Shared via Rust Playground

playground.rs

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

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

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

impl ToInt for i32 {fn to_int(self) -> i32 {self}}

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() {
   print!("{:?}\n", plus!(to_peano!(3), to_peano!(2)));
  
}