My first non-hello rust program

calc.rs

use std::os;

enum Token {
    Add,
    Sub,
    Mul,
    Div,
    Mod,
    Func {name: ~str, args: ~[~str]},
    Num (int),
    Nil
}

fn parse_token(text: &str) -> Token {
    match FromStr::from_str::<int>(text) {
        Some(x) => Num(x),
        None => match text {
            "+" => Add,
            "-" => Sub,
            "*" => Mul,
            "/" => Div,
            "%" => Mod,
            _ => Nil
        }
    }
}

fn handle_token(tok: Token, stack: &[int]) -> Option<~[int]> {
    match tok {
        Num(x)  => Some(~[x] + stack),
        Add     => match stack {
            [a, b, ..rest]  => Some(~[a+b] + rest),
            _               => None
        },
        Sub     => match stack {
            [a, b, ..rest]  => Some(~[a-b] + rest),
            _               => None
        },
        Mul     => match stack {
            [a, b, ..rest]  => Some(~[a*b] + rest),
            _               => None
        },
        Div     => match stack {
            [a, b, ..rest]  => Some(~[a/b] + rest),
            _               => None
        },
        Mod     => match stack {
            [a, b, ..rest]  => Some(~[a%b] + rest),
            _               => None
        },
        _ => None
    }
}

fn main() {
    let mut stack : ~[int] = ~[];
    let args = os::args();
    for args.iter().advance |arg| {
        let tok = parse_token(*arg);
        //println(fmt!("%?", tok));
        let res : Option<~[int]> = handle_token(tok, stack);
        match res {
            Some(r) => stack = r,
            None    => ()
        }
    }
    match stack {
        [a] => println(fmt!("Result: %i", a)),
        _   => println("No result?")
    }
}