F# Online Compiler - Run F# Without Visual Studio

F# is the functional-first language on .NET that doesn't get the attention it deserves. It has the pipe operator that Elixir fans love, the pattern matching that OCaml is known for, and full access to the entire .NET ecosystem. You can call any C# library from F#. You can build web APIs, process data, write scripts.

The problem is getting started. You need the .NET SDK. Depending on your OS, that's either straightforward or a 20-minute yak shave. And if you just want to try F# or test a quick idea, installing an SDK feels like overkill.

That's where an F# online compiler makes sense. Open a browser, write code, run it.

Pipes and transformations

The pipe operator (|>) is F#'s signature feature. It takes the result of one expression and passes it as the last argument to the next function. Simple idea, big impact on readability.

Run this on OneCompiler:

// Analyze a list of transactions
type Transaction = { Description: string; Amount: float; Category: string }

let transactions = [
    { Description = "Coffee"; Amount = 4.50; Category = "food" }
    { Description = "Laptop stand"; Amount = 45.00; Category = "office" }
    { Description = "Lunch"; Amount = 12.00; Category = "food" }
    { Description = "Keyboard"; Amount = 89.00; Category = "office" }
    { Description = "Groceries"; Amount = 67.30; Category = "food" }
    { Description = "Monitor cable"; Amount = 15.00; Category = "office" }
]

// Spending summary per category using pipes
transactions
|> List.groupBy (fun t -> t.Category)
|> List.map (fun (cat, items) ->
    let total = items |> List.sumBy (fun t -> t.Amount)
    let count = List.length items
    (cat, total, count))
|> List.sortByDescending (fun (_, total, _) -> total)
|> List.iter (fun (cat, total, count) ->
    printfn "%s: $%.2f across %d transactions" cat total count)

printfn "\nTotal spent: $%.2f"
    (transactions |> List.sumBy (fun t -> t.Amount))

Each pipe step is one clear transformation. Group by category. Map to totals. Sort. Print. You can read it top to bottom and know exactly what's happening.

Discriminated unions for modeling

F# discriminated unions are one of the best ways to model domain logic. They're like enums that can carry data, and the compiler makes sure you handle every case.

type Shape =
    | Circle of radius: float
    | Rectangle of width: float * height: float
    | Triangle of base_: float * height: float

let area = function
    | Circle r -> System.Math.PI * r * r
    | Rectangle (w, h) -> w * h
    | Triangle (b, h) -> 0.5 * b * h

let describe shape =
    printfn "%A -> area = %.2f" shape (area shape)

[Circle 5.0; Rectangle (3.0, 4.0); Triangle (6.0, 8.0)]
|> List.iter describe

Add a new shape variant and forget to handle it in area? The compiler tells you. This is the same kind of safety that makes OCaml popular at trading firms, but in the .NET ecosystem.

Why bother with an online F# compiler

A few concrete reasons:

You're learning F#. The .NET SDK is a big install just to try a language. Running F# online lets you write real code before committing to anything.

You need to share a snippet. OneCompiler gives you a shareable URL. Drop it in a pull request comment, a Slack thread, or a Stack Overflow answer.

You're on a machine without .NET. Maybe you're on a Chromebook. Maybe you're SSH'd into a server and want to test some F# logic. A browser is all you need.

You want fast iteration. Write, run, see output. No dotnet build, no project files, no Program.fs boilerplate.

Try it

F# Online Compiler on OneCompiler. Paste either example above, modify it, see what happens. The pipe operator alone is worth the visit.