A ZRef[RA, RB, EA, EB, A, B] is a polymorphic, purely functional description of a mutable reference. The fundamental operations of a ZRef are set and get. set takes a value of type A and sets the reference to a new value, requiring an environment of type RA and potentially failing with an error of type EA. get gets the current value of the reference and returns a value of type B, requiring an environment of type RB and potentially failing with an error of type EB.

When the error and value types of the ZRef are unified, that is, it is a ZRef[R, R, E, E, A, A], the ZRef also supports atomic modify and update operations. All operations are guaranteed to be safe for concurrent access.

By default, ZRef is implemented in terms of compare and swap operations for maximum performance and does not support performing effects within update operations. If you need to perform effects within update operations you can create a ZRef.Synchronized, a specialized type of ZRef that supports performing effects within update operations at some cost to performance. In this case writes will semantically block other writers, while multiple readers can read simultaneously.

ZRef.Synchronized also supports composing multiple ZRef.Synchronized values together to form a single ZRef.Synchronized value that can be atomically updated using the zip operator. In this case reads and writes will semantically block other readers and writers.

NOTE: While ZRef provides the functional equivalent of a mutable reference, the value inside the ZRef should normally be immutable since compare and swap operations are not safe for mutable values that do not support concurrent access. If you do need to use a mutable value ZRef.Synchronized will guarantee that access to the value is properly synchronized.