RPN Interpreter

rpn.rs

#[derive(Debug)]
enum Error {
    InvalidToken(String),
    InvalidProgram,
}

#[derive(Debug)]
enum Token {
    Add,
    Sub,
    Mul,
    Div,
    Number(i32),
}

fn lex(code: &str) -> Result<Vec<Token>, Error> {
    let mut tokens = vec![];

    for substring in code
        .split(' ')
        .filter(|s| s.len() > 0)
    {
        match substring {
            "+" => tokens.push(Token::Add),
            "-" => tokens.push(Token::Sub),
            "*" => tokens.push(Token::Mul),
            "/" => tokens.push(Token::Div),
            n => if let Ok(n) = n.parse::<i32>() {
                tokens.push(Token::Number(n));
            } else {
                return Err(Error::InvalidToken(substring.to_owned()));
            },
        }
    }

    Ok(tokens)
}

fn eval(tokens: &[Token]) -> Result<(i32, usize), Error> {
    if let Some(first_token) = tokens.first() {
        match first_token {
            Token::Add => {
                let (a, a_skip) = eval(&tokens[1..])?;
                let (b, b_skip) = eval(&tokens[1 + a_skip..])?;
                Ok((a + b, 1 + a_skip + b_skip))
            },
            Token::Sub => {
                let (a, a_skip) = eval(&tokens[1..])?;
                let (b, b_skip) = eval(&tokens[1 + a_skip..])?;
                Ok((a - b, 1 + a_skip + b_skip))
            },
            Token::Mul => {
                let (a, a_skip) = eval(&tokens[1..])?;
                let (b, b_skip) = eval(&tokens[1 + a_skip..])?;
                Ok((a * b, 1 + a_skip + b_skip))
            },
            Token::Div => {
                let (a, a_skip) = eval(&tokens[1..])?;
                let (b, b_skip) = eval(&tokens[1 + a_skip..])?;
                Ok((a / b, 1 + a_skip + b_skip))
            },
            Token::Number(n) => Ok((*n, 1)),
        }
    } else {
        Err(Error::InvalidProgram)
    }
}

fn main() {
    let s = "- + 5 * 8 12 * 42 2";

    let tokens = lex(s).unwrap();

    println!("Tokens: {:?}", tokens);

    let result = eval(&tokens);

    println!("Result: {:?}", result);
}