Kestrel Programming Language

What is Kestrel?

Kestrel is a compiled programming language with deterministic memory management — no garbage collector, no borrow checker. It compiles to native code via Cranelift (default) or LLVM and ships with a full ecosystem: package manager, web framework, HTTP client, VS Code extension, and more — many written in Kestrel itself.

Currently in its first preview release, Kestrel can be used to write 2D games, CLI tools, and web apps.

See it live: wall.kestrel-lang.com is a public community sticky-note wall built entirely in Kestrel — Perch backend, SQLite, and an htmx frontend.

Quick Start

# Install Kestrel
curl -fsSL https://kestrel-lang.com/install | sh

# Compile and run a program
kestrel build hello.ks && ./hello

# Or use Flock (package manager)
mkdir myproject && cd myproject && flock init && flock run

Or build from source if you prefer.

Contents

• A Taste of Kestrel
• Features
• Ecosystem
• Examples
• Editor Support
• Standard Library
• Building from Source

A Taste of Kestrel

module Cafe

enum Roast {
    case Light
    case Dark
    case Custom(String)
}

struct Order {
    let drink: String
    let roast: Roast
    let shots: Int64

    var price: Int64 { self.shots * 250 }

    func receipt() -> String {
        let roast = match self.roast {
            .Light => "light",
            .Dark => "dark",
            .Custom(name) => name
        }
        "\(self.drink) (\(roast)) — $\(self.price / 100)"
    }
}

@main
func main() {
    let orders = [
        Order(drink: "Cortado", roast: .Dark, shots: 2),
        Order(drink: "Oat Latte", roast: .Light, shots: 3),
    ];

    orders.filter { it.shots > 2 }.forEach { println(it.receipt()) };
}

Features

Type System

• Protocols and extensions — polymorphism with retroactive conformance
• Monomorphized generics — zero-cost abstractions with where clause constraints
• Algebraic data types — enums with associated values and exhaustive pattern matching
• Type inference — bidirectional constraint-based inference

Memory Model

• Value semantics — copy-on-assignment, not Copyable for move-only types
• References — &T borrowed views with compile-time escape checking — no lifetime annotations, ever
• Copy-on-write collections — Array, Dictionary, Set
• RAII — deterministic cleanup via deinit
• No GC, no borrow checker — ownership is simple and predictable

Ergonomics

• Error handling — throws / try sugar over Result[T, E]
• Closures — trailing closure syntax, implicit it parameter
• String interpolation — "\(expr)" via the Formattable protocol
• Iterators — for-in loops with 20+ adapters (map, filter, zip, scan, take, ...)
• C interop — @extern(.C) for calling C functions and linking native libraries
• Parameter labels — named parameters for readable call sites
• Flexible entry points — mark any function @main; return ExitCode, an integer, (), or a throwing Result via the Exitable protocol

Ecosystem

Kestrel ships with Flock, a package manager written in Kestrel:

mkdir myproject && cd myproject
flock init && flock run

Install tools globally with flock install — it builds optimized binaries (pass --debug to opt out) and drops them into ~/.flock/bin:

flock install <org>/<pkg>   # install from the registry
flock install               # install the current package's binaries

See the Flock README for the full command reference.

Available packages:

Package	Description
kestrel/perch	Web framework — routing, middleware, generic context
kestrel/swoop	HTTP/HTTPS client
kestrel/clutch	CLI argument parser
kestrel/quill	Serialization framework
kestrel/quill-json	JSON support for Quill
kestrel/quill-toml	TOML support for Quill
kestrel/http	Shared HTTP types
kestrel/plume	Template engine
kestrel/talon-sqlite	SQLite Wrapper
kestrel/uuid	UUID Type
kestrel/html-builder	HTML DSL
kestrel/sdl	SDL Bindings
kestrel/crypto

Also included: Jessup (toolchain version manager, like rustup).

Examples

Example	Description	Complexity
Notes App	Full-stack web app with htmx, perch backend, and SQLite database	Advanced
Weather Dashboard	Full-stack web app with Perch, htmx, and Open-Meteo API	Advanced
Pokédex	Kanto Pokédex using PokéAPI, Perch, and Plume	Advanced
Wordle	Wordle clone with shareable URL state	Intermediate
APOD	NASA Astronomy Picture of the Day viewer	Intermediate
Counter	HTMX counter app with Perch	Beginner
Game of Life	Conway's Game of Life with SDL2	Intermediate
Breakout	Terminal brick breaker with Iterator-based game loop	Intermediate
Snake	Terminal snake with RAII terminal management	Intermediate
Pong	Terminal pong with AI opponent

Editor Support

Kestrel ships with a language server (kestrel-lsp) with diagnostics, completions, go-to-definition, rename, and more. Install the VS Code extension to get started.

Standard Library

All public stdlib types are auto-imported — no import statements needed. See the full Standard Library Reference.

Status

Kestrel is in early preview — expect breaking changes between releases.

• macOS is the primary platform; Linux is supported but less tested
• Two codegen backends — Cranelift for unoptimized builds (-O0, fast compiles) and LLVM 18 for optimized builds (-O1, -O2)
• Optimized builds — kestrel build -O 2 for release binaries; flock install builds optimized by default
• Windows is not currently supported

Building from Source

Requires Rust 2024 edition (1.85+).

git clone https://github.com/kestrellang/kestrel
cd kestrel
cargo install --path .

License

Apache-2.0 — see LICENSE.