Discussion on: [Challenge] Add numbers without (+-*/)

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Replies for: I spent an unfortunate amount of time on this... fn add(a: u32, b: u32) -> u32 { let mut carry = false; let mut result = 0; for i...

Though I guess I'll be a bit more pedantic with this one: for i in 0..32 smuggles in some integer addition. So here's the set theoretic implementation, which has—less reliance on plus signs:

#[derive(Debug)]
enum Num {
    S(Box<Num>),
    Z,
}

fn s(n: Num) -> Num {
    Num::S(Box::new(n))
}

fn z() -> Num {
    Num::Z
}

fn set_add(a: Num, b: Num) -> Num {
    let mut result = a;
    let mut work = b;
    loop {
        match work {
            Num::S(next) => {
                result = s(result);
                work = *next;
            },
            Num::Z => return result,
        }
    }
}

fn main() {
    let a = s(s(s(z()))); // i.e., 3
    let b = s(s(s(s(s(z()))))); // i.e., 5
    println!("3 + 5 = {:?}", set_add(a, b));
}

Output:

3 + 5 = S(S(S(S(S(S(S(S(Z))))))))

Z means zero, and S means increment. So this is zero incremented eight times.