🪻 A Type-Safe, Purely Functional Effect System for F#

FIO is an IO monad plus lightweight fibers (green threads) for building concurrent and asynchronous F# applications. Effects are described as pure, lazy values and run by a pluggable runtime — so your program is a composable description that stays referentially transparent until you hand it to a runtime. The API takes its cues from ZIO.

• Typed effects — FIO<'A, 'E> tracks both the success value and the error in the type
• Fibers & channels — green threads via .Fork() / .Join() and typed message passing
• Structured concurrency — fail-fast parallel combinators that interrupt losers automatically
• Finalizer guarantees — Ensuring finalizers run on success, error, and interruption
• Composable — the fio { } computation expression plus a rich set of operators

dotnet add package FSharp.FIO

open FIO.DSL
open FIO.App
open FIO.Console

type App() =
    inherit FIOApp<unit, exn>()

    override _.effect = fio {
        do! Console.printLine "What is your name?" id
        let! name = Console.readLine id
        do! Console.printLine $"Hello, {name}!" id
    }

[<EntryPoint>]
let main _ = App().Run()

Fork effects onto fibers, run them in parallel, and compose the results — losers are interrupted automatically on the first failure.

open FIO.DSL

// Run two effects in parallel with <&> and collect both results as a tuple.
let taskA = FIO.succeed "Task A completed! ✅"
let taskB = FIO.succeed (200, "Task B OK ✅")
let both  = taskA <&> taskB

// Or fork/join explicitly.
let forked = FIO.succeed("Hello, concurrency! 🚀").Fork() >>= fun fiber -> fiber.Join()

More in examples/ — the DSL, App, HTTP, Sockets, and WebSockets tours.

• Effects — lazy, composable FIO<'A, 'E> with typed errors
• Fibers — green threads for scalable concurrency
• Channels — typed message passing between fibers
• Structured concurrency — fail-fast ZipPar, Race, and forEachPar that interrupt losers automatically
• Composition — fio { } CE, operators (>>=, <&>, <|>), combinators
• Modules — Console

Effects are interpreted by a runtime. Pick one explicitly, or use DefaultRuntime.

DefaultRuntime = WorkStealingRuntime — FIOApp uses it unless you override _.runtime.

Each extension library has its own README with API details: Http · Sockets · WebSockets.

Issues and pull requests welcome. See CONTRIBUTING.md, the Code of Conduct, and the Security Policy.

MIT