expr.rs

use untwine::{parser, parser_repl};

fn operate(left: f64, op: char, right: f64) -> f64 {
    match op {
        '+' => left + right,
        '-' => left - right,
        '/' => left / right,
        '*' => left * right,
        _ => unreachable!(),
    }
}

parser! {
    [recover = true]
    sep = #{char::is_ascii_whitespace}*;
    num: num=<'0'-'9'+ ("." '0'-'9'+)?> -> f64 { num.parse().unwrap() }
    term = (num | negation | "(" sep expr sep ")") -> f64;
    negation: "-" value=term -> f64 { -value }
    add: first=mul sep ops=(["+-"] sep mul)* -> f64 { ops.into_iter().fold(first, |left, (op, right)| operate(left, op, right)) }
    mul: first=term sep ops=(["*/"] sep term)* -> f64 { ops.into_iter().fold(first, |left, (op, right)| operate(left, op, right)) }
    pub expr = add -> f64;
}

fn main() {
    parser_repl(expr);
}