outputwhich

Outputwhich is a term used in computing to describe a construct for routing outputs from a processing unit to multiple destinations according to a set of selection criteria. It functions as a demultiplexing or routing operator that, for each input item, evaluates a selector and dispatches the item to the corresponding output channel. The name combines "output" with "which," signaling that the mechanism decides which output is used.

Origin and scope: The term is not standardized and appears primarily in experimental programming libraries and data-processing domain-specific languages. It is conceptually related to demultiplexers, routers, and fan-out patterns.

Concept and mechanics: An outputwhich construct typically takes two elements: a mapping from criteria or predicates to outputs, and a default output. On processing each item, it tests the predicates in order and sends the item to the first output whose condition is satisfied; if none matches, the item goes to the default. In some implementations, multiple outputs can be active per item, or the selector returns a priority, weighting, or a single target.

Typical use cases: Event routing in stream processing, log routing by severity, data transformation pipelines where different data types go to different sinks, or user interface event handling where actions are dispatched to handlers.

Example: Define routes = { level >= 5 -> fileSink, isAlert -> alertService, otherwise -> console }. For each record r, outputwhich(r, routes, default=console). Notes: Performance and determinism depend on predicate complexity; design often favors simple, precomputed routing tables.

See also: demultiplexer, routing, fan-out, predicate-based routing, data pipeline.