Fable

Effects can be used in F# compiled to JavaScript with Fable: Effect, eff and the Effect module work in Fable 5, for JavaScript. Support is experimental: what's there behaves as on .NET, and is tested on both, but it covers the core of Orsak, not all of it.

Setup

Reference the Orsak package, as on .NET:

<PackageReference Include="Orsak" Version="..." />

The package carries the sources Fable compiles, and Fable chooses them itself. An effect written once, in a project shared by a .NET server and a Fable client, compiles on both:

type IGreeter =
    abstract Greet: string -> string

let greet name = Effect.Create(fun (g: #IGreeter) -> g.Greet name)

let greetAll names = eff {
    let! greetings = names |> List.map greet |> Effect.par
    return greetings
}

// in the Fable client
task {
    match! greetAll [ "Fable"; "Node" ] |> Effect.run (Greeter()) with
    | Ok greetings -> printfn "%A" greetings
    | Error e -> printfn "failed: %s" e
}
|> ignore

What differs from .NET

• Effects run on Task, which Fable compiles to a JavaScript promise, rather than ValueTask, which Fable doesn't have: Effect.run returns a Task<Result<'a, 'e>>. Code that uses AsyncResult<'a, 'e>, the type of an effect's result, compiles on both.
• *eff is an ordinary computation expression,* rather than a compiled state machine, as Fable can't translate F#'s resumable code. It binds effects, tasks, Async and Result, and has if, while, for over sequences, try, use and and!.
• Concurrent, not parallel. JavaScript runs one thing at a time, so Effect.par, whenAll, parLimited and race run their effects concurrently, interleaved where they wait, but never in parallel. What they return is the same as on .NET.
• No type tests against interfaces. In JavaScript, :? IClock compiles to false, without a warning, so an effect that checks whether its environment implements an interface never finds it. Require the interface instead, as Effect.Create(fun (clock: #IClock) -> ...) does.

What is .NET only

• Effect.fanOut, effect sequences (effSeq, EffSeq) and Effect.Create of a ValueTask;
• for over an IAsyncEnumerable, and use of an IAsyncDisposable;
• the base effects (Time, Log, CancellationSource, ...), Orsak.Resilience, scopes and CancellationScope;
• Orsak.AspNetCore, Orsak.AspNetCore.OpenApi and Orsak.Myriad.

Some may follow; Supported frameworks lists what each package targets.