Embedded Scheme Interpreter Based on Rust

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Steel is a lightweight, high-performance Scheme interpreter designed for embedding in Rust applications. It implements core language features of Scheme (a subset of R7RS) and provides seamless interaction with Rust, making it suitable for scenarios that require scripted extensions or safe execution of dynamic code.

1. Core Features

1. Minimal Embedding

•Low Overhead: The compiled static library is less than 500KB, with no external dependencies.•Bidirectional Interaction: Supports calling Scheme functions from Rust, and also allows calling Rust functions from Scheme.•Memory Safety: Utilizes Rust’s ownership mechanism to avoid memory leaks and dangling pointers.

2. High-Performance Execution

•Mixed Execution Mode：

•Interpretation Mode: Directly traverses the AST, suitable for quick startup. •Bytecode Compilation (optional): Enabled via --features bytecode, with a performance boost of 40%.

•Zero-Cost FFI: Rust native types (such as i32, String) can be passed to Scheme without conversion.

3. Language Compatibility

•Scheme Standards: Supports lambda, continuations, hygienic macros, and other core features of R7RS.•Extended Syntax：

;; Supports Rust-style closure capture(let ((x 10))   (lambda [y] (+ x y))) ;; Square brackets indicate closure capture

4. Safe Sandbox

•Resource Limits：

let engine = Engine::new()    .max_memory(1024 * 1024) // 1MB memory limit    .max_ops(1_000_000);     // Prevent infinite loops

•Permission Control：

engine.allow_io(false)    // Disable file operations  .allow_net(false);    // Disable network access

2. Integration and Usage

1. Install Dependencies

Add to Cargo.toml:

[dependencies]steel = { git = "https://github.com/mattwparas/steel", features = ["async"] }

2. Basic Example

Rust → Scheme：

use steel::{Engine, Value};
fn main() {    let mut engine = Engine::new();    let result = engine.eval("(+ 1 (* 2 3))").unwrap();    assert_eq!(result, Value::Int(7));}

Scheme → Rust：

// Define Rust function#[steel::export]fn rust_add(a: i64, b: i64) -&gt; i64 { a + b }
// Register to Scheme environmentengine.add_fn("add", rust_add);
// Scheme callengine.eval("(add 42 58)"); // Returns 100

3. Asynchronous Support

#[steel::export]async fn fetch_url(url: String) -&gt; Result&lt;String, reqwest::Error&gt; {    reqwest::get(&amp;url).await?.text().await}
// Asynchronous call in Schemeengine.eval_async(r#"  (display (fetch-url "https://api.example.com/data"))"#).await;

3. Performance Data

Test Item	Steel (Rust)	Chibi-Scheme (C)	Comparison
Fibonacci Sequence (n=30)	650ms	720ms	+10%
List Processing (10k elements)	110ms	150ms	+36%
Startup Time	0.8ms	0.5ms	-37%

4. Application Scenarios

1.Game Scripting Engine

Write NPC AI or story branches in Scheme:

(define (update-npc player-pos)  (if (&lt; (distance player-pos npc-pos) 10)      (begin        (play-sound "alert.wav")        (move-towards player-pos))      (idle-animation)))

2.Dynamic Configuration System

Parse Scheme files for runtime configuration:

   (config     (log-level 'debug)     (retry (max-attempts 5             delay-ms 1000))     (services       (db (host "db.example.com")           (port 5432))))

3.Rule Engine

Safely execute user-submitted business rules:

   (rule (discount-for-vip user)     (if (and (&gt; (user-purchase user) 1000)              (eq? (user-level user) 'vip))         (* (order-amount) 0.8)         (order-amount)))

5. Comparison with Similar Tools

Steel	Rhai	WASM
Syntax	Scheme	JavaScript-like	Multiple languages compiled
Startup Overhead	0.8ms	0.3ms	2ms+
FFI Overhead	Zero-cost	Requires type conversion	Requires binding
Sandboxing Capability	Memory/CPU control	Basic restrictions	Depends on WASI

6. Resources

•GitHub Repository: mattwparas/steel^[1]•Documentation: API Reference^[2]•Example Project: Embedded Scripting Engine Demo^[3]

References

[1] mattwparas/steel: https://github.com/mattwparas/steel[2] API Reference: https://docs.rs/steel/latest/steel/[3] Embedded Scripting Engine Demo: https://github.com/mattwparas/steel/tree/main/examples/game-scripting

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