Shared via Rust Playground

playground.rs

#![feature(trace_macros)]
trace_macros!(true);

/// Piano numbers
#[derive(Debug, Clone, Copy)]
struct S<T>(T);

trait ToInt {
    fn to_int(self) -> i32;
}

impl ToInt for i32 {
    fn to_int(self) -> i32 {
        self
    }
}
impl<T> ToInt for S<T>
    where T: ToInt
{
    fn to_int(self) -> i32 {
        self.0.to_int()+1
    }
}

macro_rules! to_peano {
   (0) => (0);
   (1) => (S(0));
   (2) => (S(S(0)));
   (3) => (S(((0))));
   (4) => (S(to_peano!(3)));
   (5) => (S(to_peano!(4)));
   (6) => (S(to_peano!(5)));
   (7) => (S(to_peano!(6)));
   (8) => (S(to_peano!(7)));
}

macro_rules! plus {
  ($n:expr, 0) => ($n);
  ($n:expr, S($($m:tt)*)) => (plus!(S($n),$($m)*));
  //($n:expr, $($m:tt)*) => (plus!($n,to_peano!($($m)*)))
}

fn main() {
 let x : S<S<S<i32>>> = plus!(S(0), S(S(0)));
    print!("{:?} = {}\n", x, x.to_int());

}