trampoline_fac.rs

enum FacResult {
    Become(i32,i32),
    Return(i32),
}

fn fac_tail(n: i32, accu: i32) -> FacResult {
    if n < 2 {
        // result known
        FacResult::Return(accu)
    } else {
        // the recursion case
        FacResult::Become(n-1, accu*n)
    }
}

fn fac_trampoline(n: i32) -> i32 {
    let mut result = FacResult::Become(n,1);
    while let FacResult::Become(n, accu) = result {
        result = fac_tail(n, accu);
    }
    if let FacResult::Return(v) = result {
        v
    } else {
        unreachable!()
    }
}

fn main() {
    println!("fac(5) = {}", fac_trampoline(5));
}