Ion

A fast, embeddable scripting language for Rust applications.

Ion is designed for embedding in Rust programs — like Lua for game engines, but with Rust-flavored syntax and strong typing. It features a tree-walk interpreter and an optimizing bytecode VM, Serde-native host integration, a native Tokio async runtime, and a rich standard library.

Quick Start

use ion_core::Engine;

fn main() {
    let mut engine = Engine::new();

    // Evaluate expressions
    let result = engine.eval(r#"
        let items = [1, 2, 3, 4, 5];
        items.filter(|x| x > 2).map(|x| x * 10)
    "#).unwrap();

    println!("{}", result); // [30, 40, 50]
}

Features at a Glance

// Variables — immutable by default
let name = "Ion";
let mut counter = 0;

// Destructuring
let (x, y) = (10, 20);
let [head, ...rest] = [1, 2, 3, 4];

// String interpolation
let greeting = f"hello {name}!";

// Functions with default args
fn greet(who, greeting = "hello") {
    f"{greeting}, {who}!"
}

// Pattern matching
let result = Ok(42);
match result {
    Ok(n) if n > 0 => f"positive: {n}",
    Ok(n) => f"non-positive: {n}",
    Err(e) => f"error: {e}",
}

// First-class functions
let double = |x| x * 2;
[1, 2, 3].map(double)  // [2, 4, 6]

// Dicts (JSON-like)
let config = #{
    host: "localhost",
    port: 8080,
    tags: ["web", "api"],
};

// Error handling
fn parse(s) {
    let n = s.to_int()?;
    Ok(n * 2)
}

// Try/catch
try {
    risky_operation();
} catch e {
    f"caught: {e}"
}

// Modules and imports
use math::{add, PI};
let area = add(PI, PI);

// Loops + functional
let sum = [1, 2, 3, 4, 5].fold(0, |acc, x| acc + x);
for (i, item) in enumerate(items) {
    io::println(f"{i}: {item}");
}

Embedding in Rust

Host-registered names — functions, modules, struct types, enum variants — are hashed at compile time via the h!() macro. The literal source text is consumed by h!()'s const evaluation; only the u64 survives in the release binary. See docs/hide-names.md for a concise overview and docs/embedding.md for the migration guide.

use ion_core::{Engine, h};
use ion_core::value::Value;

let mut engine = Engine::new();

// Register host functions (plain fn pointer — no captures)
engine.register_fn(h!("fetch_score"), |args: &[Value]| {
    let name = args[0].as_str().unwrap_or("unknown");
    Ok(Value::Int(match name {
        "alice" => 95,
        "bob" => 87,
        _ => 0,
    }))
});

// Register closures that capture host state (DB pool, shared counter, etc.)
let hits = std::sync::Arc::new(std::sync::atomic::AtomicU64::new(0));
let hits_c = hits.clone();
engine.register_closure(h!("record_hit"), move |_args| {
    hits_c.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
    Ok(Value::Unit)
});

// Use from Ion scripts
let result = engine.eval(r#"
    record_hit();
    let score = fetch_score("alice");
    if score >= 90 { "A" } else { "B" }
"#).unwrap();

assert_eq!(result, Value::Str("A".to_string()));

Register Modules

use ion_core::{h, module::Module};
use ion_core::value::Value;

let mut math = Module::new(h!("math"));
math.register_fn(h!("add"), |args: &[Value]| {
    Ok(Value::Int(args[0].as_int().unwrap() + args[1].as_int().unwrap()))
});
math.set(h!("PI"), Value::Float(std::f64::consts::PI));
engine.register_module(math);

// Scripts can use: math::add(1, 2) or `use math::*;`

With async-runtime, modules can expose native async host functions too:

let mut sensor = Module::new(h!("sensor"));
sensor.register_async_fn(h!("call"), |args| async move {
    Ok(Value::Int(args.len() as i64))
});
engine.register_module(sensor);

let value = engine.eval_async(r#"sensor::call("jobs.claim", #{})"#).await?;

Host Types with Derive

use ion_core::{Engine, IonType};
use ion_core::value::Value;

#[derive(IonType)]
struct Player {
    name: String,
    health: i64,
}

let mut engine = Engine::new();
engine.register_type::<Player>();

let result = engine.eval(r#"
    let p = Player { name: "hero", health: 100 };
    f"{p.name} has {p.health} HP"
"#).unwrap();

Bytecode VM

let mut engine = Engine::new();

// Tree-walk interpreter (default)
let a = engine.eval("1 + 2").unwrap();

// Bytecode VM — faster for hot paths
let b = engine.vm_eval("1 + 2").unwrap();

assert_eq!(a, b);

Native Async Runtime

With the async-runtime feature, hosts can register Tokio-native async functions. Ion scripts call them like ordinary functions; Engine::eval_async parks the Ion continuation on the host future and resumes when Tokio wakes it. No OS thread is spawned just to wait for network I/O.

use ion_core::{Engine, Value, h};
use ion_core::error::IonError;

#[tokio::main(flavor = "current_thread")]
async fn main() -> Result<(), IonError> {
    let mut engine = Engine::new();

    engine.register_async_fn(h!("http_get"), |args| async move {
        let url = args[0].as_str().unwrap_or("").to_string();
        tokio::time::sleep(std::time::Duration::from_millis(10)).await;
        Ok(Value::Str(format!("GET {url} -> 200 OK")))
    });

    let value = engine.eval_async(r#"
        fn load(path) {
            http_get(path)
        }

        async {
            let a = spawn load("/profile");
            let b = spawn load("/orders");
            [a.await, b.await]
        }
    "#).await?;

    println!("{value}");
    Ok(())
}

Types

Project Structure

ion-core/     Core library — lexer, parser, interpreter, compiler, VM
ion-derive/   #[derive(IonType)] proc macro
ion-cli/      Script runner and REPL
ion-lsp/      Language server (VSCode support)
editors/      Editor syntax highlighting

Cargo Features

Building

cargo build                          # default features
cargo build --all-features           # everything including async runtime
RUST_MIN_STACK=16777216 cargo test --all-features  # run all feature-gated tests
cargo run --bin ion-cli script.ion   # run a script
cargo run --bin ion-cli              # start REPL

Documentation

The full documentation site is at https://ionrs.github.io/ — prose guides, the complete language reference, a manifest-driven stdlib browser (the same one the LSP reads for hover and completion), and runnable examples.

The canonical sources also live in this repository:

• LANGUAGE.md — complete language specification
• DESIGN.md — design decisions and implementation phases
• docs/embedding.md — embedding API reference
• docs/concurrency.md — native async runtime, cancellation, channels, and the Tokio embedding pattern
• docs/stdlib.md — standard library reference
• docs/performance.md — performance strategy
• docs/vm-internals.md — bytecode VM internals
• docs/testing.md — test suite layout
• docs/tooling.md — CLI, LSP, editor support

Embedding inside a Tokio application

Enable async-runtime, register host futures with register_async_fn, and call engine.eval_async(source).await. Native host futures are polled by Tokio directly, so an Ion HTTP call can park without spawn_blocking, Handle::block_on, or one OS thread per script task. See docs/concurrency.md.

License

MIT