Declarative Operator Composition

Declarative composition chains pure functions like map, filter, and scan to build stream-processing pipelines, replacing imperative loops with readable data transformations. RxJS formalizes this with pipe() for type-safe pipelines, while Reactor's Flux uses method chaining: Flux.from(publisher).map(...).filter(...).scan(...). Kotlin Flow's operators are suspend functions, safe for coroutines. The TC39 proposal brings map and filter to native async iterators, standardizing composition at the platform level. Apache Pekko Streams' Source/Flow/Sink triad provides 200+ composable stages, enabling fan-out and broadcast. Testing with RxJS's TestScheduler or Reactor's StepVerifier requires no mocking or DI. A concrete data point: reactive pipelines reduce code volume by 50% compared to imperative async callbacks, improving readability and reliability.