example.dart

void main() {
  apiMethod('foo');
  apiMethod('foo', (value) => print('Hello $value'));
}

typedef Callback = void Function(String value);

void apiMethod(String value, [Callback? callback]) {
  // What I would love to do instead of this if/else is something like:
  // `backingMethod(value, callback ?? default)`, where `default` is a
  // reserved word.
  if (callback == null) {
    backingMethod(value);
  } else {
    backingMethod(value, callback);
  }
}

// Imagine these methods are in another package, so _default is not
// visible to the frontend API.

void backingMethod(String value, [Callback callback = _default]) {
  callback(value);
} 
  
void _default(String value) {
  print('value is $value');
}