Step 1 - Throw

void pay(double amount) {
  if (amount.isNaN || amount.isInfinite) {
    throw ArgumentError('amount must be finite');
  }
}

void main() {
  try {
    pay(double.nan);
  } catch (e) {
    print(e);
  }
}

Step 2 - try / catch / finally

int parseLen(String raw) {
  try {
    final n = int.parse(raw);
    return n;
  } on FormatException {
    return 0;
  } finally {
    print('parse attempt done');
  }
}

void main() {
  print(parseLen('12'));
  print(parseLen('nope'));
}

• on Type narrows what you catch. A bare catch (e, st) catches everything (use sparingly).

Step 3 - Stack trace preserved (optional)

• catch (e, st) gives you the stack st if you log or wrap with Error.throwWithStackTrace.
• In app code, often log e and st at the outer boundary, not inside every helper.

Good habit

• Catch at boundaries (CLI entry, HTTP handler), not in every tiny helper.

Practice tasks

• Wrap int.parse in a function that throws FormatException with a custom message including raw.
• Add a finally block that increments a attemptCount variable and assert it runs on success and failure.