Fix constant example.

doc/src/language/constants.md

@@ -34,23 +34,28 @@ It is very useful to have a constant value hold a [custom type], which essential
 as a [_singleton_](../patterns/singleton.md).
 
 ```rust
-use rhai::{Engine, Scope};
+use rhai::{Engine, Scope, RegisterFn};
 
+#[derive(Debug, Clone)]
 struct TestStruct(i64);                                     // custom type
 
-let engine = Engine::new()
+let mut engine = Engine::new();
+
+engine
     .register_type_with_name::<TestStruct>("TestStruct")    // register custom type
-    .register_get_set("value",
+    .register_get("value", |obj: &mut TestStruct| obj.0),   // property getter
-        |obj: &mut TestStruct| obj.0,                       // property getter
+    .register_fn("update_value",
-        |obj: &mut TestStruct, value: i64| obj.0 = value    // property setter
+        |obj: &mut TestStruct, value: i64| obj.0 = value    // mutating method
     );
 
 let mut scope = Scope::new();                               // create custom scope
 
 scope.push_constant("MY_NUMBER", TestStruct(123_i64));      // add constant variable
 
-engine.consume_with_scope(&mut scope, r"
+// Beware: constant objects can still be modified via a method call!
-    MY_NUMBER.value = 42;                                   // constant objects can be modified
+engine.consume_with_scope(&mut scope,
+r"
+    MY_NUMBER.update_value(42);
     print(MY_NUMBER.value);                                 // prints 42
 ")?;
 ```

tests/constants.rs

@@ -1,4 +1,4 @@
-use rhai::{Engine, EvalAltResult, ParseErrorType, Scope, INT};
+use rhai::{Engine, EvalAltResult, ParseErrorType, RegisterFn, Scope, INT};
 
 #[test]
 fn test_constant() -> Result<(), Box<EvalAltResult>> {
@@ -62,3 +62,36 @@ fn test_var_is_def() -> Result<(), Box<EvalAltResult>> {
 
     Ok(())
 }
+
+#[cfg(not(feature = "no_object"))]
+#[test]
+fn test_constant_mut() -> Result<(), Box<EvalAltResult>> {
+    #[derive(Debug, Clone)]
+    struct TestStruct(INT); // custom type
+
+    let mut engine = Engine::new();
+
+    engine
+        .register_type_with_name::<TestStruct>("TestStruct")
+        .register_get("value", |obj: &mut TestStruct| obj.0)
+        .register_fn("update_value", |obj: &mut TestStruct, value: INT| {
+            obj.0 = value
+        });
+
+    let mut scope = Scope::new();
+
+    scope.push_constant("MY_NUMBER", TestStruct(123));
+
+    assert_eq!(
+        engine.eval_with_scope::<INT>(
+            &mut scope,
+            r"
+                MY_NUMBER.update_value(42);
+                MY_NUMBER.value
+            ",
+        )?,
+        42
+    );
+
+    Ok(())
+}