lisp-rs 2.0.2

Lisp interpreter in Rust
Documentation 
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use crate::lexer::*;
use crate::object::*;
use std::error::Error;
use std::fmt;
use std::rc::Rc;

#[derive(Debug)]
pub struct ParseError {
  err: String,
}

impl fmt::Display for ParseError {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    write!(f, "Parse error: {}", self.err)
  }
}

impl Error for ParseError {}

pub fn parse(program: &str) -> Result<Object, ParseError> {
  let token_result = tokenize(program);
  if token_result.is_err() {
    return Err(ParseError {
      err: format!("{}", token_result.err().unwrap()),
    });
  }

  let mut tokens = token_result
    .unwrap()
    .into_iter()
    .rev()
    .collect::<Vec<_>>();
  let parsed_list = parse_list(&mut tokens)?;
  Ok(parsed_list)
}

fn parse_list(
  tokens: &mut Vec<Token>,
) -> Result<Object, ParseError> {
  let token = tokens.pop();
  if token != Some(Token::LParen) {
    return Err(ParseError {
      err: format!("Expected LParen, found {:?}", token),
    });
  }

  let mut list: Vec<Object> = Vec::new();
  while !tokens.is_empty() {
    let token = tokens.pop();
    if token.is_none() {
      return Err(ParseError {
        err: "Did not find enough tokens".to_string(),
      });
    }
    let t = token.unwrap();
    match t {
      Token::Keyword(k) => list.push(Object::Keyword(k)),
      Token::BinaryOp(b) => list.push(Object::BinaryOp(b)),
      Token::Integer(n) => list.push(Object::Integer(n)),
      Token::Float(f) => list.push(Object::Float(f)),
      Token::String(s) => list.push(Object::String(s)),
      Token::Symbol(s) => list.push(Object::Symbol(s)),
      Token::LParen => {
        tokens.push(Token::LParen);
        let sub_list = parse_list(tokens)?;
        list.push(sub_list);
      }
      Token::RParen => {
        return Ok(Object::List(Rc::new(list)));
      }
    }
  }

  Ok(Object::List(Rc::new(list)))
}

#[cfg(test)]
mod tests {
  use super::*;

  #[test]
  fn test_add() {
    let list = parse("(+ 1 2)").unwrap();
    assert_eq!(
      list,
      Object::List(Rc::new(vec![
        Object::BinaryOp("+".to_string()),
        Object::Integer(1),
        Object::Integer(2),
      ]))
    );
  }

  #[test]
  fn test_area_of_a_circle() {
    let program = "(
                         (define r 10)
                         (define pi 314)
                         (* pi (* r r))
                       )";
    let list = parse(program).unwrap();
    assert_eq!(
      list,
      Object::List(Rc::new(vec![
        Object::List(Rc::new(vec![
          Object::Keyword("define".to_string()),
          Object::Symbol("r".to_string()),
          Object::Integer(10),
        ])),
        Object::List(Rc::new(vec![
          Object::Keyword("define".to_string()),
          Object::Symbol("pi".to_string()),
          Object::Integer(314),
        ])),
        Object::List(Rc::new(vec![
          Object::BinaryOp("*".to_string()),
          Object::Symbol("pi".to_string()),
          Object::List(Rc::new(vec![
            Object::BinaryOp("*".to_string()),
            Object::Symbol("r".to_string()),
            Object::Symbol("r".to_string()),
          ])),
        ])),
      ]))
    );
  }
}