Atomiceither

Atomiceither is a theoretical construct in computer science that combines the semantics of the Either data type with atomicity guarantees in concurrent programming. In its simplest form, an atomiceither represents a value that is either a Left, typically signaling an error or alternate type, or a Right, signaling success or a primary payload, with updates and reads performed atomically.

Conceptually, atomiceither models operations that may fail or succeed while ensuring that all threads observe a consistent state. The atomicity requirement means that a read, write, or swap operation on the value happens as an indivisible step, preventing intermediate or partially updated states from being observed by other threads. This makes atomiceither useful for lock-free error handling, state machines, or pipelines where concurrent components must agree on a single outcome.

Implementation approaches often rely on an atomic reference to a tagged union or a similar primitive, with a small API to manipulate the two variants. Common operations include constructing Left or Right values, testing which variant is current, retrieving the payload, and performing mappings or flat-maps that preserve atomicity. Update methods typically rely on compare-and-swap or transactional memory techniques to ensure safe transitions between states.

Typical use cases involve concurrent task pools, error propagation in asynchronous workflows, and coordination of stateful actors where a single coherent result must be exposed to all observers. Limitations include added overhead, potential complexity in reasoning about atomic state, and subtleties related to memory ordering.

See also: Either, atomic references, lock-free data structures, software transactional memory.