counting

src/count.rs

@@ -0,0 +1,42 @@
+use std::marker::PhantomData;
+
+trait Nat {
+    fn int() -> i32;
+}
+
+struct Z;
+
+impl Nat for Z {
+    fn int() -> i32 {
+        0
+    }
+}
+
+struct S<N: Nat>(PhantomData<N>);
+
+impl<N: Nat> Nat for S<N> {
+    fn int() -> i32 {
+        N::int() + 1
+    }
+}
+
+
+trait Count: Nat {
+    fn count() -> String;
+}
+
+impl Count for Z {
+    fn count() -> String {
+        "0".to_string()
+    }
+}
+
+impl<N: Count> Count for S<N> {
+    fn count() -> String {
+        format!("{} {}", N::count(), Self::int())
+    }
+}
+
+pub fn count() {
+    println!("{}", <S<S<S<Z>>> as Count>::count());
+}

src/main.rs

@@ -1,3 +1,5 @@
+mod count;
+
 fn main() {
-    println!("Hello, world!");
+    count::count();
 }