lux/docs/JS_WASM_BACKEND_PLAN.md

# Lux JavaScript/WASM Backend Plan

## Goal

Enable Lux to compile to JavaScript and WebAssembly, allowing:
1. **Browser execution** - Run Lux in web browsers
2. **Self-hosted website** - Build lux-lang.org in Lux itself (like Elm)
3. **Universal deployment** - Same code runs on server (native) and client (JS/WASM)

## Research Summary

### How Gleam Does It

[Gleam compiles to JavaScript](https://gleam.run/news/v0.16-gleam-compiles-to-javascript/) with these characteristics:
- **No runtime overhead** - Generated JS looks like human-written code
- **Promise-based concurrency** - Uses native JS promises, not Erlang actors
- **Good interop** - Gleam functions callable from JS/TS directly
- [30% performance improvement](https://gleam.run/news/gleam-javascript-gets-30-percent-faster/) in v1.11.0 through optimization

### How Elm Does It

[Elm compiles to JavaScript](https://guide.elm-lang.org/interop/) with:
- **Virtual DOM** - Efficient updates via diffing
- **Ports** - Explicit interop boundary with JavaScript
- **Small runtime** - Scheduler and virtual DOM
- **Dead code elimination** - Only includes used code
- **Name mangling** - Prefixes with underscore to avoid collisions

---

## Implementation Strategy

### Approach: Parallel to C Backend

Create `src/codegen/js_backend.rs` mirroring `c_backend.rs` structure:

```
src/codegen/
├── c_backend.rs     # Existing: Lux → C → Native
├── js_backend.rs    # New: Lux → JavaScript
└── wasm_backend.rs  # Future: Lux → WASM (via C or direct)
```

### Type Mappings

| Lux Type | JavaScript Type |
|----------|-----------------|
| `Int` | `number` (BigInt for large values) |
| `Float` | `number` |
| `Bool` | `boolean` |
| `String` | `string` |
| `Unit` | `undefined` |
| `List<T>` | `Array` |
| `Option<T>` | `{tag: "Some", value: T} \| {tag: "None"}` |
| `Result<T, E>` | `{tag: "Ok", value: T} \| {tag: "Err", error: E}` |
| `Closure` | `function` (closure environment captured naturally) |
| `ADT` | `{tag: "VariantName", field0: ..., field1: ...}` |

### Code Generation Examples

#### Functions

```lux
fn add(a: Int, b: Int): Int = a + b
```

Generates:
```javascript
function add_lux(a, b) {
    return a + b;
}
```

#### Closures

```lux
fn makeAdder(x: Int): (Int) -> Int = {
    fn(y: Int): Int => x + y
}
```

Generates:
```javascript
function makeAdder_lux(x) {
    return function(y) {
        return x + y;
    };
}
```

#### Pattern Matching

```lux
fn describe(opt: Option<Int>): String = {
    match opt {
        Some(n) => "Got " + toString(n),
        None => "Nothing"
    }
}
```

Generates:
```javascript
function describe_lux(opt) {
    if (opt.tag === "Some") {
        const n = opt.value;
        return "Got " + String(n);
    } else {
        return "Nothing";
    }
}
```

#### ADTs

```lux
type Tree =
    | Leaf
    | Node(Int, Tree, Tree)
```

Generates:
```javascript
// Constructor functions
function Leaf_lux() {
    return { tag: "Leaf" };
}

function Node_lux(value, left, right) {
    return { tag: "Node", field0: value, field1: left, field2: right };
}
```

#### Effects

Effects compile to async/await:

```lux
fn fetchData(): String with Http = {
    Http.get("https://api.example.com/data")
}
```

Generates:
```javascript
async function fetchData_lux(handlers) {
    return await handlers.Http.get("https://api.example.com/data");
}
```

---

## Implementation Phases

### Phase 1: Core Language (2-3 weeks)

| Feature | Effort | Notes |
|---------|--------|-------|
| Basic types (Int, Float, Bool, String) | 2 days | Direct mapping |
| Arithmetic and comparison operators | 1 day | |
| Functions and calls | 2 days | |
| Let bindings | 1 day | |
| If expressions | 1 day | Ternary or if/else |
| Pattern matching (basic) | 3 days | Tag checks, destructuring |
| ADT definitions and constructors | 2 days | Object literals |
| Closures | 2 days | Native JS closures |
| Lists | 2 days | Map to Array |

**Milestone:** Can compile `fib.lux` to JS and run in Node.js

### Phase 2: Standard Library (1-2 weeks)

| Module | Effort | JS Implementation |
|--------|--------|-------------------|
| Console | 1 day | `console.log` |
| String | 2 days | Native string methods |
| List | 2 days | Array methods |
| Math | 1 day | `Math.*` |
| Option/Result | 1 day | Pattern matching |
| JSON | 2 days | `JSON.parse/stringify` |

**Milestone:** Standard library examples work in browser

### Phase 3: Effects in JS (2 weeks)

| Effect | Effort | JS Implementation |
|--------|--------|-------------------|
| Console | 1 day | `console.log`, `prompt()` |
| Http | 3 days | `fetch()` API |
| File | 2 days | Not available in browser (Node.js only) |
| Time | 1 day | `Date.now()`, `setTimeout` |
| Random | 1 day | `Math.random()` |
| DOM (new) | 5 days | New effect for browser manipulation |

**Milestone:** HTTP requests work in browser

### Phase 4: Browser/DOM Support (2-3 weeks)

New `Dom` effect for browser manipulation:

```lux
effect Dom {
    fn querySelector(selector: String): Option<Element>
    fn createElement(tag: String): Element
    fn appendChild(parent: Element, child: Element): Unit
    fn addEventListener(el: Element, event: String, handler: () -> Unit): Unit
    fn setTextContent(el: Element, text: String): Unit
    fn setAttribute(el: Element, name: String, value: String): Unit
    fn getInputValue(el: Element): String
}
```

| Feature | Effort | Notes |
|---------|--------|-------|
| Basic DOM queries | 2 days | querySelector, getElementById |
| Element creation | 2 days | createElement, appendChild |
| Event handling | 3 days | addEventListener with closures |
| Attribute manipulation | 2 days | setAttribute, classList |
| Form handling | 2 days | Input values, form submission |
| Virtual DOM (optional) | 5 days | Efficient updates |

**Milestone:** Can build interactive web page in Lux

### Phase 5: CLI Integration (1 week)

```bash
# Compile to JavaScript
lux compile app.lux --target js -o app.js

# Compile to JavaScript module (ES modules)
lux compile app.lux --target js --module -o app.mjs

# Compile with bundled runtime
lux compile app.lux --target js --bundle -o app.bundle.js

# Run in Node.js
lux run app.lux --target js
```

### Phase 6: WASM Backend (3-4 weeks)

Options:
1. **Lux → C → Emscripten → WASM** (easiest, reuse C backend)
2. **Lux → WASM directly** (more control, harder)
3. **Lux → AssemblyScript → WASM** (middle ground)

Recommended: Start with Emscripten approach, direct WASM later.

---

## Architecture

### JS Backend Module Structure

```rust
// src/codegen/js_backend.rs

pub struct JsBackend {
    output: String,
    indent: usize,
    functions: HashSet<String>,
    name_counter: usize,
}

impl JsBackend {
    pub fn new() -> Self { ... }

    pub fn compile(program: &[Decl]) -> Result<String, JsGenError> { ... }

    fn emit_decl(&mut self, decl: &Decl) -> Result<(), JsGenError> { ... }
    fn emit_function(&mut self, func: &Function) -> Result<(), JsGenError> { ... }
    fn emit_expr(&mut self, expr: &Expr) -> Result<String, JsGenError> { ... }
    fn emit_pattern(&mut self, pattern: &Pattern, value: &str) -> Result<String, JsGenError> { ... }
    fn emit_adt(&mut self, adt: &TypeDecl) -> Result<(), JsGenError> { ... }

    // JS-specific
    fn emit_runtime(&mut self) { ... }  // Minimal runtime helpers
    fn emit_effect_handlers(&mut self, effects: &[Effect]) { ... }
}
```

### Runtime (Minimal)

```javascript
// Lux JS Runtime (embedded in generated code)
const Lux = {
    // Option helpers
    Some: (value) => ({ tag: "Some", value }),
    None: () => ({ tag: "None" }),

    // Result helpers
    Ok: (value) => ({ tag: "Ok", value }),
    Err: (error) => ({ tag: "Err", error }),

    // List helpers
    Cons: (head, tail) => [head, ...tail],
    Nil: () => [],

    // Effect handler invoker
    handle: async (computation, handlers) => {
        return await computation(handlers);
    }
};
```

---

## Browser Integration

### Entry Point

```lux
// app.lux
fn main(): Unit with Dom = {
    let button = Dom.createElement("button")
    Dom.setTextContent(button, "Click me!")
    Dom.addEventListener(button, "click", fn(): Unit => {
        Dom.setTextContent(button, "Clicked!")
    })
    let body = Dom.querySelector("body")
    match body {
        Some(el) => Dom.appendChild(el, button),
        None => ()
    }
}
```

Compiles to:
```javascript
async function main_lux(handlers) {
    const button = handlers.Dom.createElement("button");
    handlers.Dom.setTextContent(button, "Click me!");
    handlers.Dom.addEventListener(button, "click", function() {
        handlers.Dom.setTextContent(button, "Clicked!");
    });
    const body = handlers.Dom.querySelector("body");
    if (body.tag === "Some") {
        handlers.Dom.appendChild(body.value, button);
    }
}

// Browser handler
const BrowserDom = {
    createElement: (tag) => document.createElement(tag),
    setTextContent: (el, text) => { el.textContent = text; },
    addEventListener: (el, event, handler) => el.addEventListener(event, handler),
    querySelector: (sel) => {
        const el = document.querySelector(sel);
        return el ? Lux.Some(el) : Lux.None();
    },
    appendChild: (parent, child) => parent.appendChild(child)
};

// Initialize
main_lux({ Dom: BrowserDom });
```

### HTML Integration

```html
<!DOCTYPE html>
<html>
<head>
    <title>Lux App</title>
</head>
<body>
    <script src="app.js"></script>
    <script>
        // Generated initialization code
        Lux.main({ Dom: BrowserDom });
    </script>
</body>
</html>
```

---

## Self-Hosting the Website

Once the JS backend is complete, the lux-lang.org website can be built in Lux:

```lux
// website/src/Main.lux

import Html.{div, h1, p, button, text}
import App.{Model, Msg, init, update, view}

fn main(): Unit with Dom = {
    let model = init()
    let root = Dom.querySelector("#app")
    match root {
        Some(el) => render(el, model),
        None => Console.print("No #app element found")
    }
}

fn render(root: Element, model: Model): Unit with Dom = {
    let html = view(model)
    Dom.setInnerHtml(root, html)
}
```

---

## Testing Strategy

### Unit Tests

```rust
#[test]
fn test_js_function_generation() {
    let input = "fn add(a: Int, b: Int): Int = a + b";
    let js = JsBackend::compile(input).unwrap();
    assert!(js.contains("function add_lux(a, b)"));
    assert!(js.contains("return a + b"));
}
```

### Integration Tests

Run generated JS in Node.js and compare output:

```rust
#[test]
fn test_fibonacci_js() {
    let lux_code = include_str!("../../examples/fibonacci.lux");
    let js_code = JsBackend::compile(lux_code).unwrap();

    let output = Command::new("node")
        .arg("-e")
        .arg(&js_code)
        .output()
        .unwrap();

    assert!(String::from_utf8_lossy(&output.stdout).contains("fib(10) = 55"));
}
```

### Browser Tests

Use Playwright/Puppeteer to test DOM manipulation:

```javascript
test('button click works', async ({ page }) => {
    await page.goto('http://localhost:3000/test.html');
    await page.click('button');
    expect(await page.textContent('button')).toBe('Clicked!');
});
```

---

## Timeline

| Phase | Duration | Milestone |
|-------|----------|-----------|
| Phase 1: Core Language | 2-3 weeks | Fibonacci runs in Node.js |
| Phase 2: Standard Library | 1-2 weeks | Examples work in browser |
| Phase 3: Effects | 2 weeks | HTTP works in browser |
| Phase 4: DOM Support | 2-3 weeks | Interactive page in Lux |
| Phase 5: CLI Integration | 1 week | `lux compile --target js` |
| Phase 6: WASM | 3-4 weeks | WASM execution |

**Total: 11-15 weeks** for full JS/WASM support

---

## Success Criteria

1. **Correctness**: All existing tests pass when targeting JS
2. **Performance**: Within 2x of hand-written JS for benchmarks
3. **Size**: Generated JS is reasonable size (< 2x hand-written equivalent)
4. **Interop**: Easy to call Lux from JS and JS from Lux
5. **Self-hosting**: lux-lang.org runs entirely on Lux-compiled JS

---

## References

- [Gleam JS Compilation](https://gleam.run/news/v0.16-gleam-compiles-to-javascript/)
- [Elm JavaScript Interop](https://guide.elm-lang.org/interop/)
- [Koka JavaScript Backend](https://koka-lang.github.io/koka/doc/book.html)
- [PureScript Code Generation](https://github.com/purescript/purescript/tree/master/src/Language/PureScript/CodeGen)