What is this?

This is a Lisp interpreter, written in Rust, intended to be embeddable as a library in a larger application for scripting purposes. Goals:

• Small footprint (both code size and memory usage)
• No runtime dependencies [1]
• Easy, ergonomic interop with native Rust functions
• Small but practical set of Lisp functionality

[1] cfg-if is build-time, num-traits add (I believe) no runtime presence, and num-bigint is entirely opt-in (at build time)

Basic Usage

[dependencies]
rust_lisp = "0.16.1"

use std::{cell::RefCell, rc::Rc};

use rust_lisp::default_env;
use rust_lisp::parser::parse;
use rust_lisp::interpreter::eval;

fn main() {

    // create a base environment
    let env = Rc::new(RefCell::new(default_env()));

    // parse into an iterator of syntax trees (one for each root)
    let mut ast_iter = parse("(+ \"Hello \" \"world!\")");
    let first_expression = ast_iter.next().unwrap().unwrap();

    // evaluate
    let evaluation_result = eval(env.clone(), &first_expression).unwrap();

    // use result
    println!("{}", &evaluation_result);
}

As you can see, the base environment is managed by the user of the library, as is the parsing stage. This is to give the user maximum control, including error-handling by way of Results.

The data model

The heart of the model is Value, an enum encompassing every type of valid Lisp value. Most of these are trivial, but Value::List is not. It holds a recursive List data structure which functions internally like a linked-list. into_iter() and from_iter() have been implemented for List, and there is also a lisp! macro (see below) which makes working with Lists, in particular, much more convenient.

Value does not implement Copy because of cases like Value::List, so if you read the source you'll see lots of value.clone(). This almost always amounts to copying a primitive, except in the Value::List case where it means cloning an internal Rc pointer. In all cases, it's considered cheap enough to do liberally.

The environment and exposing Rust functions

The base environment is managed by the user of the library mainly so that it can be customized. default_env() prepopulates the environment with a number of common functions, but these can be omitted (or pared down) if you wish. Adding an entry to the environment is also how you would expose your Rust functions to your scripts, which can take the form of either regular functions or closures:

fn my_func(env: Rc<RefCell<Env>>, args: &Vec<Value>) -> Result<Value, RuntimeError> {
  println!("Hello world!");
  return Ok(Value::NIL);
}

...

env.borrow_mut().define(
  Symbol::from("sayhello"),
  Value::NativeFunc(my_func)
);

env.borrow_mut().define(
  Symbol::from("sayhello"),
  Value::NativeFunc(
    |env, args| {
      println!("Hello world!");
      return Ok(Value::NIL);
    })
);

In either case, a native function must have the following function signature:

type NativeFunc = fn(env: Rc<RefCell<Env>>, args: &Vec<Value>) -> Result<Value, RuntimeError>;

The first argument is the environment at the time and place of calling (closures are implemented as environment extensions). The second argument is the Vec of evaluated argument values. For convenience, utility functions (require_arg(), require_int_parameter(), etc) have been provided for doing basic argument retrieval with error messaging. See default_environment.rs for examples.

The lisp! macro

A Rust macro, named lisp!, is provided which allows the user to embed sanitized Lisp syntax inside their Rust code, which will be converted to an AST at compile-time:

fn parse_basic_expression() {
  let ast = parse("
    (list 
      (* 1 2)  ;; a comment
      (/ 6 3 \"foo\"))").next().unwrap().unwrap();

  assert_eq!(ast, lisp! {
    (list 
      (* 1 2)
      (/ 6 3 "foo"))
  });
}

Note that this just gives you a syntax tree (in the form of a Value). If you want to actually evaluate the expression, you would need to then pass it to eval().

The macro also allows Rust expressions (of type Value) to be embedded within the lisp code using { }:

fn parse_basic_expression() {
  let ast = parse("
    (+ 3 1)").next().unwrap().unwrap();

  let n = 2;

  assert_eq!(ast, lisp! {
    (+ { Value::Int(n + 1) } 1)
  });
}

NOTE: When parsing lisp code from a string, dashes (-) are allowed to be used in identifiers. However, due to the limitations of declarative Rust macros, these cannot be handled correctly by lisp! {}. So it's recommended that you use underscores in your identifiers instead, which the macro will be able to handle correctly. The built-in functions follow this convention.

The ForeignValue trait

Sometimes if you're wanting to script an existing system, you don't want to convert your data to and from lisp-compatible values. This can be tedious, and more importantly super inefficient.

If you have some type - say a struct - that you want to be able to work with directly from your lisp code, you can implement ForeignValue for it:

struct Foo {
    some_prop: f32,
}

impl ForeignValue for Foo {
    fn command(
        &mut self,
        env: Rc<RefCell<Env>>,
        command: &str,
        args: &[Value],
    ) -> Result<Value, RuntimeError> {
        match command {
            "get_some_prop" => Ok(self.some_prop.into()),
            "set_some_prop" => {
                let new_value = require_typed_arg::<FloatType>("set_some_prop", args, 0)?;

                self.some_prop = new_value;

                Ok(new_value.into())
            }
            _ => Err(RuntimeError {
                msg: format!("Unexpected command {}", command),
            }),
        }
    }
}

And then call out to it from your lisp code:

(set x (cmd my_foo get_some_prop))

(cmd my_foo set_some_prop 3.14)

Included functionality

Special forms: define, set, defun, defmacro, lambda, quote, let, begin, cond, if, and, or

Functions (in default_env()): print, is_null, is_number, is_symbol, is_boolean, is_procedure, is_pair, car, cdr, cons, list, nth, sort, reverse, map, filter, length, range, hash, hash_get, hash_set, +, -, *, /, truncate, not, ==, !=, <, <=, >, >=, apply, eval

Other features:

• Quoting with comma-escapes
• Lisp macros
• Tail-call optimization