Generic structs and enums

Structs and enums may declare type parameters after their names. Each concrete argument produces a checked instantiation that keeps the nominal identity of the declaration.

Expected arguments also guide contextual variants and empty collection payloads, so Box<int> keeps int even when a variant carries no value.

enum Box<T> {
  Value(T),
  Empty,
}

fn read(box: Box<int>) -> Optional<int> {
  match box { .Value(value) => some(value), .Empty => none }
}

fn main() { println(read(.Value(7))) }