Dart:
Writing to standard error

How to:

In Dart, writing to stderr is straightforward using the stderr object available in dart:io. Here’s a basic example:

import 'dart:io';

void main() {
  stderr.writeln('This is an error message.');
}

Output when run:

This is an error message.

This message is sent to the stderr stream, which is typically displayed in the console or terminal.

To demonstrate more complexity, such as logging an exception, Dart’s rich set of features allows for concise and effective error handling:

import 'dart:io';

void riskyOperation() {
  try {
    // Simulate an operation that might throw
    throw Exception('Something went wrong!');
  } catch (e) {
    stderr.writeln('Error: $e');
  }
}

void main() {
  riskyOperation();
}

Output when run:

Error: Exception: Something went wrong!

This pattern is especially useful for applications that need to separate normal logs from error logs, making it easier to monitor and debug applications.

While Dart’s standard library is quite comprehensive, many programs don’t require third-party libraries for writing to stderr. However, if your application needs more sophisticated logging capabilities (e.g., to files, over the network, formatting), the logging package is a popular choice. Here’s a quick peek at using logging for errors:

import 'dart:io';
import 'package:logging/logging.dart';

final logger = Logger('MyAppLogger');

void setupLogging() {
  logger.onRecord.listen((record) {
    if (record.level >= Level.SEVERE) {
      stderr.writeln('${record.level.name}: ${record.time}: ${record.message}');
    }
  });
}

void main() {
  setupLogging();
  logger.severe('Severe Error: Something significantly bad happened.');
}

Output when run:

SEVERE: 2023-04-01 00:00:00.000: Severe Error: Something significantly bad happened.

This method offers a higher degree of customization and control over what gets logged as an error and how it’s formatted, which can be very helpful in larger, more complex applications.