loon

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

Try it in your browser →

[fn greet [name]
[println "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.
• Multi-shot continuations — Handlers resume a captured continuation zero, one, or many times: nondeterministic search, backtracking, and durable replay from one program.
• Handler towers — One program runs unchanged under different handlers: deterministic offline tests, a recording replay, or real production IO. Swap the tower, not the code.
• Async as effects — Concurrency is ordinary effectful code with the scheduler as a handler: cooperative spawn/yield/cancel with structured concurrency. No function coloring.
• Pattern Matching — Positional pairs with destructuring and expression guards.
• Type Methods — Define methods inside type declarations with automatic dispatch.
• 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.

Effect-native libraries

Two libraries share one effect/runtime substrate (src/eff, src/http, src/agent). The same program runs unchanged under prod / test / replay handler towers.

• HTTP framework (src/http) — routes are effectful functions whose inferred effect rows advertise their capabilities; a route can't touch the database without Db in its signature. Serves real HTTP over a TCP socket, or runs fully offline and deterministic under a test tower. Response bodies are moved and consumed on send, so a double-send is a compile error.
• Agent framework (src/agent) — the control loop is plain code performing Llm/Tool/Approval/Memory effects; the tower supplies the model, tools, approval policy, and memory. The same loop runs as a deterministic offline eval, a full trace, a human-in-the-loop approval round-trip, a multi-shot explorer over every approval outcome, durably (replay-then-resume), or served over HTTP.

Quick Start

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

loon run hello.oo # Run a program
loon repl # Interactive REPL with time-travel
loon fmt hello.oo # 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 servercd web && npm run dev
# Build language server
cargo build -p loon-lsp --release

License

MIT