> For the complete documentation index, see [llms.txt](https://osl.mistium.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://osl.mistium.com/introduction/readme.md).

# What is OSL?

**OSL** is a small, fast, compiled programming language. You write `.osl` source files in a clean, readable syntax and the OSL compiler turns them into a **single self-contained native binary** - no interpreter, no dependencies to ship.

```javascript
import "osl/serve"

string port = "8080"
*serve.Router app = serve.new()

app.GET("/", def(*serve.Context c) -> (
  c.string(200, "Hello from OSL!")
))

log "Listening on http://localhost:" ++ port
app.serve(":" ++ port)
```

Compile it and run the binary:

```bash
osl compile server.osl   # produces ./server
./server
```

## What is it good for?

OSL is designed for writing **servers, command-line tools, scripts, and small applications** quickly, with a batteries-included standard library. Out of the box you get HTTP servers and clients, WebSockets, an embedded SQL database, JSON/YAML/CSV/XML, cryptography, file-system and process utilities, a terminal-UI toolkit, image/PDF/QR generation, and much more - all documented in the [Packages](/packages/packages.md) section.

Because programs compile to a native binary, they start instantly and run fast, while the language itself stays approachable and dynamic-feeling.

## Key ideas

* **Readable, command-style syntax.** Code reads top-to-bottom. There is no `main()` to write - the file *is* the program (see [The Execution Model](/introduction/getting-started.md#the-execution-model)).
* **Optional types.** Variables and function parameters can be untyped (`x = 5`) or typed (`int x = 5`). Types are checked at compile time when you use them.
* **A rich standard library.** Capabilities are grouped into [packages](/packages/packages.md) you pull in with `import "osl/<name>"`.
* **Methods and functions everywhere.** Values have methods (`"hi".toUpper()`, `[1,2,3].map(...)`), and there is a large set of built-in [functions](/builtins/math.md).

## Where to go next

| If you want to…                          | Read                                                                                                                                                                  |
| ---------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Install OSL and write your first program | [Getting Started](/introduction/getting-started.md)                                                                                                                   |
| Learn the language                       | [Types](/the-language/types.md) · [Syntax](/the-language/syntax.md) · [Functions](/functions-and-classes/functions.md) · [Classes](/functions-and-classes/classes.md) |
| Browse the standard library              | [Packages](/packages/packages.md)                                                                                                                                     |
| Build a web server                       | [`osl/serve`](/packages/web/serve.md)                                                                                                                                 |
| See the old originOS graphical OSL       | [Legacy OSL](/legacy-osl-originos/legacy-osl.md)                                                                                                                      |

***

> **A note on history.** OSL began as the scripting language of **originOS**, a graphical desktop environment, where every program was a window. This documentation covers **OSL.go**, the modern compiler that brings the same syntax to standalone native programs. Most language features carry over; the windowing/graphics features now live in the [`osl/window`](/graphics-osl-window/window.md) package, and originOS-only features are preserved for reference under [Legacy OSL](/legacy-osl-originos/legacy-osl.md).