Quick links:

• About the library
• How to get latest version
• API docs (fs2-core), API docs (fs2-io)
• Docs and getting help

FS2 is a streaming I/O library. The design goals are compositionality, expressiveness, resource safety, and speed. Here's a simple example of its use:

import fs2.{io, text, Task}
import java.nio.file.Paths

def fahrenheitToCelsius(f: Double): Double =
  (f - 32.0) * (5.0/9.0)

val converter: Task[Unit] =
  io.file.readAll[Task](Paths.get("testdata/fahrenheit.txt"), 4096)
    .through(text.utf8Decode)
    .through(text.lines)
    .filter(s => !s.trim.isEmpty && !s.startsWith("//"))
    .map(line => fahrenheitToCelsius(line.toDouble).toString)
    .intersperse("\n")
    .through(text.utf8Encode)
    .through(io.file.writeAll(Paths.get("testdata/celsius.txt")))
    .run

// at the end of the universe...
val u: Unit = converter.unsafeRun()

This will construct a Task, converter, which reads lines incrementally from testdata/fahrenheit.txt, skipping blanklines and commented lines. It then parses temperatures in degrees fahrenheit, converts these to celsius, UTF-8 encodes the output and writes incrementally to testdata/celsius.txt, using constant memory. The input and output files will be closed in the event of normal termination or exceptions.

The library supports a number of other interesting use cases:

• Zipping and merging of streams: A streaming computations may read from multiple sources in a streaming fashion, zipping or merging their elements using a arbitrary Tee. In general, clients have a great deal of flexibility in what sort of topologies they can define--source, sinks, and effectful channels are all first-class concepts in the library.
• Dynamic resource allocation: A streaming computation may allocate resources dynamically (for instance, reading a list of files to process from a stream built off a network socket), and the library will ensure these resources get released in the event of normal termination or when errors occur.
• Nondeterministic and concurrent processing: A computation may read from multiple input streams simultaneously, using whichever result comes back first, and a pipeline of transformation can allow for nondeterminism and queueing at each stage.

• The official guide is a good starting point for learning more about the library.
• Also feel free to come discuss and ask/answer questions in the gitter channel and/or on StackOverflow using the tag FS2.

Blog posts and other external resources are listed on the Additional Resources page.

The 0.9 release is out and we recommend upgrading. You may want to first read the migration guide if you are upgrading from 0.8 or earlier. To get 0.9, add the following to your SBT build:

// available for Scala 2.11.8, 2.12.0-RC1
libraryDependencies += "co.fs2" %% "fs2-core" % "0.9.1"

// optional I/O library
libraryDependencies += "co.fs2" %% "fs2-io" % "0.9.1"

The fs2-core library is also supported on Scala.js:

// available for Scala 2.11.8, 2.12.0-RC1
libraryDependencies += "co.fs2" %%% "fs2-core" % "0.9.1"

API docs:

• The core library
• The io library, FS2 bindings for NIO-based file I/O and TCP/UDP networking

The previous stable release is 0.8.4 (source).

If you have a project you'd like to include in this list, either open a PR or let us know in the gitter channel and we'll add a link to it here.

• doobie: Pure functional JDBC built on fs2.
• http4s: Minimal, idiomatic Scala interface for HTTP services using fs2.
• scodec-stream: A library for streaming binary decoding and encoding, built using fs2 and scodec.
• streamz: A library that allows a Stream to consume from and produce to Apache Camel endpoints, Akka Persistence journals and snapshot stores and Akka Stream flows (reactive streams) with full back-pressure support.

FS2 has evolved from earlier work on streaming APIs in Scala and Haskell and in Scala. Some influences:

• Machines, a Haskell library by Ed Kmett, which spawned scala-machines
• The FP in Scala stream processing library developed for the book FP in Scala
• Reflex, an FRP library in Haskell, by Ryan Trinkle
• There are various other iteratee-style libraries for doing compositional, streaming I/O in Scala, notably the scalaz/iteratee package and iteratees in Play.

See Additional resources.