main.rs

use std::io;

struct Stack(Vec<f32>);

fn main() -> io::Result<()> {
    let stdin = io::stdin();
    let mut stack = Stack::new();
    let mut line = String::new();

    'quit: loop {
        line.clear();
        stdin.read_line(&mut line)?;
        for word in line.split(char::is_whitespace) {
            if let Ok(num) = word.parse::<f32>() {
                stack.push(num);
            } else {
                match word {
                    "" => continue,
                    "exit" => break 'quit,
                    "+" => stack.apply(|a, b| a + b),
                    "-" => stack.apply(|a, b| a - b),
                    "/" => stack.apply(|a, b| a / b),
                    "*" => stack.apply(|a, b| a * b),
                    "%" => stack.apply(|a, b| a % b),
                    "." => println!("{}", stack.pop()),
                    _ => println!("? {}", word)
                }
            }
        }
    }

    println!("Bye!");
    Ok(())
}

impl Stack {
    fn new() -> Self { Self(Vec::new()) }

    fn push(&mut self, val: f32) {
        self.0.push(val);
    }

    fn pop(&mut self) -> f32 {
        return self.0.pop().expect("Stack underflow");
    }
    fn apply<F: Fn(f32, f32) -> f32>(&mut self, op: F) {
        let b = self.0.pop().expect("Stack underflow");
        let a = self.0.pop().expect("Stack underflow");
        self.0.push(op(a, b));
    }
}