hare-lang

name	hare-lang
description	Hare language skill for simple systems programming. Use when building with hare build/test/run, Hare stdlib, C FFI with @extern, tagged union error handling, or comparing Hare vs C/Zig. Activates on queries about Hare language, hare build, @extern, tagged union, or Hare stdlib.

Hare

Purpose

Guide agents through the Hare programming language: design philosophy (simple, stable, compiled), hare build/test/run workflows, stdlib overview, C FFI with @extern, the type system (tagged unions, slices), error handling with (T | error!), and comparison with C and Zig for systems utilities.

When to Use

• Writing small system utilities with C-like control and modern safety
• Building CLI tools or daemons with minimal dependencies
• Calling C libraries from Hare or exporting Hare functions to C
• Preferring explicit error handling over exceptions
• Evaluating Hare vs C or Zig for a new project
• Needing a stable, auditable codebase without heavy runtime

Workflow

1. Install Hare

# Linux/macOS — build from source
git clone https://git.sr.ht/~sircmpwn/hare
cd hare
make check   # builds and runs tests
sudo make install

hare version

# Create project (Hare has no project generator — lay out manually)
mkdir -p mytool && cd mytool
cat > hare.mod << 'EOF'
module mytool
EOF

2. Hello world and build

// main.ha
use fmt;

export fn main() void = {
	fmt::println("Hello, Hare!")!;
};

hare build -o mytool
./mytool

hare run .        # build and run
hare test         # run tests

3. Stdlib overview

Module	Purpose
fmt	Formatted I/O (printf, println)
io	Reader/writer interfaces
os	Files, environment, args
strings	String manipulation
bufio	Buffered I/O
encoding	JSON, hex, etc.
net	TCP/UDP networking
time	Dates and durations
mem	Memory helpers
types	Platform integer types

use os;
use fmt;
use strings;

export fn main() void = {
	const args = os::args;
	for (let i = 0z; i < len(args); i += 1) {
		fmt::println(args[i])!;
	};
};

4. Error handling

// Errors are tagged union values — explicit propagation with !
fn read_file(path: str) (str | os::error) = {
	const file = os::open(path)?;
	defer os::close(file);
	let buf: []u8 = [];
	io::readall(file, &buf)?;
	return strings::fromutf8(buf)!;
};

export fn main() void = {
	match (read_file("config.txt")) {
	case let s: str =>
		fmt::println(s)!;
	case let err: os::error =>
		fmt::fatalf("error: {}", err)!;
	};
};

? propagates errors; ! asserts success in infallible context; match for handling.

5. Type system

// Tagged union
type color = (u8 | u16 | void);

fn get_color(c: color) u16 = {
	match (c) {
	case let v: u8 => return v: u16;
	case let v: u16 => return v;
	case => abort();
	};
};

// Slices — pointer + length
fn sum(nums: []i32) i32 = {
	let total = 0i32;
	for (let i = 0z; i < len(nums); i += 1) {
		total += nums[i];
	};
	return total;
};

No implicit conversions — explicit casts required.

6. C FFI — @extern

// Link against C library
use c;

@extern("c") fn strlen(s: *const u8) size;

export fn main() void = {
	const s = "hello";
	fmt::println(len(s))!;  // Hare strlen via strings module
};

Export Hare to C:

// Exported C ABI function
export fn my_add(a: i32, b: i32) i32 = {
	return a + b;
};

hare build -o libmytool.a
# Link from C with generated headers or manual declarations

// cgo-style module linking in hare.mod
module mytool
require (
    libc
)

7. Testing

@test fn test_add() void = {
	assert(sum(&[1i32, 2, 3]) == 6);
};

@test fn test_error() void = {
	match (read_file("/nonexistent")) {
	case => abort("expected error");
	case let err: os::error => void;
	};
};

hare test -v

8. Hare vs C vs Zig

Aspect	Hare	C	Zig
Memory safety	Some (no null, tagged errors)	Manual	Manual + optional safety
Compile time	Fast	Fast	Comptime heavy
Stdlib	Minimal, stable	libc	Extensive
C interop	@extern	Native	@cImport
Generics	No (comptime limited)	No	Comptime generics
Best for	Utilities, tools	Everything	Systems with metaprogramming

9. Use cases

Good Hare fits
├── CLI utilities (grep-like, init tools)
├── Build tools and scripts replacing shell
├── Network daemons with simple protocol
└── Auditable security-sensitive code

Consider C/Zig instead when
├── Heavy generic metaprogramming needed (Zig)
├── Existing massive C ecosystem glue
└── GPU/kernel domains with immature Hare support

Common Problems

Symptom	Cause	Fix
unknown type	Missing import	Add use module;
FFI link error	Library not in hare.mod	Add require; correct -l flags
Error not handled	Missing ? or match	Propagate or handle all cases
UTF-8 error	Invalid bytes in string	Validate with strings::fromutf8
Test not found	Missing @test	Name fn with @test attribute
Platform syscall missing	Hare stdlib gap	Use @extern to libc

Related Skills

• skills/zig/zig-compiler — Zig as alternative systems language
• skills/zig/zig-cinterop — Zig C interop comparison
• skills/compilers/gcc — C toolchain alongside Hare
• skills/languages/carbon-lang — other emerging systems languages
• skills/build-systems/make — integrating Hare into Makefiles
• skills/runtime-safety/sanitizers — C interop safety testing