Which language versions?

F# 8, 9, 10 — all backed by the current .NET SDK.

No network — bring source only.

F# · online compiler

Run F# online

Run F# online — functional-first .NET, browser-hosted.

Available versions

Firecracker microVM live per-test verdicts multi-file compile

F# is Microsoft's functional-first .NET language: strong types, discriminated unions, pattern matching, type providers, and the whole .NET ecosystem underneath. Loved for data pipelines and financial modeling.

ls-judge runs F# through the .NET SDK inside a Firecracker microVM.

Starter code

What you get in F#

Clicking "Run now" opens the playground with this exact code and a matching test case pre-loaded — passes on first Run.

F# 10

[<EntryPoint>]
let main _ =
    let n = System.Console.ReadLine().Trim() |> int
    printfn "%d" (n * 2)
    0

stdin →

expected →

FAQ

Common questions about F# on ls-judge

Which language versions?: F# 8, 9, 10 — all backed by the current .NET SDK.
NuGet packages?: No network — bring source only.
FAKE / Paket?: Not in the base sandbox.
Free?: Yes.