Step 1 - Field error alias

typedef FieldError = String?;

Step 2 - Present check

FieldError requirePresent(String? value) {
  if (value == null || value.trim().isEmpty) {
    return 'required';
  }
  return null;
}

Step 3 - Email

FieldError validateEmail(String? value) {
  if (value == null || value.trim().isEmpty) {
    return 'required';
  }
  final v = value.trim();
  if (!v.contains('@')) return 'invalid email';
  return null;
}

Step 4 - Password rules

FieldError validatePassword(String? value) {
  if (value == null || value.trim().isEmpty) {
    return 'required';
  }
  final v = value.trim();
  if (v.length < 8) return 'min length 8';
  if (!RegExp(r'[0-9]').hasMatch(v)) return 'need a digit';
  return null;
}

Step 5 - Aggregate

Map<String, FieldError> validateSignUp({String? email, String? password}) {
  return {
    'email': validateEmail(email),
    'password': validatePassword(password),
  };
}

void main() {
  final errors = validateSignUp(email: 'bad', password: 'short');
  print(errors);
}

Practice tasks

• Return a record (bool ok, Map<String, String> errors) where the map only contains keys that failed.
• Add validatePasswordConfirm(String? password, String? confirm).
• Add maximum length checks with one shared helper FieldError? lengthLimit(String trimmed, int max).