Step 1 - Result type

class Result<T> {
  final T? value;
  final String? error;

  const Result._({this.value, this.error});

  const Result.ok(T value) : this._(value: value, error: null);

  const Result.err(String error) : this._(value: null, error: error);

  bool get isOk => error == null;
}

Step 2 - Email as Result<String>

Result<String> validateEmail(String? raw) {
  if (raw == null || raw.trim().isEmpty) {
    return const Result.err('email required');
  }
  final email = raw.trim();
  if (!email.contains('@')) {
    return const Result.err('email invalid');
  }
  return Result.ok(email);
}

Step 3 - Parse age

Result<int> parseAge(String? raw) {
  if (raw == null || raw.trim().isEmpty) {
    return const Result.err('age required');
  }
  final n = int.tryParse(raw.trim());
  if (n == null) return const Result.err('age not a number');
  if (n < 0 || n > 130) return const Result.err('age out of range');
  return Result.ok(n);
}

void main() {
  final e = validateEmail('a@b.com');
  final a = parseAge('20');
  if (e.isOk) print(e.value);
  if (a.isOk) print(a.value);
}

Practice tasks

• Add Result<void> or Result<Unit>-style success (use Result<bool> with true or a tiny sealed success marker if you prefer).
• Combine Result<String> email + Result<int> age into Result<SignUp> with a function Result<SignUp> validateSignUp(...) that stops on first error or returns both values.
• Add extension<T> on Result<T> { Result<U> map<U>(U Function(T) f); } that propagates errors.