Named and optional parameters

Wrap trailing positional parameters in [ ] to make them optional: String greet(String name, [String? title]). Callers may omit them, and an omitted nullable one is null.

Named parameters go in { } and are passed by name: box(width: 2, height: 3). Order at the call site does not matter, which makes calls with many flags readable.

A named parameter is optional unless marked required. Non-nullable optional parameters need a default: {int retries = 3}. Defaults must be compile-time constants.

Mixing is limited: a function can have optional positional or named parameters, not both. Required positional parameters always come first.

Misses: forgetting required on a non-nullable named parameter with no default (compile error), and passing a named argument positionally.

String box({required int width, int height = 1}) =>
    '${width}x$height';

String greet(String name, [String? title]) =>
    title == null ? 'Hi $name' : 'Hi $title $name';

void main() {
  print(box(width: 4));
  print(box(height: 2, width: 3));
  print(greet('Ada'));
  print(greet('Ada', 'Dr'));
}