feature/issue-305-parallel-async-result-validation

[njlr]

Proposed Changes

• Add Async.parallelMap2
• Add Async.parallelMap3
• Add Async.parallelZip
• Add parallelAsyncResult
• Add parallelAsyncValidation
• Add tests

The main motivation is to be able to run Async<Result<_, _>> and AsyncValidation<_> workflows concurrently:

parallelAsyncResult {
  let! a = foo
  and! b = bar

  return a, b
}

parallelAsyncValidation {
  let! a = foo
  and! b = bar

  return a, b
}

I appreciate this is a pretty big PR - can split it up if needed!

Types of changes

What types of changes does your code introduce to FsToolkit.ErrorHandling?
Put an x in the boxes that apply and remove ones that don't apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Further comments

Deviating from what was discussed here, I did not use Async.StartChild or Async.StartImmediateAsTask, instead opting for an Async.Parallel based implementation. Why?

Async.StartChild

This has the issue of not failing fast in the case of an exception.

let zipWithStartChild (a1: Async<'left>) (a2: Async<'right>) : Async<'left * 'right> =
    async.Bind(
        Async.StartChild a1,
        fun c1 ->
            async.Bind(
                Async.StartChild a2,
                fun c2 ->
                    async.Bind(c1, (fun r1 -> async.Bind(c2, (fun r2 -> async.Return(r1, r2)))))
            )
    )

open System

async {
    let! _ =
        zipWithStartChild 
            (Async.Sleep(TimeSpan.FromHours 1)) 
            (async { return failwith "Kaboom" })

    ()
}
|> Async.RunSynchronously

This terminates in 1 hour (probably).

It's order sensitive, which I think is a bit of a foot-gun; this terminates immediately:

async {
    let! _ =
        zipWithStartChild 
            (async { return failwith "Kaboom" }) 
            (Async.Sleep(TimeSpan.FromHours 1))

    ()
}
|> Async.RunSynchronously

In contrast, this fails immediately regardless of ordering:

open System

async {
    let! _ =
        Async.Parallel [|
            Async.Sleep(TimeSpan.FromHours 1)
            async { return failwith "Kaboom" }
        |]

    ()
}
|> Async.RunSynchronously

Async.StartImmediateAsTask

Tasks are not available in JavaScript Fable environments, whereas Async.Parallel is well supported across Fable targets.

[njlr]

@TheAngryByrd Are you able to kill https://github.com/demystifyfp/FsToolkit.ErrorHandling/actions/runs/13728714344 ? Seems to be stuck

[TheAngryByrd]

Fantastic work ❤️.

Would you mind adding a few pages to the gitbook/ showing off a few examples in ce.md, map2.md, and map3.md?

[njlr]

Fantastic work ❤️.

Would you mind adding a few pages to the gitbook/ showing off a few examples in ce.md, map2.md, and map3.md?

Done 👍

[TheAngryByrd]

Thank you!