Fix README example for on_print and on_debug.

tests/print.rs

@@ -0,0 +1,31 @@
+use rhai::{Engine, EvalAltResult};
+use std::sync::{Arc, RwLock};
+
+#[test]
+fn test_print() -> Result<(), Box<EvalAltResult>> {
+    let mut engine = Engine::new();
+
+    let logbook = Arc::new(RwLock::new(Vec::<String>::new()));
+
+    // Redirect print/debug output to 'log'
+    let log = logbook.clone();
+    engine.on_print(move |s| log.write().unwrap().push(format!("entry: {}", s)));
+
+    let log = logbook.clone();
+    engine.on_debug(move |s| log.write().unwrap().push(format!("DEBUG: {}", s)));
+
+    // Evaluate script
+    engine.eval::<()>("print(40 + 2)")?;
+    engine.eval::<()>(r#"debug("hello!")"#)?;
+
+    // 'logbook' captures all the 'print' and 'debug' output
+    assert_eq!(logbook.read().unwrap().len(), 2);
+    assert_eq!(logbook.read().unwrap()[0], "entry: 42");
+    assert_eq!(logbook.read().unwrap()[1], r#"DEBUG: "hello!""#);
+
+    for entry in logbook.read().unwrap().iter() {
+        println!("{}", entry);
+    }
+
+    Ok(())
+}