diff --git a/RELEASES.md b/RELEASES.md
index aec6810f..ac4ddd40 100644
--- a/RELEASES.md
+++ b/RELEASES.md
@@ -8,6 +8,7 @@ Bug fixes
 ---------
 
 * Constants are no longer propagated by the optimizer if shadowed by a non-constant variable.
+* Constants passed as the `this` parameter to Rhai functions now throws an error if assigned to.
 
 
 Version 0.19.7
diff --git a/src/ast.rs b/src/ast.rs
index 14156b88..140a7cd8 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -1,6 +1,6 @@
 //! Module defining the AST (abstract syntax tree).
 
-use crate::dynamic::{AccessType, Union};
+use crate::dynamic::{AccessMode, Union};
 use crate::fn_native::shared_make_mut;
 use crate::module::NamespaceRef;
 use crate::stdlib::{
@@ -942,7 +942,7 @@ impl Expr {
             Self::StringConstant(x, _) => x.clone().into(),
             Self::FnPointer(x, _) => Dynamic(Union::FnPtr(
                 Box::new(FnPtr::new_unchecked(x.clone(), Default::default())),
-                AccessType::Constant,
+                AccessMode::ReadOnly,
             )),
             Self::BoolConstant(x, _) => (*x).into(),
             Self::Unit(_) => ().into(),
@@ -954,7 +954,7 @@ impl Expr {
                     x.len(),
                 ));
                 arr.extend(x.iter().map(|v| v.get_constant_value().unwrap()));
-                Dynamic(Union::Array(Box::new(arr), AccessType::Constant))
+                Dynamic(Union::Array(Box::new(arr), AccessMode::ReadOnly))
             }
 
             #[cfg(not(feature = "no_object"))]
@@ -967,7 +967,7 @@ impl Expr {
                     x.iter()
                         .map(|(k, v)| (k.name.clone(), v.get_constant_value().unwrap())),
                 );
-                Dynamic(Union::Map(Box::new(map), AccessType::Constant))
+                Dynamic(Union::Map(Box::new(map), AccessMode::ReadOnly))
             }
 
             _ => return None,
diff --git a/src/dynamic.rs b/src/dynamic.rs
index 750c07c6..e5880ea5 100644
--- a/src/dynamic.rs
+++ b/src/dynamic.rs
@@ -116,21 +116,21 @@ impl dyn Variant {
     }
 }
 
-/// Type of an entry in the Scope.
+/// Modes of access.
 #[derive(Debug, Eq, PartialEq, Hash, Copy, Clone)]
-pub enum AccessType {
-    /// Normal value.
-    Normal,
-    /// Immutable constant value.
-    Constant,
+pub enum AccessMode {
+    /// Mutable.
+    ReadWrite,
+    /// Immutable.
+    ReadOnly,
 }
 
-impl AccessType {
-    /// Is the access type [`Constant`]?
-    pub fn is_constant(self) -> bool {
+impl AccessMode {
+    /// Is the access type [`ReadOnly`]?
+    pub fn is_read_only(self) -> bool {
         match self {
-            Self::Normal => false,
-            Self::Constant => true,
+            Self::ReadWrite => false,
+            Self::ReadOnly => true,
         }
     }
 }
@@ -142,25 +142,25 @@ pub struct Dynamic(pub(crate) Union);
 ///
 /// Most variants are boxed to reduce the size.
 pub enum Union {
-    Unit((), AccessType),
-    Bool(bool, AccessType),
-    Str(ImmutableString, AccessType),
-    Char(char, AccessType),
-    Int(INT, AccessType),
+    Unit((), AccessMode),
+    Bool(bool, AccessMode),
+    Str(ImmutableString, AccessMode),
+    Char(char, AccessMode),
+    Int(INT, AccessMode),
     #[cfg(not(feature = "no_float"))]
-    Float(FLOAT, AccessType),
+    Float(FLOAT, AccessMode),
     #[cfg(not(feature = "no_index"))]
-    Array(Box<Array>, AccessType),
+    Array(Box<Array>, AccessMode),
     #[cfg(not(feature = "no_object"))]
-    Map(Box<Map>, AccessType),
-    FnPtr(Box<FnPtr>, AccessType),
+    Map(Box<Map>, AccessMode),
+    FnPtr(Box<FnPtr>, AccessMode),
     #[cfg(not(feature = "no_std"))]
-    TimeStamp(Box<Instant>, AccessType),
+    TimeStamp(Box<Instant>, AccessMode),
 
-    Variant(Box<Box<dyn Variant>>, AccessType),
+    Variant(Box<Box<dyn Variant>>, AccessMode),
 
     #[cfg(not(feature = "no_closure"))]
-    Shared(crate::Shared<crate::Locked<Dynamic>>, AccessType),
+    Shared(crate::Shared<crate::Locked<Dynamic>>, AccessMode),
 }
 
 /// Underlying [`Variant`] read guard for [`Dynamic`].
@@ -506,27 +506,27 @@ impl Clone for Dynamic {
     /// The cloned copy is marked [`AccessType::Normal`] even if the original is constant.
     fn clone(&self) -> Self {
         match self.0 {
-            Union::Unit(value, _) => Self(Union::Unit(value, AccessType::Normal)),
-            Union::Bool(value, _) => Self(Union::Bool(value, AccessType::Normal)),
-            Union::Str(ref value, _) => Self(Union::Str(value.clone(), AccessType::Normal)),
-            Union::Char(value, _) => Self(Union::Char(value, AccessType::Normal)),
-            Union::Int(value, _) => Self(Union::Int(value, AccessType::Normal)),
+            Union::Unit(value, _) => Self(Union::Unit(value, AccessMode::ReadWrite)),
+            Union::Bool(value, _) => Self(Union::Bool(value, AccessMode::ReadWrite)),
+            Union::Str(ref value, _) => Self(Union::Str(value.clone(), AccessMode::ReadWrite)),
+            Union::Char(value, _) => Self(Union::Char(value, AccessMode::ReadWrite)),
+            Union::Int(value, _) => Self(Union::Int(value, AccessMode::ReadWrite)),
             #[cfg(not(feature = "no_float"))]
-            Union::Float(value, _) => Self(Union::Float(value, AccessType::Normal)),
+            Union::Float(value, _) => Self(Union::Float(value, AccessMode::ReadWrite)),
             #[cfg(not(feature = "no_index"))]
-            Union::Array(ref value, _) => Self(Union::Array(value.clone(), AccessType::Normal)),
+            Union::Array(ref value, _) => Self(Union::Array(value.clone(), AccessMode::ReadWrite)),
             #[cfg(not(feature = "no_object"))]
-            Union::Map(ref value, _) => Self(Union::Map(value.clone(), AccessType::Normal)),
-            Union::FnPtr(ref value, _) => Self(Union::FnPtr(value.clone(), AccessType::Normal)),
+            Union::Map(ref value, _) => Self(Union::Map(value.clone(), AccessMode::ReadWrite)),
+            Union::FnPtr(ref value, _) => Self(Union::FnPtr(value.clone(), AccessMode::ReadWrite)),
             #[cfg(not(feature = "no_std"))]
             Union::TimeStamp(ref value, _) => {
-                Self(Union::TimeStamp(value.clone(), AccessType::Normal))
+                Self(Union::TimeStamp(value.clone(), AccessMode::ReadWrite))
             }
 
             Union::Variant(ref value, _) => (***value).clone_into_dynamic(),
 
             #[cfg(not(feature = "no_closure"))]
-            Union::Shared(ref cell, _) => Self(Union::Shared(cell.clone(), AccessType::Normal)),
+            Union::Shared(ref cell, _) => Self(Union::Shared(cell.clone(), AccessMode::ReadWrite)),
         }
     }
 }
@@ -540,29 +540,29 @@ impl Default for Dynamic {
 
 impl Dynamic {
     /// A [`Dynamic`] containing a `()`.
-    pub const UNIT: Dynamic = Self(Union::Unit((), AccessType::Normal));
+    pub const UNIT: Dynamic = Self(Union::Unit((), AccessMode::ReadWrite));
     /// A [`Dynamic`] containing a `true`.
-    pub const TRUE: Dynamic = Self(Union::Bool(true, AccessType::Normal));
+    pub const TRUE: Dynamic = Self(Union::Bool(true, AccessMode::ReadWrite));
     /// A [`Dynamic`] containing a [`false`].
-    pub const FALSE: Dynamic = Self(Union::Bool(false, AccessType::Normal));
+    pub const FALSE: Dynamic = Self(Union::Bool(false, AccessMode::ReadWrite));
     /// A [`Dynamic`] containing the integer zero.
-    pub const ZERO: Dynamic = Self(Union::Int(0, AccessType::Normal));
+    pub const ZERO: Dynamic = Self(Union::Int(0, AccessMode::ReadWrite));
     /// A [`Dynamic`] containing the integer one.
-    pub const ONE: Dynamic = Self(Union::Int(1, AccessType::Normal));
+    pub const ONE: Dynamic = Self(Union::Int(1, AccessMode::ReadWrite));
     /// A [`Dynamic`] containing the integer negative one.
-    pub const NEGATIVE_ONE: Dynamic = Self(Union::Int(-1, AccessType::Normal));
+    pub const NEGATIVE_ONE: Dynamic = Self(Union::Int(-1, AccessMode::ReadWrite));
     /// A [`Dynamic`] containing the floating-point zero.
     #[cfg(not(feature = "no_float"))]
-    pub const FLOAT_ZERO: Dynamic = Self(Union::Float(0.0, AccessType::Normal));
+    pub const FLOAT_ZERO: Dynamic = Self(Union::Float(0.0, AccessMode::ReadWrite));
     /// A [`Dynamic`] containing the floating-point one.
     #[cfg(not(feature = "no_float"))]
-    pub const FLOAT_ONE: Dynamic = Self(Union::Float(1.0, AccessType::Normal));
+    pub const FLOAT_ONE: Dynamic = Self(Union::Float(1.0, AccessMode::ReadWrite));
     /// A [`Dynamic`] containing the floating-point negative one.
     #[cfg(not(feature = "no_float"))]
-    pub const FLOAT_NEGATIVE_ONE: Dynamic = Self(Union::Float(-1.0, AccessType::Normal));
+    pub const FLOAT_NEGATIVE_ONE: Dynamic = Self(Union::Float(-1.0, AccessMode::ReadWrite));
 
-    /// Get the [`AccessType`] for this [`Dynamic`].
-    pub(crate) fn access_type(&self) -> AccessType {
+    /// Get the [`AccessMode`] for this [`Dynamic`].
+    pub(crate) fn access_mode(&self) -> AccessMode {
         match self.0 {
             Union::Unit(_, access)
             | Union::Bool(_, access)
@@ -584,8 +584,8 @@ impl Dynamic {
             Union::Shared(_, access) => access,
         }
     }
-    /// Set the [`AccessType`] for this [`Dynamic`].
-    pub(crate) fn set_access_type(&mut self, typ: AccessType) {
+    /// Set the [`AccessMode`] for this [`Dynamic`].
+    pub(crate) fn set_access_mode(&mut self, typ: AccessMode) {
         match &mut self.0 {
             Union::Unit(_, access)
             | Union::Bool(_, access)
@@ -615,7 +615,7 @@ impl Dynamic {
     /// if its value is going to be modified. This safe-guards constant values from being modified
     /// from within Rust functions.
     pub fn is_read_only(&self) -> bool {
-        self.access_type().is_constant()
+        self.access_mode().is_read_only()
     }
     /// Create a [`Dynamic`] from any type.  A [`Dynamic`] value is simply returned as is.
     ///
@@ -733,7 +733,7 @@ impl Dynamic {
             }
         }
 
-        Self(Union::Variant(Box::new(boxed), AccessType::Normal))
+        Self(Union::Variant(Box::new(boxed), AccessMode::ReadWrite))
     }
     /// Turn the [`Dynamic`] value into a shared [`Dynamic`] value backed by an [`Rc`][std::rc::Rc]`<`[`RefCell`][std::cell::RefCell]`<`[`Dynamic`]`>>`
     /// or [`Arc`][std::sync::Arc]`<`[`RwLock`][std::sync::RwLock]`<`[`Dynamic`]`>>` depending on the `sync` feature.
@@ -750,7 +750,7 @@ impl Dynamic {
     /// Panics under the `no_closure` feature.
     #[inline(always)]
     pub fn into_shared(self) -> Self {
-        let _access = self.access_type();
+        let _access = self.access_mode();
 
         #[cfg(not(feature = "no_closure"))]
         return match self.0 {
@@ -1338,38 +1338,38 @@ impl Dynamic {
 impl From<()> for Dynamic {
     #[inline(always)]
     fn from(value: ()) -> Self {
-        Self(Union::Unit(value, AccessType::Normal))
+        Self(Union::Unit(value, AccessMode::ReadWrite))
     }
 }
 impl From<bool> for Dynamic {
     #[inline(always)]
     fn from(value: bool) -> Self {
-        Self(Union::Bool(value, AccessType::Normal))
+        Self(Union::Bool(value, AccessMode::ReadWrite))
     }
 }
 impl From<INT> for Dynamic {
     #[inline(always)]
     fn from(value: INT) -> Self {
-        Self(Union::Int(value, AccessType::Normal))
+        Self(Union::Int(value, AccessMode::ReadWrite))
     }
 }
 #[cfg(not(feature = "no_float"))]
 impl From<FLOAT> for Dynamic {
     #[inline(always)]
     fn from(value: FLOAT) -> Self {
-        Self(Union::Float(value, AccessType::Normal))
+        Self(Union::Float(value, AccessMode::ReadWrite))
     }
 }
 impl From<char> for Dynamic {
     #[inline(always)]
     fn from(value: char) -> Self {
-        Self(Union::Char(value, AccessType::Normal))
+        Self(Union::Char(value, AccessMode::ReadWrite))
     }
 }
 impl<S: Into<ImmutableString>> From<S> for Dynamic {
     #[inline(always)]
     fn from(value: S) -> Self {
-        Self(Union::Str(value.into(), AccessType::Normal))
+        Self(Union::Str(value.into(), AccessMode::ReadWrite))
     }
 }
 #[cfg(not(feature = "no_index"))]
@@ -1378,7 +1378,7 @@ impl<T: Variant + Clone> From<crate::stdlib::vec::Vec<T>> for Dynamic {
     fn from(value: crate::stdlib::vec::Vec<T>) -> Self {
         Self(Union::Array(
             Box::new(value.into_iter().map(Dynamic::from).collect()),
-            AccessType::Normal,
+            AccessMode::ReadWrite,
         ))
     }
 }
@@ -1388,7 +1388,7 @@ impl<T: Variant + Clone> From<&[T]> for Dynamic {
     fn from(value: &[T]) -> Self {
         Self(Union::Array(
             Box::new(value.iter().cloned().map(Dynamic::from).collect()),
-            AccessType::Normal,
+            AccessMode::ReadWrite,
         ))
     }
 }
@@ -1405,26 +1405,26 @@ impl<K: Into<ImmutableString>, T: Variant + Clone> From<crate::stdlib::collectio
                     .map(|(k, v)| (k.into(), Dynamic::from(v)))
                     .collect(),
             ),
-            AccessType::Normal,
+            AccessMode::ReadWrite,
         ))
     }
 }
 impl From<FnPtr> for Dynamic {
     #[inline(always)]
     fn from(value: FnPtr) -> Self {
-        Self(Union::FnPtr(Box::new(value), AccessType::Normal))
+        Self(Union::FnPtr(Box::new(value), AccessMode::ReadWrite))
     }
 }
 impl From<Box<FnPtr>> for Dynamic {
     #[inline(always)]
     fn from(value: Box<FnPtr>) -> Self {
-        Self(Union::FnPtr(value, AccessType::Normal))
+        Self(Union::FnPtr(value, AccessMode::ReadWrite))
     }
 }
 #[cfg(not(feature = "no_std"))]
 impl From<Instant> for Dynamic {
     #[inline(always)]
     fn from(value: Instant) -> Self {
-        Self(Union::TimeStamp(Box::new(value), AccessType::Normal))
+        Self(Union::TimeStamp(Box::new(value), AccessMode::ReadWrite))
     }
 }
diff --git a/src/engine.rs b/src/engine.rs
index 7fcb0dbd..e42d06ac 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -1,7 +1,7 @@
 //! Main module defining the script evaluation [`Engine`].
 
 use crate::ast::{Expr, FnCallExpr, Ident, IdentX, ReturnType, Stmt};
-use crate::dynamic::{map_std_type_name, AccessType, Union, Variant};
+use crate::dynamic::{map_std_type_name, AccessMode, Union, Variant};
 use crate::fn_call::run_builtin_op_assignment;
 use crate::fn_native::{CallableFunction, Callback, IteratorFn, OnVarCallback};
 use crate::module::NamespaceRef;
@@ -850,7 +850,7 @@ impl Engine {
 
                     // Module variables are constant
                     let mut target = target.clone();
-                    target.set_access_type(AccessType::Constant);
+                    target.set_access_mode(AccessMode::ReadOnly);
                     Ok((target.into(), name, *pos))
                 }
                 // Normal variable access
@@ -905,7 +905,7 @@ impl Engine {
             if let Some(mut result) =
                 resolve_var(name, index, &context).map_err(|err| err.fill_position(*pos))?
             {
-                result.set_access_type(AccessType::Constant);
+                result.set_access_mode(AccessMode::ReadOnly);
                 return Ok((result.into(), name, *pos));
             }
         }
@@ -1732,7 +1732,7 @@ impl Engine {
                 for item in x.as_ref() {
                     arr.push(self.eval_expr(scope, mods, state, lib, this_ptr, item, level)?);
                 }
-                Ok(Dynamic(Union::Array(Box::new(arr), AccessType::Normal)))
+                Ok(Dynamic(Union::Array(Box::new(arr), AccessMode::ReadWrite)))
             }
 
             #[cfg(not(feature = "no_object"))]
@@ -1745,7 +1745,7 @@ impl Engine {
                         self.eval_expr(scope, mods, state, lib, this_ptr, expr, level)?,
                     );
                 }
-                Ok(Dynamic(Union::Map(Box::new(map), AccessType::Normal)))
+                Ok(Dynamic(Union::Map(Box::new(map), AccessMode::ReadWrite)))
             }
 
             // Normal function call
@@ -2289,8 +2289,8 @@ impl Engine {
             // Let/const statement
             Stmt::Let(var_def, expr, export, _) | Stmt::Const(var_def, expr, export, _) => {
                 let entry_type = match stmt {
-                    Stmt::Let(_, _, _, _) => AccessType::Normal,
-                    Stmt::Const(_, _, _, _) => AccessType::Constant,
+                    Stmt::Let(_, _, _, _) => AccessMode::ReadWrite,
+                    Stmt::Const(_, _, _, _) => AccessMode::ReadOnly,
                     _ => unreachable!(),
                 };
 
@@ -2384,7 +2384,7 @@ impl Engine {
             #[cfg(not(feature = "no_closure"))]
             Stmt::Share(x) => {
                 match scope.get_index(&x.name) {
-                    Some((index, AccessType::Normal)) => {
+                    Some((index, AccessMode::ReadWrite)) => {
                         let val = scope.get_mut(index);
 
                         if !val.is_shared() {
diff --git a/src/optimize.rs b/src/optimize.rs
index 77a60e46..f04dda40 100644
--- a/src/optimize.rs
+++ b/src/optimize.rs
@@ -1,7 +1,7 @@
 //! Module implementing the [`AST`] optimizer.
 
 use crate::ast::{Expr, ScriptFnDef, Stmt};
-use crate::dynamic::AccessType;
+use crate::dynamic::AccessMode;
 use crate::engine::{KEYWORD_DEBUG, KEYWORD_EVAL, KEYWORD_PRINT, KEYWORD_TYPE_OF};
 use crate::fn_call::run_builtin_binary_op;
 use crate::parser::map_dynamic_to_expr;
@@ -59,7 +59,7 @@ struct State<'a> {
     /// Has the [`AST`] been changed during this pass?
     changed: bool,
     /// Collection of constants to use for eager function evaluations.
-    variables: Vec<(String, AccessType, Expr)>,
+    variables: Vec<(String, AccessMode, Expr)>,
     /// An [`Engine`] instance for eager function evaluation.
     engine: &'a Engine,
     /// [Module] containing script-defined functions.
@@ -102,7 +102,7 @@ impl<'a> State<'a> {
     }
     /// Add a new constant to the list.
     #[inline(always)]
-    pub fn push_var(&mut self, name: &str, access: AccessType, value: Expr) {
+    pub fn push_var(&mut self, name: &str, access: AccessMode, value: Expr) {
         self.variables.push((name.into(), access, value))
     }
     /// Look up a constant from the list.
@@ -110,7 +110,7 @@ impl<'a> State<'a> {
     pub fn find_constant(&self, name: &str) -> Option<&Expr> {
         for (n, access, expr) in self.variables.iter().rev() {
             if n == name {
-                return if access.is_constant() {
+                return if access.is_read_only() {
                     Some(expr)
                 } else {
                     None
@@ -163,14 +163,18 @@ fn optimize_stmt_block(
     statements.iter_mut().for_each(|stmt| match stmt {
         // Add constant literals into the state
         Stmt::Const(var_def, Some(expr), _, _) if expr.is_constant() => {
-            state.push_var(&var_def.name, AccessType::Constant, mem::take(expr));
+            state.push_var(&var_def.name, AccessMode::ReadOnly, mem::take(expr));
         }
         Stmt::Const(var_def, None, _, _) => {
-            state.push_var(&var_def.name, AccessType::Constant, Expr::Unit(var_def.pos));
+            state.push_var(&var_def.name, AccessMode::ReadOnly, Expr::Unit(var_def.pos));
         }
         // Add variables into the state
         Stmt::Let(var_def, _, _, _) => {
-            state.push_var(&var_def.name, AccessType::Normal, Expr::Unit(var_def.pos));
+            state.push_var(
+                &var_def.name,
+                AccessMode::ReadWrite,
+                Expr::Unit(var_def.pos),
+            );
         }
         // Optimize the statement
         _ => optimize_stmt(stmt, state, preserve_result),
@@ -749,11 +753,11 @@ fn optimize_top_level(
     // Add constants and variables from the scope
     scope.iter().for_each(|(name, constant, value)| {
         if !constant {
-            state.push_var(name, AccessType::Normal, Expr::Unit(Position::NONE));
+            state.push_var(name, AccessMode::ReadWrite, Expr::Unit(Position::NONE));
         } else if let Some(val) = map_dynamic_to_expr(value, Position::NONE) {
-            state.push_var(name, AccessType::Constant, val);
+            state.push_var(name, AccessMode::ReadOnly, val);
         } else {
-            state.push_var(name, AccessType::Constant, Expr::Unit(Position::NONE));
+            state.push_var(name, AccessMode::ReadOnly, Expr::Unit(Position::NONE));
         }
     });
 
@@ -774,7 +778,7 @@ fn optimize_top_level(
                     optimize_expr(value_expr, &mut state);
 
                     if value_expr.is_constant() {
-                        state.push_var(&var_def.name, AccessType::Constant, value_expr.clone());
+                        state.push_var(&var_def.name, AccessMode::ReadOnly, value_expr.clone());
                     }
 
                     // Keep it in the global scope
@@ -784,10 +788,14 @@ fn optimize_top_level(
                     }
                 }
                 Stmt::Const(var_def, None, _, _) => {
-                    state.push_var(&var_def.name, AccessType::Constant, Expr::Unit(var_def.pos));
+                    state.push_var(&var_def.name, AccessMode::ReadOnly, Expr::Unit(var_def.pos));
                 }
                 Stmt::Let(var_def, _, _, _) => {
-                    state.push_var(&var_def.name, AccessType::Normal, Expr::Unit(var_def.pos));
+                    state.push_var(
+                        &var_def.name,
+                        AccessMode::ReadWrite,
+                        Expr::Unit(var_def.pos),
+                    );
                 }
                 _ => {
                     // Keep all variable declarations at this level
diff --git a/src/parser.rs b/src/parser.rs
index 956e197b..34c4f848 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -3,7 +3,7 @@
 use crate::ast::{
     BinaryExpr, CustomExpr, Expr, FnCallExpr, Ident, IdentX, ReturnType, ScriptFnDef, Stmt,
 };
-use crate::dynamic::{AccessType, Union};
+use crate::dynamic::{AccessMode, Union};
 use crate::engine::{KEYWORD_THIS, MARKER_BLOCK, MARKER_EXPR, MARKER_IDENT};
 use crate::module::NamespaceRef;
 use crate::optimize::optimize_into_ast;
@@ -48,7 +48,7 @@ struct ParseState<'e> {
     /// Interned strings.
     strings: HashMap<String, ImmutableString>,
     /// Encapsulates a local stack with variable names to simulate an actual runtime scope.
-    stack: Vec<(ImmutableString, AccessType)>,
+    stack: Vec<(ImmutableString, AccessMode)>,
     /// Size of the local variables stack upon entry of the current block scope.
     entry_stack_len: usize,
     /// Tracks a list of external variables (variables that are not explicitly declared in the scope).
@@ -1290,11 +1290,11 @@ fn make_assignment_stmt<'a>(
                 },
             ) = x.as_ref();
             match state.stack[(state.stack.len() - index.unwrap().get())].1 {
-                AccessType::Normal => {
+                AccessMode::ReadWrite => {
                     Ok(Stmt::Assignment(Box::new((lhs, fn_name.into(), rhs)), pos))
                 }
                 // Constant values cannot be assigned to
-                AccessType::Constant => {
+                AccessMode::ReadOnly => {
                     Err(PERR::AssignmentToConstant(name.to_string()).into_err(*name_pos))
                 }
             }
@@ -1317,11 +1317,11 @@ fn make_assignment_stmt<'a>(
                     },
                 ) = x.as_ref();
                 match state.stack[(state.stack.len() - index.unwrap().get())].1 {
-                    AccessType::Normal => {
+                    AccessMode::ReadWrite => {
                         Ok(Stmt::Assignment(Box::new((lhs, fn_name.into(), rhs)), pos))
                     }
                     // Constant values cannot be assigned to
-                    AccessType::Constant => {
+                    AccessMode::ReadOnly => {
                         Err(PERR::AssignmentToConstant(name.to_string()).into_err(*name_pos))
                     }
                 }
@@ -1810,7 +1810,7 @@ fn parse_custom_syntax(
             // Variable searches stop at the first empty variable name.
             state.stack.resize(
                 state.stack.len() + delta as usize,
-                ("".into(), AccessType::Normal),
+                ("".into(), AccessMode::ReadWrite),
             );
         }
         delta if delta < 0 && state.stack.len() <= delta.abs() as usize => state.stack.clear(),
@@ -2109,7 +2109,7 @@ fn parse_for(
 
     let loop_var = state.get_interned_string(name.clone());
     let prev_stack_len = state.stack.len();
-    state.stack.push((loop_var, AccessType::Normal));
+    state.stack.push((loop_var, AccessMode::ReadWrite));
 
     settings.is_breakable = true;
     let body = parse_block(input, state, lib, settings.level_up())?;
@@ -2124,7 +2124,7 @@ fn parse_let(
     input: &mut TokenStream,
     state: &mut ParseState,
     lib: &mut FunctionsLib,
-    var_type: AccessType,
+    var_type: AccessMode,
     export: bool,
     mut settings: ParseSettings,
 ) -> Result<Stmt, ParseError> {
@@ -2155,16 +2155,16 @@ fn parse_let(
 
     match var_type {
         // let name = expr
-        AccessType::Normal => {
+        AccessMode::ReadWrite => {
             let var_name = state.get_interned_string(name.clone());
-            state.stack.push((var_name, AccessType::Normal));
+            state.stack.push((var_name, AccessMode::ReadWrite));
             let var_def = Ident::new(name, pos);
             Ok(Stmt::Let(Box::new(var_def), init_expr, export, token_pos))
         }
         // const name = { expr:constant }
-        AccessType::Constant => {
+        AccessMode::ReadOnly => {
             let var_name = state.get_interned_string(name.clone());
-            state.stack.push((var_name, AccessType::Constant));
+            state.stack.push((var_name, AccessMode::ReadOnly));
             let var_def = Ident::new(name, pos);
             Ok(Stmt::Const(Box::new(var_def), init_expr, export, token_pos))
         }
@@ -2231,13 +2231,13 @@ fn parse_export(
     match input.peek().unwrap() {
         (Token::Let, pos) => {
             let pos = *pos;
-            let mut stmt = parse_let(input, state, lib, AccessType::Normal, true, settings)?;
+            let mut stmt = parse_let(input, state, lib, AccessMode::ReadWrite, true, settings)?;
             stmt.set_position(pos);
             return Ok(stmt);
         }
         (Token::Const, pos) => {
             let pos = *pos;
-            let mut stmt = parse_let(input, state, lib, AccessType::Constant, true, settings)?;
+            let mut stmt = parse_let(input, state, lib, AccessMode::ReadOnly, true, settings)?;
             stmt.set_position(pos);
             return Ok(stmt);
         }
@@ -2392,7 +2392,7 @@ fn parse_stmt(
     lib: &mut FunctionsLib,
     mut settings: ParseSettings,
 ) -> Result<Option<Stmt>, ParseError> {
-    use AccessType::{Constant, Normal};
+    use AccessMode::{ReadOnly, ReadWrite};
 
     let (token, token_pos) = match input.peek().unwrap() {
         (Token::EOF, pos) => return Ok(Some(Stmt::Noop(*pos))),
@@ -2520,9 +2520,9 @@ fn parse_stmt(
 
         Token::Try => parse_try_catch(input, state, lib, settings.level_up()).map(Some),
 
-        Token::Let => parse_let(input, state, lib, Normal, false, settings.level_up()).map(Some),
+        Token::Let => parse_let(input, state, lib, ReadWrite, false, settings.level_up()).map(Some),
         Token::Const => {
-            parse_let(input, state, lib, Constant, false, settings.level_up()).map(Some)
+            parse_let(input, state, lib, ReadOnly, false, settings.level_up()).map(Some)
         }
 
         #[cfg(not(feature = "no_module"))]
@@ -2636,7 +2636,7 @@ fn parse_fn(
                         return Err(PERR::FnDuplicatedParam(name, s).into_err(pos));
                     }
                     let s = state.get_interned_string(s);
-                    state.stack.push((s.clone(), AccessType::Normal));
+                    state.stack.push((s.clone(), AccessMode::ReadWrite));
                     params.push((s, pos))
                 }
                 (Token::LexError(err), pos) => return Err(err.into_err(pos)),
@@ -2769,7 +2769,7 @@ fn parse_anon_fn(
                             return Err(PERR::FnDuplicatedParam("".to_string(), s).into_err(pos));
                         }
                         let s = state.get_interned_string(s);
-                        state.stack.push((s.clone(), AccessType::Normal));
+                        state.stack.push((s.clone(), AccessMode::ReadWrite));
                         params.push((s, pos))
                     }
                     (Token::LexError(err), pos) => return Err(err.into_err(pos)),
diff --git a/src/scope.rs b/src/scope.rs
index 42216538..93b4ba7d 100644
--- a/src/scope.rs
+++ b/src/scope.rs
@@ -1,6 +1,6 @@
 //! Module that defines the [`Scope`] type representing a function call-stack scope.
 
-use crate::dynamic::{AccessType, Variant};
+use crate::dynamic::{AccessMode, Variant};
 use crate::stdlib::{borrow::Cow, boxed::Box, iter, string::String, vec::Vec};
 use crate::{Dynamic, StaticVec};
 
@@ -156,7 +156,7 @@ impl<'a> Scope<'a> {
         name: impl Into<Cow<'a, str>>,
         value: impl Variant + Clone,
     ) -> &mut Self {
-        self.push_dynamic_value(name, AccessType::Normal, Dynamic::from(value))
+        self.push_dynamic_value(name, AccessMode::ReadWrite, Dynamic::from(value))
     }
     /// Add (push) a new [`Dynamic`] entry to the [`Scope`].
     ///
@@ -172,7 +172,7 @@ impl<'a> Scope<'a> {
     /// ```
     #[inline(always)]
     pub fn push_dynamic(&mut self, name: impl Into<Cow<'a, str>>, value: Dynamic) -> &mut Self {
-        self.push_dynamic_value(name, value.access_type(), value)
+        self.push_dynamic_value(name, value.access_mode(), value)
     }
     /// Add (push) a new constant to the [`Scope`].
     ///
@@ -195,7 +195,7 @@ impl<'a> Scope<'a> {
         name: impl Into<Cow<'a, str>>,
         value: impl Variant + Clone,
     ) -> &mut Self {
-        self.push_dynamic_value(name, AccessType::Constant, Dynamic::from(value))
+        self.push_dynamic_value(name, AccessMode::ReadOnly, Dynamic::from(value))
     }
     /// Add (push) a new constant with a [`Dynamic`] value to the Scope.
     ///
@@ -218,18 +218,18 @@ impl<'a> Scope<'a> {
         name: impl Into<Cow<'a, str>>,
         value: Dynamic,
     ) -> &mut Self {
-        self.push_dynamic_value(name, AccessType::Constant, value)
+        self.push_dynamic_value(name, AccessMode::ReadOnly, value)
     }
     /// Add (push) a new entry with a [`Dynamic`] value to the [`Scope`].
     #[inline]
     pub(crate) fn push_dynamic_value(
         &mut self,
         name: impl Into<Cow<'a, str>>,
-        access: AccessType,
+        access: AccessMode,
         mut value: Dynamic,
     ) -> &mut Self {
         self.names.push((name.into(), Box::new(Default::default())));
-        value.set_access_type(access);
+        value.set_access_mode(access);
         self.values.push(value.into());
         self
     }
@@ -287,14 +287,14 @@ impl<'a> Scope<'a> {
     }
     /// Find an entry in the [`Scope`], starting from the last.
     #[inline(always)]
-    pub(crate) fn get_index(&self, name: &str) -> Option<(usize, AccessType)> {
+    pub(crate) fn get_index(&self, name: &str) -> Option<(usize, AccessMode)> {
         self.names
             .iter()
             .enumerate()
             .rev() // Always search a Scope in reverse order
             .find_map(|(index, (key, _))| {
                 if name == key.as_ref() {
-                    Some((index, self.values[index].access_type()))
+                    Some((index, self.values[index].access_mode()))
                 } else {
                     None
                 }
@@ -349,8 +349,8 @@ impl<'a> Scope<'a> {
             None => {
                 self.push(name, value);
             }
-            Some((_, AccessType::Constant)) => panic!("variable {} is constant", name),
-            Some((index, AccessType::Normal)) => {
+            Some((_, AccessMode::ReadOnly)) => panic!("variable {} is constant", name),
+            Some((index, AccessMode::ReadWrite)) => {
                 *self.values.get_mut(index).unwrap() = Dynamic::from(value);
             }
         }