes adding a few brackets around.

Answer 2

If you only want the basic functionality of returning either type based on whether an exception occurred or not then you can easily create a utility class such as below.
Otherwise I recommend #SergGr's answer about using Future.sync since it gives you more monadic like pipeline.
void main() {
Try<Error, void> result = Try.it(() => Validate.isEmail("test-example.com"));
if (result is Success) {
print("Good");
} else if (result is Failure) {
print("Error: " + result.exception().toString());
}
}
typedef TryExec<V> = V Function();
abstract class Try<E extends Error, V> {
static Try<E, V> it<E extends Error, V>(TryExec<V> fn) {
try {
return Try.success(fn());
} catch (e) {
return Try.failure(e);
}
}
static Try<E, V> failure<E extends Error, V>(Error e) {
return new Failure(e);
}
static Try<E, V> success<E extends Error, V>(V v) {
return new Success(v);
}
}
class Failure<E extends Error, V> extends Try<E, V> {
final E _e;
Failure(this._e);
E exception() => _e;
}
class Success<E extends Error, V> extends Try<E, V> {
final V _v;
Success(this._v);
V value() => _v;
}