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 dependencies
- Easy, ergonomic interop with native Rust functions
- Small but practical set of Lisp functionality
[dependencies]
rust_lisp = "0.1.0"
use std::{cell::RefCell, rc::Rc};
use rust_lisp::{parse,eval,default_env};
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 Result
s.
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 conventient.
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 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().entries.insert(
String::from("sayhello"),
Value::NativeFunc(my_func));
entries.insert(
String::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_parameter()
, require_int_parameter()
, etc) have been provided for
doing basic argument retrieval with error messaging. See
default_environment.rs
for examples.
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: There is currently a problem with the macro where predicates ending in ?
cannot be used. This is because ?
cannot be a valid part of an identifier in Rust, and
so null?
for example cannot be processed by Rust as a single token. The solution will likely
involve renaming those predicates to not include ?
. This is why the project is still in version
0.X.X
:)
Special forms:
define
, set
, defun
, lambda
, quote
, let
, begin
, cond
, if
,
and
, or
Functions (in default_env()
):
print
, null?
, number?
, symbol?
, boolean?
, procedure?
, pair?
,
car
, cdr
, cons
, list
, nth
, sort
, reverse
, map
, filter
,
length
, range
, +
, -
, *
, /
, truncate
, not
, ==
, !=
, <
, <=
,
>
, >=
, apply
, eval
Other features:
- Single-tick quoting
- Tail-call optimization