lux/README.md

# Lux

A functional programming language with first-class effects, schema evolution, and behavioral types.

## Vision

Most programming languages treat three critical concerns as afterthoughts:

1. **Effects** — What can this code do? (Hidden, untraceable, untestable)
2. **Data Evolution** — Types change, data persists. (Manual migrations, runtime failures)
3. **Behavioral Properties** — Is this idempotent? Does it terminate? (Comments and hope)

Lux makes these first-class language features. The compiler knows what your code does, how your data evolves, and what properties your functions guarantee.

## Core Principles

### 1. Effects Are Explicit and Composable

```lux
fn fetchUser(id: UserId): User with {Database, Http} =
  let profile = Http.get("/users/{id}")
  let prefs = Database.query(userPrefsQuery(id))
  User.merge(profile, prefs)

-- Testing: swap real effects for mocks
test "fetchUser returns merged data" =
  run fetchUser(testId) with {
    Database = mockDb({ testId: testPrefs }),
    Http = mockHttp({ "/users/{testId}": testProfile })
  }
  |> Assert.eq(expectedUser)
```

No hidden side effects. No dependency injection boilerplate. Effects are declared, handlers are swappable, composition just works.

### 2. Schema Evolution Is Built-In

```lux
type User @v1 {
  name: String,
  email: String
}

type User @v2 {
  name: String,
  email: String,
  age: Option<Int>          -- optional field: auto-compatible
}

type User @v3 {
  fullName: String,         -- renamed: requires migration
  email: String,
  age: Option<Int>,

  from @v2 = { fullName: v2.name, ..v2 }
}
```

The compiler tracks compatibility. Breaking changes are compile errors. Migrations are code, not config.

### 3. Behavioral Types Are First-Class

```lux
fn retry<F, T>(action: F): Result<T, Error>
  where F: fn() -> T with {Fail},
  where F is idempotent                    -- enforced!
=
  match action() {
    Ok(v) => Ok(v),
    Err(_) => action()                     -- safe: we know it's idempotent
  }

fn sort<T: Ord>(list: List<T>): List<T>
  is pure,
  is total,
  where result.len == list.len,
  where result.isSorted
```

Properties like `pure`, `total`, `idempotent`, `commutative` are part of the type system. The compiler proves what it can, tests what it can't.

## Example

```lux
-- Define an effect
effect Logger {
  fn log(level: Level, msg: String): Unit
}

-- Define a versioned type
type Config @v1 {
  host: String,
  port: Int
}

type Config @v2 {
  host: String,
  port: Int,
  timeout: Duration,

  from @v1 = { timeout: Duration.seconds(30), ..v1 }
}

-- A function with explicit effects and properties
fn loadConfig(path: Path): Config @v2 with {FileSystem, Logger}
  is total
=
  Logger.log(Info, "Loading config from {path}")
  let raw = FileSystem.read(path)
  Config.parse(raw)

-- Run with handlers
fn main(): Unit with {Console} =
  let config = run loadConfig("./config.json") with {
    FileSystem = realFs,
    Logger = consoleLogger
  }
  Console.print("Loaded: {config}")
```

## Status

**Current Phase: Prototype Implementation**

The interpreter is functional with:
- Core language (functions, closures, pattern matching)
- Effect system (declare effects, use operations, handle with handlers)
- Type checking with effect tracking
- REPL for interactive development

See:
- [SKILLS.md](./SKILLS.md) — Language specification and implementation roadmap
- [docs/VISION.md](./docs/VISION.md) — Problems Lux solves and development roadmap
- [docs/OVERVIEW.md](./docs/OVERVIEW.md) — Use cases, pros/cons, complexity analysis

## Design Goals

| Goal | Approach |
|------|----------|
| **Correctness by default** | Effects, schemas, and behaviors are compiler-checked |
| **Incremental adoption** | Start simple, add properties/versions as needed |
| **Zero-cost abstractions** | Effect handlers inline, versions compile away |
| **Practical, not academic** | Familiar syntax, clear errors, gradual verification |

## Non-Goals

- Not a systems language (no manual memory management)
- Not a scripting language (static types required)
- Not a proof assistant (verification is practical, not total)

## Building

Requires Rust 1.70+:

```bash
# Build the interpreter
cargo build --release

# Run the REPL
cargo run

# Run a file
cargo run -- examples/hello.lux

# Run tests
cargo test
```

## Examples

See the `examples/` directory:

- `hello.lux` — Hello World with effects
- `factorial.lux` — Recursive functions
- `effects.lux` — Custom effects and handlers
- `datatypes.lux` — ADTs and pattern matching
- `functional.lux` — Higher-order functions and pipes

### Quick REPL Session

```
$ cargo run
Lux v0.1.0
Type :help for help, :quit to exit

lux> let x = 42
lux> x * 2
84
lux> fn double(n: Int): Int = n * 2
lux> double(21)
42
lux> [1, 2, 3] |> List.reverse
[3, 2, 1]
lux> List.map([1, 2, 3], double)
[2, 4, 6]
lux> String.split("a,b,c", ",")
["a", "b", "c"]
lux> Some(42) |> Option.map(double)
Some(84)
lux> :quit
```

## Contributing

This project is in early design. Contributions welcome in:
- Language design discussions (open an issue)
- Syntax bikeshedding
- Semantic formalization
- Compiler implementation (once design stabilizes)

## License

MIT