I loved Norvig's Lispy, and made the first version of it in Rust, to learn Rust. I would love your thoughts :)
A few specific questions:
- Is there a way to write
ensure_tonicitywithout a macro? I tried, but was having trouble typing it - What do you think of passing in
&mut RispEnvineval. I needed to do this to implementdef, as it mutates env. Is my approach the idiomatic rust way, anything I'm missing? - In my implementation of
PartialEqforRispExp, I had to use a defaultfalse. This removes the win of exhaustive typechecking, but I had to do it, I think, in order to avoid writing out all the combinations of RispExp a, with RispExp b. Is there some way I can do exhaustive typechecking here, without writing out all combinations? i.e no different enum types (RispExp::Symbol, with RispExp::Number for ex) should ever be equal. - How is my use of references, Rc, etc?
And of course, any other thoughts greatly appreciated :)
use std::collections::HashMap;
use std::io;
use std::num::ParseFloatError;
use std::rc::Rc;
/*
Types
*/
#[derive(Clone)]
enum RispExp {
Bool(bool),
Symbol(String),
Number(f64),
List(Vec<RispExp>),
Func(fn(&[RispExp]) -> Result<RispExp, RispErr>),
Lambda(RispLambda)
}
#[derive(Debug)]
enum RispErr {
Reason(String),
}
#[derive(Clone)]
struct RispEnv {
data: HashMap<String, RispExp>,
outer: Option<Rc<RispEnv>>,
}
#[derive(Clone)]
struct RispLambda {
params_exp: Rc<RispExp>,
body_exp: Rc<RispExp>,
}
impl PartialEq for RispExp {
fn eq(&self, other: &RispExp) -> bool {
match (self, other) {
(RispExp::Bool(ref a), RispExp::Bool(ref b)) => a == b,
(RispExp::Symbol(ref a), RispExp::Symbol(ref b)) => a == b,
(RispExp::Number(ref a), RispExp::Number(ref b)) => a == b,
(RispExp::List(ref a), RispExp::List(ref b)) => a == b,
_ => false,
}
}
}
/*
Print
*/
fn to_str(exp: &RispExp) -> String {
match exp {
RispExp::Symbol(s) => s.clone(),
RispExp::Number(n) => n.to_string(),
RispExp::Bool(b) => b.to_string(),
RispExp::List(list) => {
let xs: Vec<String> = list
.iter()
.map(|x| to_str(x))
.collect();
return format!("({})", xs.join(","));
},
RispExp::Func(_) => "Function {}".to_string(),
RispExp::Lambda(_) => "Lambda {}".to_string(),
}
}
/*
Env
*/
fn parse_single_float(exp: &RispExp) -> Result<f64, RispErr> {
match exp {
RispExp::Number(num) => Ok(*num),
_ => Err(
RispErr::Reason(
format!("expected a number, got form='{}'", to_str(exp))
)
),
}
}
fn parse_list_of_floats(args: &[RispExp]) -> Result<Vec<f64>, RispErr> {
return args
.iter()
.map(|x| parse_single_float(x))
.collect::<Result<Vec<f64>, RispErr>>();
}
macro_rules! ensure_tonicity {
($check_fn:expr) => {{
|args: &[RispExp]| -> Result<RispExp, RispErr> {
let floats = parse_list_of_floats(args)?;
let first = floats.first().ok_or(RispErr::Reason("expected at least one number".to_string()))?;
let rest = &floats[1..];
fn f (prev: &f64, xs: &[f64]) -> bool {
match xs.first() {
Some(x) => $check_fn(prev, x) && f(x, &xs[1..]),
None => true,
}
};
return Ok(RispExp::Bool(f(first, rest)));
}
}};
}
fn default_env() -> RispEnv {
let mut data: HashMap<String, RispExp> = HashMap::new();
data.insert(
"+".to_string(),
RispExp::Func(
|args: &[RispExp]| -> Result<RispExp, RispErr> {
let sum = parse_list_of_floats(args)?.iter().fold(0.0, |sum, a| sum + a);
return Ok(RispExp::Number(sum));
}
)
);
data.insert(
"-".to_string(),
RispExp::Func(
|args: &[RispExp]| -> Result<RispExp, RispErr> {
let floats = parse_list_of_floats(args)?;
let first = *floats.first().ok_or(RispErr::Reason("expected at least one number".to_string()))?;
let sum_of_rest = floats[1..].iter().fold(0.0, |sum, a| sum + a);
return Ok(RispExp::Number(first - sum_of_rest));
}
)
);
data.insert(
"=".to_string(),
RispExp::Func(ensure_tonicity!(|a, b| a == b))
);
data.insert(
">".to_string(),
RispExp::Func(ensure_tonicity!(|a, b| a > b))
);
data.insert(
">=".to_string(),
RispExp::Func(ensure_tonicity!(|a, b| a >= b))
);
data.insert(
"<".to_string(),
RispExp::Func(ensure_tonicity!(|a, b| a < b))
);
data.insert(
"<=".to_string(),
RispExp::Func(ensure_tonicity!(|a, b| a <= b))
);
return RispEnv {data: data, outer: None}
}
/*
Eval
*/
fn eval_if_args(arg_forms: &[RispExp], env: &mut RispEnv) -> Result<RispExp, RispErr> {
let test_form = arg_forms.first().ok_or(
RispErr::Reason(
"expected test form".to_string(),
)
)?;
let test_eval = eval(test_form, env)?;
match test_eval {
RispExp::Bool(b) => {
let form_idx = if b { 1 } else { 2 };
let res_form = arg_forms.get(form_idx)
.ok_or(RispErr::Reason(
format!("expected form idx={}", form_idx)
))?;
let res_eval = eval(res_form, env);
return res_eval;
},
_ => Err(
RispErr::Reason(format!("unexpected test form='{}'", to_str(test_form)))
)
}
}
fn eval_def(arg_forms: &[RispExp], env: &mut RispEnv) -> Result<RispExp, RispErr> {
let first_form = arg_forms.first().ok_or(
RispErr::Reason(
"expected first form".to_string(),
)
)?;
let first_str = match first_form {
RispExp::Symbol(s) => Ok(s.clone()),
_ => Err(RispErr::Reason(
"expected first form to be a symbol".to_string(),
))
}?;
let second_form = arg_forms.get(1).ok_or(
RispErr::Reason(
"expected second form".to_string(),
)
)?;
if arg_forms.len() > 2 {
return Err(
RispErr::Reason(
"def can only have two forms ".to_string(),
)
)
}
let second_eval = eval(second_form, env)?;
env.data.insert(first_str, second_eval);
return Ok(first_form.clone());
}
fn eval_lambda(arg_forms: &[RispExp]) -> Result<RispExp, RispErr> {
let params_exp = arg_forms.first().ok_or(
RispErr::Reason(
"expected args form".to_string(),
)
)?;
let body_exp = arg_forms.get(1).ok_or(
RispErr::Reason(
"expected second form".to_string(),
)
)?;
if arg_forms.len() > 2 {
return Err(
RispErr::Reason(
"fn deefinition can only have two forms ".to_string(),
)
)
}
return Ok(
RispExp::Lambda(
RispLambda {
body_exp: Rc::new(body_exp.clone()),
params_exp: Rc::new(params_exp.clone()),
}
)
);
}
fn eval_built_in_symbol(sym: String, arg_forms: &[RispExp], env: &mut RispEnv) -> Result<RispExp, RispErr> {
match sym.as_ref() {
"if" => eval_if_args(arg_forms, env),
"def" => eval_def(arg_forms, env),
"fn" => eval_lambda(arg_forms),
_ => Err(RispErr::Reason(format!("unknown built-in symbol='{}'", sym))),
}
}
fn eval_forms(arg_forms: &[RispExp], env: &mut RispEnv) -> Result<Vec<RispExp>, RispErr> {
return arg_forms
.iter()
.map(|x| eval(x, env))
.collect::<Result<Vec<RispExp>, RispErr>>();
}
fn parse_list_of_symbol_strings(form: Rc<RispExp>) -> Result<Vec<String>, RispErr> {
let list = match form.as_ref() {
RispExp::List(s) => Ok(s.clone()),
_ => Err(RispErr::Reason(
"expected args form to be a list".to_string(),
))
}?;
return list
.iter()
.map(
|x| {
return match x {
RispExp::Symbol(s) => Ok(s.clone()),
_ => Err(RispErr::Reason(
"expected symbols in the argument list".to_string(),
))
}
}
).collect::<Result<Vec<String>, RispErr>>();
}
fn env_for_lambda(
params: Rc<RispExp>,
arg_forms: &[RispExp],
outer_env: &mut RispEnv,
) -> Result<RispEnv, RispErr> {
let ks = parse_list_of_symbol_strings(params)?;
if ks.len() != arg_forms.len() {
return Err(
RispErr::Reason(
format!("expected {} arguments, got {}", ks.len(), arg_forms.len())
)
);
}
let vs = eval_forms(arg_forms, outer_env)?;
let mut data: HashMap<String, RispExp> = HashMap::new();
for (k, v) in ks.iter().zip(vs.iter()) {
data.insert(k.clone(), v.clone());
}
return Ok(
RispEnv {
data: data,
outer: Some(Rc::new(outer_env.clone())),
}
);
}
fn env_get(k: &str, env: &RispEnv) -> Option<RispExp> {
return match env.data.get(k) {
Some(exp) => Some(exp.clone()),
None => {
return match &env.outer {
Some(outer_env) => env_get(k, &outer_env),
None => None
}
}
};
}
fn eval(exp: &RispExp, env: &mut RispEnv) -> Result<RispExp, RispErr> {
match exp {
RispExp::Symbol(k) =>
env_get(k, env)
.or(Some(exp.clone()))
.ok_or(
RispErr::Reason(
format!("unexpected symbol k='{}'", k)
)
)
,
RispExp::Bool(_a) => Ok(exp.clone()),
RispExp::Number(_a) => Ok(exp.clone()),
RispExp::List(list) => {
let first_form = list
.first()
.ok_or(RispErr::Reason("expected a non-empty list".to_string()))?;
let arg_forms = &list[1..];
let first_eval = eval(first_form, env)?;
return match first_eval {
RispExp::Symbol(sym) => eval_built_in_symbol(sym, arg_forms, env),
RispExp::Func(f) => {
return f(&eval_forms(arg_forms, env)?);
},
RispExp::Lambda(lambda) => {
let new_env = &mut env_for_lambda(lambda.params_exp, arg_forms, env)?;
return eval(&lambda.body_exp, new_env);
},
_ => Err(
RispErr::Reason(
format!("first form must be a function, but got form='{}'", to_str(&first_eval))
)
),
}
},
RispExp::Func(_) => Err(
RispErr::Reason(
format!("unexpected form='{}'", to_str(exp))
)
),
RispExp::Lambda(_) => Err(
RispErr::Reason(
format!("unexpected form='{}'", to_str(exp))
)
),
}
}
/*
Parse
*/
fn read_seq(tokens: &[String], start: usize) -> Result<(RispExp, usize), RispErr> {
let mut res: Vec<RispExp> = vec![];
let mut next = start;
loop {
let next_token = tokens
.get(next)
.ok_or(RispErr::Reason("could not find closing `)`".to_string()))
?;
if next_token == ")" {
return Ok((RispExp::List(res), next + 1)) // skip `)`, head to the token after
}
let (exp, new_next) = parse(&tokens, next)?;
res.push(exp);
next = new_next;
}
}
fn parse_atom(token: &str) -> RispExp {
match token.as_ref() {
"true" => RispExp::Bool(true),
"false" => RispExp::Bool(false),
_ => {
let potential_float: Result<f64, ParseFloatError> = token.parse();
return match potential_float {
Ok(v) => RispExp::Number(v),
Err(_) => RispExp::Symbol(token.to_string().clone())
}
}
}
}
fn parse(tokens: &[String], pos: usize) -> Result<(RispExp, usize), RispErr> {
let token = tokens
.get(pos)
.ok_or(
RispErr::Reason(format!("could not get token for pos='{}'", pos))
)?;
let to_match = &token[..];
match to_match {
"(" => read_seq(tokens, pos + 1),
")" => Err(RispErr::Reason("unexpected `)`".to_string())),
_ => Ok(
(parse_atom(token), pos + 1)
),
}
}
fn tokenize(expr: String) -> Vec<String> {
return expr
.replace("(", " ( ")
.replace(")", " ) ")
.split(" ")
.map(|x| x.trim().to_string())
.filter(|x| !x.is_empty())
.collect();
}
/*
REPL
*/
fn parse_eval_print(expr: String, env: &mut RispEnv) -> Result<String, RispErr> {
let (parsed_exp, _) = parse(&tokenize(expr), 0)?;
let evaled_exp = eval(&parsed_exp, env)?;
return Ok(to_str(&evaled_exp));
}
fn slurp_expr() -> String {
let mut expr = String::new();
io::stdin().read_line(&mut expr)
.expect("Failed to read line");
return expr;
}
fn main() {
let env = &mut default_env();
loop {
println!("risp >");
let expr = slurp_expr();;
match parse_eval_print(expr, env) {
Ok(res) => println!("// 🔥 => {}", res),
Err(e) => match e {
RispErr::Reason(msg) => println!("// 🙀 => {}", msg),
},
}
}
}
```
