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use std::rc::Rc;
#[derive(Debug, Clone)]
struct Env {
global: std::collections::HashMap<String, Expression>,
}
#[allow(dead_code)]
impl Env {
pub fn new() -> Self {
Env {
global: std::collections::HashMap::new(),
}
}
}
#[derive(Debug, Clone, PartialEq)]
#[allow(dead_code)]
enum Expression {
Float(f32),
Symbol(Rc<String>),
List(Rc<Vec<Expression>>),
// (Parameters, Body)
Procedure(Rc<Vec<Expression>>, Rc<Vec<Expression>>),
}
impl From<&str> for Expression {
fn from(value: &str) -> Self {
if let Ok(v) = value.parse::<f32>() {
Expression::Float(v)
} else {
Expression::Symbol(Rc::new(String::from(value)))
}
}
}
#[allow(dead_code)]
impl Expression {
pub fn parse_string<S>(tokens: S) -> Result<Expression, String>
where
S: Into<String>,
{
let mut tokens = tokens
.into()
.replace('(', " ( ")
.replace(')', " ) ")
.split_ascii_whitespace()
.map(String::from)
.collect::<Vec<String>>();
Expression::parse(&mut tokens)
}
// TODO: Consider reworking this so that it only takes a reference to the Vec.
pub fn parse(tokens: &mut Vec<String>) -> Result<Expression, String> {
if tokens.is_empty() {
Err("unexpected EOF while parsing the tokens.".to_string())
} else {
let token = tokens.remove(0);
match token.as_str() {
"(" => {
// TODO: Rework this logic so that it does not crash upon malformed input.
let mut list = Vec::<Expression>::new();
while tokens[0] != ")" {
match Expression::parse(tokens) {
Ok(v) => list.push(v),
Err(e) => {
return Err(e);
}
}
}
tokens.remove(0);
Ok(Expression::List(Rc::new(list)))
}
")" => Err("unexpected ')' found while parsing tokens".to_string()),
_ => Ok(Expression::from(token.as_str())),
}
}
}
pub fn evaluate_without_env(&self) -> Option<Expression> {
self.evaluate(&mut Env::new())
}
pub fn evaluate(&self, env: &mut Env) -> Option<Expression> {
match self {
// Expression::Float is an atom, it cannot be evaluated further.
Expression::Float(x) => Some(Expression::Float(*x)),
// Try to find this symbol in the global environment and evaluate it.
Expression::Symbol(name) => match env.global.get(name.as_ref()) {
Some(Expression::Float(x)) => Some(Expression::Float(*x)),
// NOTE: This is like having a pointer, this symbol references other symbols.
// Therefore, we "dereference" until we get something useful -- if there is any.
Some(Expression::Symbol(x)) => Some(Expression::Symbol(x.clone())),
// NOTE: If we remove the evaluation here, this program becomes "lazy"!
Some(Expression::List(list)) => Some(Expression::List(list.clone())),
Some(Expression::Procedure(parameter_symbols, body)) => Some(
Expression::Procedure(parameter_symbols.clone(), body.clone()),
),
None => Some(Expression::Symbol(name.clone())),
},
// We try to evaluate this list using the first element as its procedure over the rest of the list.
Expression::List(list) => {
match &list.as_slice() {
[Expression::Symbol(procedure_name), ref args @ ..] => {
match procedure_name.as_str() {
// NOTE: I think the operators below are not syntactically pure enough.
// It should be possible to consume the list one by one, hence performing the folding operation organically in Lisp.
"+" => {
let result = args.iter().fold(0.0f32, |acc, x| {
if let Some(Expression::Float(x)) = x.evaluate(env) {
acc + x
} else {
acc
}
});
Some(Expression::Float(result))
}
"-" => {
let result = args.iter().fold(0.0f32, |acc, x| {
if let Some(Expression::Float(x)) = x.evaluate(env) {
acc - x
} else {
acc
}
});
Some(Expression::Float(result))
}
"*" => {
let result = args.iter().fold(1.0f32, |acc, x| {
if let Some(Expression::Float(x)) = x.evaluate(env) {
acc * x
} else {
acc
}
});
Some(Expression::Float(result))
}
"/" => {
let result = args.iter().fold(1.0f32, |acc, x| {
if let Some(Expression::Float(x)) = x.evaluate(env) {
acc / x
} else {
acc
}
});
Some(Expression::Float(result))
}
// Creates an anonymous procedure. If one wants to create a function in the imperative programming sense, one wants to
// bind this anynomous procedure to a symbol using `define`.
"lambda" => {
if let [_, Expression::List(params), Expression::List(body)] =
list.as_slice()
{
Some(Expression::Procedure(params.clone(), body.clone()))
} else {
None
}
}
// Binds an Expression to a Expression::Symbol
"define" => {
if let [_, Expression::Symbol(name), ref expression] =
list.as_slice()
{
if let Some(evaluation) = expression.evaluate(env) {
env.global.insert(name.as_ref().clone(), evaluation);
}
}
None
}
// Not any of the internal procedures above, this could be a user defined functions.
name => {
if let Some(Expression::Procedure(parameter, body)) =
env.global.get(name)
{
if parameter.len() == args.len() {
let mut env = env.clone();
let mut local_env = args.iter().zip(parameter.iter()).fold(
Env::new(),
|mut local_env, (arg_body, arg_symbol)| {
let value = arg_body.evaluate(&mut env).unwrap();
if let Expression::Symbol(arg_symbol) = arg_symbol {
local_env
.global
.insert(arg_symbol.as_ref().clone(), value);
}
local_env
},
);
Expression::List(body.clone()).evaluate(&mut local_env)
} else {
None
}
} else {
None
}
}
}
}
// NOTE: The first Expression in the list is not an instance of an Expression::Symbol.
// What should happen here? We could evaluate this as the first Expression in the list.
[..] => None,
}
}
// NOTE: User invoked a call to `lambda` without binding it to a symbol.
// What _should_ happen here?
Expression::Procedure(_, _) => None,
}
}
}
#[test]
fn parsing() {
assert_eq!(
Expression::parse(
&mut vec!["(", "define", "a", "10", ")"]
.iter()
.map(|x| String::from(x.clone()))
.collect()
)
.unwrap(),
Expression::List(std::rc::Rc::new(vec![
Expression::Symbol(std::rc::Rc::new("define".to_owned())),
Expression::Symbol(std::rc::Rc::new("a".to_owned())),
Expression::Float(10.0f32)
]))
);
}
#[test]
fn evaluations() {
let mut env = Env::new();
Expression::parse_string("(define a 10)")
.unwrap()
.evaluate(&mut env);
Expression::parse_string("(define sq (lambda (x) (* x x)))")
.unwrap()
.evaluate(&mut env);
assert_eq!(
Expression::parse_string("(sq 10)")
.unwrap()
.evaluate(&mut env),
Expression::parse_string("(sq a)")
.unwrap()
.evaluate(&mut env)
);
}