Getting started

Xo is experimental, and the language may change before 1.0.

Build the toolchain

The source code is not public yet; it will be soon. Until then, the steps below show how the toolchain will be built, and the specification describes the whole language.

The toolchain is built from source with Go 1.26 or later. From a checkout of the repository:

go build -o .tools/bin/xo ./cmd/xo
export PATH="$PWD/.tools/bin:$PATH"

A first program

Make a directory and create the module:

$ mkdir hello && cd hello
$ xo init example.com/hello
wrote xo.mod

Then add main.xo:

fn greet(name: Str) -> Str {
  "hello, ${name}"
}

test "greet" {
  expect(greet("Ada")) == "hello, Ada"
}

fn main(os: Os) {
  os.stdio.println(greet("world"))
}

Then check, run, and test it:

$ xo check .
1 module(s), 0 error(s), 0 warning(s)
$ xo run .
hello, world
$ xo test .
ok: 1 passed, 0 failed, 0 skipped

main receives the operating system as a capability (os). Code that needs the network, files, or the clock gets them from main, so a function’s signature shows what it can touch.

Diagnostics for agents

xo check --json reports every problem as JSON with a stable XOnnnn code, notes, and machine applicable fixes. xo explain <code> describes any code. Every code is listed, with an example and a fix, in the diagnostics reference.

Next

• Take the tour: twelve short steps, each with a program that checks, runs, and passes its tests.
• Read the core specification for everything in full.
• See the realistic example service in examples/notes-service in the repository: HTTP and JSON, std/sql, auth middleware, a background job, and graceful shutdown.