Add IO monad example

Callee/Callee.lean

@@ -17,3 +17,7 @@ def think (s: String): Think :=
     Think.Success (s.drop 1)
   else
     Think.Failure s.length
+
+@[export print_in_upper]
+def print_in_upper (s: String): IO Unit := do
+  IO.println s.toUpper

src/main.rs

@@ -23,6 +23,7 @@ mod callee {
 		pub fn my_function(s: lean_obj_arg) -> lean_obj_res;
 		pub fn stylize(s: lean_obj_arg) -> lean_obj_res;
 		pub fn think(s: lean_obj_arg) -> lean_obj_res;
+		pub fn print_in_upper(s: lean_obj_arg, world: lean_obj_arg) -> lean_obj_res;
 	}
 }
 
@@ -118,6 +119,22 @@ fn call_think(s: &str) -> Result<String, usize> {
 	}
 }
 
+fn call_print_in_upper()
+{
+	let c_str = std::ffi::CString::new("caller").unwrap();
+	unsafe {
+		let native_str = lean::lean_mk_string(c_str.as_ptr());
+		let res = callee::print_in_upper(native_str, lean::lean_io_mk_world());
+		if lean::lean_io_result_is_ok(res) {
+			lean::lean_dec_ref(res);
+		} else {
+			lean::lean_io_result_show_error(res);
+			lean::lean_dec(res);
+			panic!("IO Monad execution failed");
+		}
+	};
+}
+
 fn main() {
 	initialize();
 	println!("Lean initialization complete!");
@@ -127,4 +144,5 @@ fn main() {
 
 	assert_eq!(call_think("Stanford"), Ok("tanford".to_string()));
 	assert_eq!(call_think("Else"), Err(4));
+	call_print_in_upper();
 }