Lux

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

Philosophy

Make the important things visible.

Most languages hide what matters most: what code can do (effects), how data changes over time (schema evolution), and what guarantees functions provide (behavioral properties). Lux makes all three first-class, compiler-checked language features.

Principle	What it means
Explicit over implicit	Effects in types — see what code does
Composition over configuration	No DI frameworks — effects compose naturally
Safety without ceremony	Type inference + explicit signatures where they matter
Practical over academic	Familiar syntax, ML semantics, no monads
One right way	Opinionated formatter, integrated tooling, built-in test framework
Tools are the language	lux fmt/lint/check/test/compile — one binary, not seven tools

See docs/PHILOSOPHY.md for the full philosophy with language comparisons and design rationale.

Core Principles

1. Effects Are Explicit and Composable

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

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

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

-- 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

Core Language: Complete

• Full type system with Hindley-Milner inference
• Pattern matching with exhaustiveness checking
• Algebraic data types, generics, string interpolation
• Effect system with handlers
• Behavioral types (pure, total, idempotent, deterministic, commutative)
• Schema evolution with version tracking

Compilation Targets:

• Interpreter (full-featured)
• C backend (functions, closures, pattern matching, lists, reference counting)
• JavaScript backend (full language, browser & Node.js, DOM, TEA runtime)

Tooling:

• REPL with history
• LSP server (diagnostics, hover, completions, go-to-definition)
• Formatter (lux fmt)
• Package manager (lux pkg)
• Watch mode / hot reload

Standard Library:

• String, List, Option, Result, Math, JSON modules
• Console, File, Http, Random, Time, Process effects
• SQL effect (SQLite with transactions)
• PostgreSQL effect (connection pooling ready)
• DOM effect (40+ browser operations)

See:

• docs/ROADMAP.md — Development roadmap and feature status
• 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

With Nix (recommended)

# Build
nix build

# Run the REPL
nix run

# Enter development shell
nix develop

# Run tests
nix develop --command cargo test

With Cargo

Requires Rust 1.70+:

cargo build --release
./target/release/lux           # REPL
./target/release/lux file.lux  # Run a file
cargo test                     # Tests

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