1open System
2open System.Net.Http
3
4let httpClient = new HttpClient()
5
6let fetchText (url: string) =
7    async {
8        let! response = httpClient.GetStringAsync(url) |> Async.AwaitTask
9        return response
10    }

let fetchMany urls =
    urls
|> List.map fetchText |> Async.Parallel let run () = async { let urls = [ "https://example.com"; "https://example.org" ] let! pages = fetchMany urls printfn "Fetched %d pages" pages.Length } ``` This is clearer and faster than manually starting child workflows with mutable coordination. ### Propagate cancellation explicitly Long-running operations should honor cancellation tokens. ```fsharp open System.Threading let delayWork ms = async { do! Async.Sleep ms return "done" } let runWithCancellation () = use cts = new CancellationTokenSource(1000) Async.StartAsTask(delayWork 5000, cancellationToken = cts.Token) ``` Treat cancellation as a normal control path, not an exceptional edge case. ### Standardize error handling strategy Use `try/with` around external I/O and convert failures into domain-specific results where useful. ```fsharp type FetchResult = | Ok of string | Failed of string let safeFetch url = async { try let! text = fetchText url return Ok text with ex -> return Failed ex.Message } ``` This keeps call sites simple and reduces exception-driven control flow across layers. ### Bridge `Task` and `Async` intentionally Many .NET libraries return `Task`. Convert at boundaries, not everywhere. ```fsharp open System.Threading.Tasks let readAllAsync (path: string) = System.IO.File.ReadAllTextAsync(path) |> Async.AwaitTask let saveAllAsync (path: string) (content: string) = async { do! System.IO.File.WriteAllTextAsync(path, content) |> Async.AwaitTask } ``` Boundary conversion keeps internal code consistent and easier to reason about. ### Prefer pure workflow composition over manual threading Avoid starting async work imperatively unless needed. Compose workflows and run at application edges. ```fsharp let program = async { let! a = delayWork 200 let! b = delayWork 300 return a + " & " + b } Async.RunSynchronously program ``` This separation improves testability and prevents orphaned background operations. ## Common Pitfalls * Mixing `Task` and `Async` types throughout code without a clear conversion boundary. * Using `Async.RunSynchronously` deep inside libraries, causing blocking behavior in unexpected places. * Ignoring cancellation tokens for I/O-heavy workflows and leaving operations uninterruptible. * Starting many async jobs manually with `Async.Start` and losing error and completion visibility. * Treating exceptions inconsistently across modules instead of standardizing result-based handling where appropriate. ## Summary Simplifying asynchronous programming in F# is mostly about consistency: compose with `async {}`, run independent work through `Async.Parallel`, propagate cancellation, and choose a clear error model. Keep `Task` interop at boundaries and execute workflows at the application edge rather than inside core library code. These patterns reduce incidental complexity and make async behavior easier to reason about, test, and maintain. As workloads grow, add lightweight instrumentation around workflow latency and cancellation rates. Visibility into async behavior helps you tune parallelism limits and avoid resource saturation. Consistent patterns plus basic metrics provide both readability and operational confidence.