XS

A general-purpose programming language with gradual typing and multiple backends. Made privately starting 2024, published in early stage 2026.

-- no types required, but you can add them
fn greet(name) {
    print("Hello, {name}!")
}

fn greet_typed(name: str) -> str {
    return "Hello, {name}!"
}

-- closures, pattern matching, the usual
let nums = [1, 2, 3, 4, 5]
let doubled = nums.map((x) => x * 2)

-- algebraic effects
effect Ask {
    fn prompt(msg) -> str
}

let result = handle ask_user() {
    Ask.prompt(msg) => resume("World")
}

Build

make          # produces ./xs
make test     # runs the test suite
make clean    # clean build artifacts

Requires a C compiler (gcc or clang). Builds on Linux, macOS, and Windows (MinGW).

Run

./xs file.xs          # run a script
./xs                  # start the REPL
./xs run file.xs      # explicit run (same as above)

What works well

Gradual typing -- Leave types off and everything is dynamic. Add annotations and they get enforced at runtime. --strict mode requires annotations everywhere, --check does static analysis.

Tree-walk interpreter -- The default backend. Handles all language features, including the weirder ones like algebraic effects and the plugin system.

Object system -- Both struct/trait (composition) and class/inheritance (OOP). Use whichever makes sense for the problem.

Pattern matching -- match expressions with destructuring, guards, nested patterns. Works well.

What exists but is rough

Bytecode VM -- Works for basic programs. Doesn't support everything the tree-walk interpreter does yet.

JIT compiler -- x86-64 only, handles arithmetic and simple functions. Very early stage.

Transpilation -- JS backend is the most complete. C backend handles a decent chunk. WASM is very early, basically just arithmetic.

Concurrency -- spawn, async/await, actors, channels, nurseries are all there. Everything is cooperative (no real threads), spawn just runs immediately. It works, but it's not going to win any benchmarks.

Algebraic effects -- effect, perform, handle, resume. Covers the common cases, edge cases may still have bugs.

Plugin system -- Plugins can inject globals, hook into the parser, add keywords, override syntax. Pretty powerful but the API is still shifting.

Built-in tools

REPL, LSP server, formatter, linter, profiler, test runner.

Project layout

src/          # compiler and runtime source
tests/        # test files
Makefile      # build system
xs.toml       # project config
LANGUAGE.md   # language reference
COMMANDS.md   # CLI reference

Reference

• LANGUAGE.md -- language spec with examples
• COMMANDS.md -- CLI commands and flags

Author

@xs-lang0

License

Apache 2.0