church.rs

use std::rc::Rc;
use std::cell::RefCell;
use std::fmt::{ self, Display };

enum MyNumber {
    One(Rc<RefCell<MyNumber>>),
    Zero,
}

impl MyNumber {
    fn zero() -> MyNumber {
        MyNumber::Zero
    }

    fn plus(self) -> MyNumber {
        MyNumber::One(Rc::new(RefCell::new(self)))
    }

    fn value(&self) -> u8 {
        match self {
            MyNumber::Zero => 0,
            MyNumber::One(val) => {
                (*val).borrow().value() + 1
            },
        }
    }
}

impl Display for MyNumber {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.value())
    }
}

fn main() {
    println!("{}", MyNumber::zero().plus());
}