Quickstart

Your first program

Create hello.milo:

fn main(): i32 {
    print("hello, world!")
    return 0
}

Every Milo program starts at main, which returns an i32 exit code.

Run it

./milo run hello.milo

The run command compiles and executes in one step — no artifacts left behind.

Build a binary

./milo build hello.milo -o hello
./hello

The binary is standalone — no runtime needed. Typically under 300KB.

Build modes

./milo build app.milo -o app            # default: -O2
./milo build app.milo -o app --release   # -O3
./milo build app.milo -o app --debug     # -O0
./milo build app.milo -o app -g --debug  # -O0 + DWARF, for lldb

-g emits DWARF debug info and composes with any optimization level. See Debugging.

Runtime dependencies

A Milo binary links libc and nothing else — no runtime, no GC, no shared Milo library. Native libraries are linked only when you actually use them: OpenSSL for std/net TLS, sqlite for std/sqlite. Check with otool -L app (macOS) or ldd app (Linux).

Programs that use TLS link OpenSSL dynamically by default, so they pick up system security fixes without a rebuild — but they then need OpenSSL installed wherever they run. To ship one of those to a machine that may not have it:

./milo build app.milo -o app --static-deps   # bakes in openssl/sqlite; libc-only binary

The tradeoff is size (roughly +5MB) and losing system security updates for the baked-in copy — a CVE means rebuilding and redistributing. Programs that don't use TLS or sqlite need neither the flag nor the thought.

See the LLVM IR

./milo emit-ir hello.milo

Useful for understanding what the compiler generates.

Run the test suite

bun test

Something more interesting

Create greet.milo:

struct User {
    name: string,
    age: i32,
}

fn greet(user: &User): string {
    return "hi, " + user.name + "!"
}

fn main(): i32 {
    let u = User { name: "Alice", age: 30 }
    print(greet(u))
    print("age: ", u.age)
    return 0
}

$ ./milo run greet.milo
hi, Alice!
age: 30

&User borrows the value without consuming it — u is still usable after the call. Milo auto-borrows at call sites, so you write greet(u) not greet(&u).

Next: IDE Setup → or jump to Variables & Types →