risp-1.0.rs

use std::collections::HashMap;
use std::fmt;
use std::io;
use std::num::ParseFloatError;

/*
  Types
*/

#[derive(Clone)]
enum RispExp {
  Symbol(String),
  Number(f64),
  List(Vec<RispExp>),
  Func(fn(&[RispExp]) -> Result<RispExp, RispErr>),
}

impl fmt::Display for RispExp {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    let str = match self {
      RispExp::Symbol(s) => s.clone(),
      RispExp::Number(n) => n.to_string(),
      RispExp::List(list) => {
        let xs: Vec<String> = list
          .iter()
          .map(|x| x.to_string())
          .collect();
        format!("({})", xs.join(","))
      },
      RispExp::Func(_) => "Function {}".to_string(),
    };
    
    write!(f, "{}", str)
  }
}

#[derive(Debug)]
enum RispErr {
  Reason(String),
}

#[derive(Clone)]
struct RispEnv {
  data: HashMap<String, RispExp>,
}

/*
  Parse
*/

fn tokenize(expr: String) -> Vec<String> {
  expr
    .replace("(", " ( ")
    .replace(")", " ) ")
    .split_whitespace()
    .map(|x| x.to_string())
    .collect()
}

fn parse<'a>(tokens: &'a [String]) -> Result<(RispExp, &'a [String]), RispErr> {
  let (token, rest) = tokens.split_first()
    .ok_or(
      RispErr::Reason("could not get token".to_string())
    )?;
  match &token[..] {
    "(" => read_seq(rest),
    ")" => Err(RispErr::Reason("unexpected `)`".to_string())),
    _ => Ok((parse_atom(token), rest)),
  }
}

fn read_seq<'a>(tokens: &'a [String]) -> Result<(RispExp, &'a [String]), RispErr> {
  let mut res: Vec<RispExp> = vec![];
  let mut xs = tokens;
  loop {
    let (next_token, rest) = xs
      .split_first()
      .ok_or(RispErr::Reason("could not find closing `)`".to_string()))
      ?;
    if next_token == ")" {
      return Ok((RispExp::List(res), rest)) // skip `)`, head to the token after
    }
    let (exp, new_xs) = parse(&xs)?;
    res.push(exp);
    xs = new_xs;
  }
}

fn parse_atom(token: &str) -> RispExp {      
  let potential_float: Result<f64, ParseFloatError> = token.parse();
  match potential_float {
    Ok(v) => RispExp::Number(v),
    Err(_) => RispExp::Symbol(token.to_string().clone())
  }
}

/*
  Env
*/

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);
        
        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);
        
        Ok(RispExp::Number(first - sum_of_rest))
      }
    )
  );
  
  RispEnv {data}
}

fn parse_list_of_floats(args: &[RispExp]) -> Result<Vec<f64>, RispErr> {
  args
    .iter()
    .map(|x| parse_single_float(x))
    .collect::<Result<Vec<f64>, RispErr>>()
}

fn parse_single_float(exp: &RispExp) -> Result<f64, RispErr> {
  match exp {
    RispExp::Number(num) => Ok(*num),
    _ => Err(RispErr::Reason("expected a number".to_string())),
  }
}

/*
  Eval
*/

fn eval(exp: &RispExp, env: &mut RispEnv) -> Result<RispExp, RispErr> {
  match exp {
    RispExp::Symbol(k) =>
        env.data.get(k)
        .ok_or(
          RispErr::Reason(
            format!("unexpected symbol k='{}'", k)
          )
        )
        .map(|x| x.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)?;
      match first_eval {
        RispExp::Func(f) => {
          let args_eval = arg_forms
            .iter()
            .map(|x| eval(x, env))
            .collect::<Result<Vec<RispExp>, RispErr>>();
          f(&args_eval?)
        },
        _ => Err(
          RispErr::Reason("first form must be a function".to_string())
        ),
      }
    },
    RispExp::Func(_) => Err(
      RispErr::Reason("unexpected form".to_string())
    ),
  }
}

/*
  Repl
*/

fn parse_eval(expr: String, env: &mut RispEnv) -> Result<RispExp, RispErr> {
  let (parsed_exp, _) = parse(&tokenize(expr))?;
  let evaled_exp = eval(&parsed_exp, env)?;
  
  Ok(evaled_exp)
}

fn slurp_expr() -> String {
  let mut expr = String::new();
  
  io::stdin().read_line(&mut expr)
    .expect("Failed to read line");
  
  expr
}

fn main() {
  let env = &mut default_env();
  loop {
    println!("risp >");
    let expr = slurp_expr();;
    match parse_eval(expr, env) {
      Ok(res) => println!("// 🔥 => {}", res),
      Err(e) => match e {
        RispErr::Reason(msg) => println!("// 🙀 => {}", msg),
      },
    }
  }
}