# Introduction

> What Masque is, what it is for, and how the pieces fit together.

Masque is a UI language and embeddable runtime for Rust applications. You describe your interface in Masque documents (`.masque` files). Masque compiles them to bytecode, runs them on a small stack VM inside your process, and reconciles the result into a live widget tree that a renderer draws. A Skia backend ships with the crate.

## Who it is for

Masque is for Rust programs that already own a window and a frame loop and want a better way to build their interface: games, creative tools, editors, and embedded displays. It is a library you call each frame, not a framework your app moves into.

## How it fits together

Your application owns all the state and logic. A Masque document is a projection of that state:

- **Host state flows in.** You hand the runtime values by name, and the document reads them as plain identifiers.
- **Events flow out.** The document calls `event("name", ...)`, and your registered handler decides what happens.
- **The document never mutates your app.** It cannot reach in, so every change to your data happens in your Rust code.

Inside the runtime, a document passes through four stages, each depending only on the one before it:

| Stage | Produces |
| --- | --- |
| Scanner and parser | An AST |
| Compiler | Bytecode, compiled once per load |
| VM | Widget descriptors, re-run when state changes |
| Builder and reconciler | Updates to the live widget tree, applied in place |

The widget tree does layout (flexbox and grid), springs, hit-testing, and events. Only painting goes through the `Surface` trait, which is why the renderer can be swapped.

## Where to go next

- [Installation](/docs/installation): add the crate and its native dependencies.
- [Quick start](/docs/quick-start): from an empty project to a hot-reloading window.
- [Design principles](/docs/principles): the rules a document can rely on.