Add Scope::is_constant.

CHANGELOG.md

@@ -22,6 +22,7 @@ Enhancements
 * Added `log10()` for `Decimal`.
 * `ln` for `Decimal` is now checked and won't panic.
 * `Scope::set_value` now takes anything that implements `Into<Cow<str>>`.
+* Added `Scope::is_constant` to check if a variable is constant.
 
 
 Version 1.0.2

src/scope.rs

@@ -339,6 +339,30 @@ impl<'a> Scope<'a> {
             .find(|(_, (key, _))| name == key.as_ref())
             .and_then(|(index, _)| self.values[index].flatten_clone().try_cast())
     }
+    /// Check if the named entry in the [`Scope`] is constant.
+    ///
+    /// Search starts backwards from the last, stopping at the first entry matching the specified name.
+    ///
+    /// Returns [`None`] if no entry matching the specified name is found.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use rhai::Scope;
+    ///
+    /// let mut my_scope = Scope::new();
+    ///
+    /// my_scope.push_constant("x", 42_i64);
+    /// assert_eq!(my_scope.is_constant("x"), Some(true));
+    /// assert_eq!(my_scope.is_constant("y"), None);
+    /// ```
+    #[inline]
+    pub fn is_constant(&self, name: &str) -> Option<bool> {
+        self.get_index(name).and_then(|(_, access)| match access {
+            AccessMode::ReadWrite => None,
+            AccessMode::ReadOnly => Some(true),
+        })
+    }
     /// Update the value of the named entry in the [`Scope`].
     ///
     /// Search starts backwards from the last, and only the first entry matching the specified name is updated.

tests/custom_syntax.rs

@@ -157,13 +157,12 @@ fn test_custom_syntax() -> Result<(), Box<EvalAltResult>> {
             // Evaluate the expression
             let value = context.eval_expression_tree(expr)?;
 
-            // Push new variable into the scope if it doesn't already exist.
-            // Otherwise set its value.
-            // WARNING - This panics if 'var_name' already exists and is constant!
-            //         - In a real implementation, check this before doing anything!
-            context.scope_mut().set_value(var_name, value);
-
-            Ok(Dynamic::UNIT)
+            if !context.scope().is_constant(&var_name).unwrap_or(false) {
+                context.scope_mut().set_value(var_name, value);
+                Ok(Dynamic::UNIT)
+            } else {
+                Err(format!("variable {} is constant", var_name).into())
+            }
         },
     )?;