Hello, World
printfn "Hello, World!"Program.fs
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F# 10.0.302ReadyProgram.fsLn 1, Col 1Spaces: 4UTF-8
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Program.fs · output appears here
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Output streams here as your program runs.
F# is a strongly typed, functional-first programming language on the .NET platform. It combines concise, expression-oriented syntax with powerful type inference, immutability by default, discriminated unions, pattern matching, and seamless interop with the rest of .NET. CompileBytes builds and runs F# with the .NET SDK toolchain version 5.0.201 in an isolated hosted runtime, so you can write idiomatic functional code without configuring a project.
While F# embraces functional programming, it is fully multi-paradigm and supports objects, classes, and imperative code when needed. Features such as pipelines with the |> operator, lightweight records, and computation expressions make it expressive for data processing, scripting, and application logic. Because it runs on .NET, F# programs have access to the large Base Class Library and the broader NuGet ecosystem.
Program.fs is the entry point for each run.
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printfn "Hello, World!"let square x = x * x
printfn "%d" (square 7)[1..5]
|> List.map (fun x -> x * x)
|> List.iter (printfn "%d")let describe n =
match n with
| 0 -> "zero"
| x when x < 0 -> "negative"
| _ -> "positive"
printfn "%s" (describe -3)let rec factorial n =
if n <= 1 then 1
else n * factorial (n - 1)
printfn "%d" (factorial 6)type Shape =
| Circle of float
| Rectangle of float * float
let area shape =
match shape with
| Circle r -> 3.14159 * r * r
| Rectangle (w, h) -> w * h
printfn "%.2f" (area (Circle 2.0))type Person = { Name: string; Age: int }
let p = { Name = "Ada"; Age = 36 }
printfn "%s is %d" p.Name p.Agelet safeDiv a b =
if b = 0 then None
else Some (a / b)
match safeDiv 10 2 with
| Some r -> printfn "%d" r
| None -> printfn "cannot divide"let nums = [1; 2; 3; 4; 5]
let total = List.fold (+) 0 nums
printfn "Sum: %d" totalThe editor uses Monaco, the engine behind VS Code, with language-aware editing tools and familiar keyboard controls.
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F# is one of 46 single-file compiler scratchpads. Each run submits only Program.fs; local sibling-file imports are not enabled.
Linked-project languages (23): Node.js, TypeScript, Deno, Python, C, Go, Dart, Swift, Rust, Ruby, PHP, Java, Kotlin, C++, C#, Nim, Crystal, Haxe, Zig, Scala, D, Bun, Objective-C.
Single-file scratchpads (46): Matplotlib, NumPy, pandas, Haskell, Clojure, Perl, Lua, Tcl, jq, Raku, Elixir, Bash, Zsh, Nushell, R, Assembly (NASM), Julia, Octave, Groovy, PowerShell, Visual Basic .NET, Odin, Pascal, Lisp, Erlang, CoffeeScript, COBOL, BASIC, V, Racket, OCaml, Prolog, AWK, Emacs Lisp, Emojicode, Forth, F#, Fortran, Assembly (NASM64), Solidity, Mojo, Ada, WASM (WAT), Gleam, Shell Script (sh), Scheme.
Download file exports the active F# source file.