builtin: add Nat.Sub

Author: Defelo

src/par/builtin/Builtin.par

@@ -1,6 +1,7 @@
 /// Nat
 
 dec Nat.Add   : [Nat, Nat] Nat
+dec Nat.Sub   : [Nat, Int] Nat
 dec Nat.Mul   : [Nat, Nat] Nat
 dec Nat.Div   : [Nat, Nat] Nat
 dec Nat.Mod   : [Nat, Nat] Nat

src/par/builtin/nat.rs

@@ -22,6 +22,11 @@ pub fn external_module() -> Module<Arc<process::Expression<()>>> {
                 Type::function(Type::nat(), Type::function(Type::nat(), Type::nat())),
                 |handle| Box::pin(nat_add(handle)),
             ),
+            Definition::external(
+                "Sub",
+                Type::function(Type::nat(), Type::function(Type::int(), Type::nat())),
+                |handle| Box::pin(nat_sub(handle)),
+            ),
             Definition::external(
                 "Mul",
                 Type::function(Type::nat(), Type::function(Type::nat(), Type::nat())),
@@ -136,6 +141,12 @@ async fn nat_add(mut handle: Handle) {
     handle.provide_nat(x + y);
 }
 
+async fn nat_sub(mut handle: Handle) {
+    let x = handle.receive().nat().await;
+    let y = handle.receive().int().await;
+    handle.provide_nat((x - y).max(0.into()));
+}
+
 async fn nat_mul(mut handle: Handle) {
     let x = handle.receive().nat().await;
     let y = handle.receive().nat().await;