Property capabilities#

Properties and indexers declared readonly or writeonly grant read and write access independently. This lets a type describe the authority an API actually needs instead of turning every member into a read-write slot.

A Snapshot can read value but cannot assign it. An Output can assign the member but cannot observe its current value. The same syntax works in records and structural shapes:

The two declarations name one runtime property. They may use different types: here a write accepts string | integer, while every read produces string. This models normalizing setters and declaration-file APIs without weakening reads to the setter's broader input type. Construction may initialize a read-only record field; the capability governs access through the constructed view, not creation of the value.

An unmodified property is shorthand for matching read and write capabilities:

Variance#

Readonly types are covariant. If Dog fits Animal, a {readonly value: Dog} fits a {readonly value: Animal} because every value read is still an animal.

Writeonly types are contravariant. A {writeonly value: Animal} fits a {writeonly value: Dog} because it accepts every dog the narrower view may write.

An ordinary property has both constraints, so it is invariant. A stored {value: Dog} does not fit {value: Animal}: code using the latter view could write another kind of animal and break the former type. Fresh table literals may initialize a contextual type because no narrower stored view exists yet.

Indexers#

Indexers use the same capabilities and may appear in shapes, interfaces, and records:

Reading a map-like indexer remains optional because a key may be absent. The write type describes a present value accepted by assignment.

Relationship to other qualifiers#

Property capabilities are member-level access views:

• const T makes the whole value read-only rather than selecting members.
• borrows and exclusive govern lifetime and aliasing, not whether a member may be read or written.
• A const binding prevents rebinding the local name; it does not by itself make the referenced table immutable.

Access diagnostics#

NUPP2009 reports a read through a write-only view or an assignment through a read-only view. Compound assignment needs both capabilities because it first reads the old value and then writes the result.

NUPP2118 reports duplicate capabilities, an ordinary property combined with a separate capability of the same name, and capability properties on structs. Struct fields are fixed C memory slots and remain ordinary invariant fields.