loon

A functional language with invisible types, safe ownership, and algebraic effects.

Try it in your browser →

[fn greet [name]
  [str "Hello, " name "!"]]

[pipe [range 1 10]
  [filter [fn [n] [> n 4]]]
  [map [fn [n] [* n n]]]
  [each println]]

Features

• Type Inference — Full Hindley-Milner with let-polymorphism and row types. No annotations required.
• Ownership — Rust-style move semantics and borrow checking, inferred from dataflow. No lifetimes.
• Algebraic Effects — Declare, perform, and handle effects. Replaces exceptions, async, and mutable state.
• Pattern Matching — Match on literals, ADT constructors, and wildcards with destructuring.
• Macros — Template macros with quasiquoting. Procedural macros with compile-time IO.
• Pipes — Thread data through transformation chains. No nesting, no temp variables.
• Language Server — Diagnostics, hover types, go-to-definition, completions, inlay hints.
• WASM — Compiles to WebAssembly with closures, ADTs, and tree-shaking.

Quick Start

curl -fsSL https://loonlang.com/install.sh | sh

loon run hello.loon        # Run a program
loon repl                  # Interactive REPL with time-travel
loon fmt hello.loon        # Format source
loon explain E0201         # Interactive error tutorial

Architecture

loon/
├── crates/
│   ├── loon-lang/        # Core: parser, type checker, interpreter
│   ├── loon-cli/         # CLI: run, repl, fmt, explain
│   ├── loon-lsp/         # Language server protocol
│   └── loon-wasm/        # WASM bindings for browser
├── web/                  # Website (written in Loon)
│   ├── public/           # Static assets, WASM bootstrap
│   └── src/              # Loon source: pages, components, router
├── tree-sitter-loon/     # Tree-sitter grammar
├── samples/              # Example programs
└── tests/                # Test suite

Development

# Run tests
cargo test --workspace

# Build WASM + dev server
cd web && npm run dev

# Build language server
cargo build -p loon-lsp --release

License

MIT