Your first useful FScript in five minutes

You will build a small pricing rule, run it, and see the language features that make FScript useful inside a host application.

Zero-install route

Want to touch the language before installing anything? Open the browser playground, change a number in the first example, and press Run.

1. Install the CLI

Choose one route. The .NET tool works anywhere supported by the .NET SDK; Homebrew is the shortest path on macOS.

.NET tool

dotnet tool install --global MagnusOpera.FScript

Homebrew

brew install magnusopera/tap/fscript

Check the installation:

fscript version

Building from source instead

git clone https://github.com/MagnusOpera/FScript.git
cd FScript
make build

Run scripts with dotnet run --project src/FScript -- your-script.fss.

2. Create a rule

Create pricing.fss:

type Plan =
  | Starter
  | Team
  | Scale

let pricePerSeat plan =
  match plan with
  | Starter -> 9
  | Team -> 19
  | Scale -> 29

let quote plan seats =
  pricePerSeat plan * seats

let total = quote Scale 12
Console.writeLine $"12 Scale seats: ${total}/month"

Run it:

fscript pricing.fss

You should see:

12 Scale seats: $348/month

You have already used four important ideas:

• type Plan names the only valid plan choices.
• match handles every choice explicitly.
• let quote plan seats declares a function without type boilerplate.
• the compiler infers the types and checks the program before it runs.

Try removing the Scale branch or calling quote "Scale" 12. The type checker will stop the mistake at the script boundary.

3. Make the rule host-callable

An embedded script exposes only the bindings you mark as part of its host contract. Add the export annotation:

[<export>] let quote plan seats =
  pricePerSeat plan * seats

Your .NET or JavaScript host can now load the script and invoke quote by name. Everything else remains an implementation detail of the script.

• Follow the complete .NET host example.
• Use FScript from JavaScript or a browser host.
• Learn how typed externs let scripts call capabilities supplied by your application.

4. Pick your next move

If you want to…	Go here
Learn the core syntax quickly	Language Tour
Change and run examples	Browser Playground
Build something recognizable	Guided Examples
Put FScript inside an application	Embedding Overview

REPL and script arguments

Run fscript with no file to start an interactive session:

fscript

Pass script arguments after --:

fscript pricing.fss -- acme scale

The script receives them as the string list value Env.Arguments.

Editor setup

The official VS Code extension adds diagnostics, completion, hover, go-to-definition, references, rename, semantic tokens, and inlay hints.

• VS Code Marketplace
• Open VSX

See Editor Setup for the full walkthrough.