show a prompt

src/main.rs

@@ -7,6 +7,12 @@ fn main() {
 
     let mut line = String::new();
     loop {
+        if let Err(_) = stdout.write(b"> ") {
+            break;
+        }
+        if let Err(_) = stdout.flush() {
+            break;
+        }
         match stdin.read_line(&mut line) {
             Ok(_) => {
                 if let Err(_) = stdout.write(b"ok\n") {