Stak Scheme

The miniature, embeddable R7RS Scheme implementation in Rust

The full documentation is here.

Install

Library

To install Stak Scheme as a library in your Rust project, run:

cargo add stak

Command line tools

To install the Scheme interpreter and alike as command line tools, run:

# Install the Scheme interpreter.
cargo install stak

# Install the minimal Scheme interpreter (6 times smaller!)
cargo install mstak

# Install the Scheme-to-bytecode compiler and bytecode interpreter.
cargo install stak-compile
cargo install stak-interpret

Examples

Embedding Scheme scripts in Rust

First, prepare a Scheme script at src/hello.scm.

(import (scheme base))

(write-string "Hello, world!\n")

Then, add a build script at build.rs to build the Scheme source file into bytecodes.

use stak_build::{build_r7rs, BuildError};

fn main() -> Result<(), BuildError> {
    build_r7rs()
}

Now, you can include the Scheme script into a program in Rust using the stak::include_module macro.

use core::error::Error;
use stak::{
    device::StdioDevice,
    file::VoidFileSystem,
    include_module,
    process_context::VoidProcessContext,
    module::{Module, UniversalModule},
    r7rs::{SmallError, SmallPrimitiveSet},
    time::VoidClock,
    vm::Vm,
};

const HEAP_SIZE: usize = 1 << 16;

// Include a Scheme script in the bytecode format built by the build script above.
static MODULE: UniversalModule = include_module!("hello.scm");

fn main() -> Result<(), Box<dyn Error>> {
    run(&MODULE.bytecode())?;

    Ok(())
}

fn run(bytecodes: &[u8]) -> Result<(), SmallError> {
    // Prepare a heap memory of a virtual machine.
    let mut heap = [Default::default(); HEAP_SIZE];
    // Create a virtual machine with its heap memory primitive procedures.
    let mut vm = Vm::new(
        &mut heap,
        SmallPrimitiveSet::new(
            // Attach standard input, output, and error of this process to a virtual machine.
            StdioDevice::new(),
            // Use void system interfaces for security because we don't need them for this example.
            VoidFileSystem::new(),
            VoidProcessContext::new(),
            VoidClock::new(),
        ),
    )?;

    // Initialize a virtual machine with bytecodes.
    vm.initialize(bytecodes.iter().copied())?;
    // Run bytecodes on a virtual machine.
    vm.run()
}

Communication between Scheme and Rust

Currently, in-memory standard input (stdin) and output (stdout) to Scheme scripts are the only way to communicate information between Rust programs and Scheme scripts.

use core::{error::Error, ffi::CStr, str::FromStr};
use stak::{
    device::ReadWriteDevice,
    file::VoidFileSystem,
    include_module,
    process_context::VoidProcessContext,
    module::{Module, UniversalModule},
    r7rs::{SmallError, SmallPrimitiveSet},
    time::VoidClock,
    vm::Vm,
};

const BUFFER_SIZE: usize = 1 << 8;
const HEAP_SIZE: usize = 1 << 16;

static MODULE: UniversalModule = include_module!("fibonacci.scm");

fn main() -> Result<(), Box<dyn Error>> {
    let mut input = 24;
    let mut output = [0u8; BUFFER_SIZE];
    let mut error = [0u8; BUFFER_SIZE];

    run(&MODULE.bytecode(), input.to_string().as_bytes(), &mut output, &mut error)?;

    let error = decode_buffer(&error)?;

    // If stderr is not empty, we assume that some error has occurred.
    if !error.is_empty() {
        return Err(error.into());
    }

    // Decode and print the output.
    println!("Answer: {}", isize::from_str(&decode_buffer(&output)?)?);

    Ok(())
}

fn run(
    bytecodes: &[u8],
    input: &[u8],
    output: &mut [u8],
    error: &mut [u8],
) -> Result<(), SmallError> {
    let mut heap = [Default::default(); HEAP_SIZE];
    let mut vm = Vm::new(
        &mut heap,
        SmallPrimitiveSet::new(
            // Create and attach an in-memory I/O device.
            ReadWriteDevice::new(input, output, error),
            VoidFileSystem::new(),
            VoidProcessContext::new(),
            VoidClock::new(),
        ),
    )?;

    vm.initialize(bytecodes.iter().copied())?;
    vm.run()
}

fn decode_buffer(buffer: &[u8]) -> Result<String, Box<dyn Error>> {
    Ok(CStr::from_bytes_until_nul(buffer)
        .map_err(|error| error.to_string())?
        .to_string_lossy()
        .into())
}

License

MIT