Concurrency vs. Parallelism

Concurrency vs. Parallelism

In this lesson — part of Concurrency Basics — you'll learn concurrency vs. parallelism in F# and why it matters in real work.

Why it matters

Modern programs wait on networks and disks — async lets them stay responsive.

Key ideas

• Blocking vs. non-blocking
• Callbacks, promises, async/await
• Concurrency vs. parallelism
• Error handling in async code

In practice

Here's how it looks in idiomatic F#:

open System.Net.Http

// 'async { }' builds a cold computation; nothing runs until started
let fetchLength (url: string) = async {
    use client = new HttpClient()
    let! html = client.GetStringAsync url |> Async.AwaitTask  // let! = await
    return html.Length
}

// Run several in parallel, collect results
[ "https://example.com"; "https://www.fsharp.org" ]
|> List.map fetchLength
|> Async.Parallel
|> Async.RunSynchronously
|> printfn "%A"

F# note: F#'s async blocks are cold and composable — they don't start until Async.Start/RunSynchronously, unlike C#'s hot Tasks — and let! is the await that propagates cancellation automatically.

Try it yourself

Exercise: In F#, fetch two things concurrently and combine the results.

Recap

You now understand concurrency vs. parallelism and can apply it in F#. Mark this lesson complete and continue to the next one.