如何在 F# 中使用 Async 和 AsyncResult

Question

Using the FsToolkit.ErrorHandling library, let's take an example:

let doStuff =
    result {
        let! x = doSomething "hello"
        let! y = doAnotherthing "world"
        let  z = combineTwo x y
        return z
    }

now, I have an async version of combinedTwo called combinedTwoAsync:

let doStuff =
    result {
        let! x = doSomething "hello"
        let! y = doAnotherthing "world"
        let  z = combineTwoAsync x y |> Async.AwaitTask |> Async.RunSynchronously
        return z
    }

but I saw that they have an asyncResult expression which returns an Async<Result<'a, 'b>> type

however, I can't do something like:

let doStuff =
    asyncResult {
        let! x = doSomething "hello"
        let! y = doAnotherthing "world"
        let! z = combineTwoAsync x y
        return z
    }

since doSomethingAsync is just an async function that doesn't return a Result object

I guess this was planned with the idea that all async functions would return an Async<Result<>> type, but I'm trying to figure out how to use async calls (which are to external libs) within a result computation expression to return an Async<Result<>>..

hopefully this makes some sense:)

Answer 1

In general, for functors (of which async is one), one could use map to apply some sort of transformation to the functor's value, such as wrapping it in a Result.Ok , for example:

// xAsyncResult : Async<Result<_,_>>
let xAsyncResult = doSomething "hello" |> Async.map Result.Ok

Surprisingly, Async.map is a not a thing. There's even an issue about it .

But no matter: it's easy enough to implement:

module Async =
  map f a = async {
    let! x = a
    return f a
  }

And then:

let doStuff =
    asyncResult {
        let! x = doSomething "hello" |> Async.map Result.Ok
        let! y = doAnotherthing "world" |> Async.map Result.Ok
        let! z = combineTwoAsync x y
        return z
    }