Std.Unit

The unit type — exactly one value, carrying no information.

Written the Swift way: the type is Unit and its sole value is (). () is to Unit what 0 is to Int — the one canonical inhabitant. A function returning Unit is one called only for its effect; a field of type Unit stores nothing.

Unit is also the empty-telescope terminator: a Tuple(T1, …, Tn) unfolds to the unit-terminated Σ chain Sigma(T1, … Sigma(Tn, ())), and the empty tuple Tuple() is exactly Unit. The compiler seeds Unit into every module so that machinery always has it; this file is its one visible, documented definition (mirroring Std.Bool/Std.Nat, which are likewise both seeded and defined here in surface Cure).

= () is reserved: only Unit may be defined this way. Writing type Foo = () for any other name is a compile error — () denotes the one built-in unit type, it is not a spelling other types may borrow.