Fastingeither

Fastingeither is a hypothetical functional programming construct used in discussions of concurrent error handling. It describes a race between two computations that each yield an Either value and returns the result of the computation that completes first.

Semantics are simple and design-driven. The two computations run concurrently. The first to finish yields either a Right value or a Left error. If the result is Right a, fastingeither returns Right a. If it is Left e, fastingeither returns Left e. The other computation is cancelled or ignored. This pattern prioritizes speed and responsiveness over guaranteed completion, and it is not part of standard language features but a conceptual tool for illustrating latency versus reliability trade-offs in error-handling code.

Implementation considerations include how cancellation and resource cleanup are handled. In languages with asynchronous primitives, fastingeither can be implemented by starting two tasks, awaiting the first completion, returning its result, and cancelling the other task. In languages like Haskell, similar behavior can be approached with existing concurrency abstractions, though care must be taken to ensure proper cancellation and to avoid leaking resources if the other computation performs side effects. Designers should also consider deterministic behavior in the presence of exceptions or asynchronous interrupts.

Applications for fastingeither are typically in latency-sensitive contexts where a quick result is preferable to guaranteeing a successful outcome in every case. This can arise in user interfaces, fast-path routing, or competitive deployment scenarios where time to first result matters. Related concepts include the Either monad, race patterns, and general concurrency and cancellation strategies.