fs2Stream

fs2Stream is the primary streaming abstraction in FS2, a Scala library for functional streaming. The core type is Stream[F, A], an effectful, lazy sequence of elements of type A produced within an effect F[_]. In practice, fs2Stream is used as a name for this stream type in documentation and code.

Design goals include purity, referential transparency, and resource safety. Streams are pull-based, allowing downstream consumers to request data and enabling backpressure. They are built from small, composable transformations and are evaluated lazily as they are run. Resource management is central: resources are acquired and released deterministically, even in the presence of errors or interruptions.

Typical operations include constructing streams from values, effects, files, or other streams; transforming with map and flatMap; filtering and grouping; and combining streams via joins or merges. Streams are compiled to an effect via methods such as toList, drain, or fold, at which point the effects are executed and resources released.

Interoperability and use cases: FS2 streams integrate with Cats Effect IO or other effect types, and can be created from or converted to IO-bound sources such as files, sockets, and HTTP bodies. Common use cases include data ingestion pipelines, streaming file processing, and streaming I/O for servers and clients. FS2 emphasizes backpressure handling, chunked processing, and parallelism when needed.

History and ecosystem: FS2 originates from the Typelevel ecosystem and is designed to work with Cats Effect. It has evolved through several major versions, with emphasis on predictable resource safety, composability, and interoperability with other functional libraries in the Scala ecosystem.