Implement index setters.

README.md

@@ -472,7 +472,7 @@ The follow packages are available:
 Packages typically contain Rust functions that are callable within a Rhai script.
 All functions registered in a package is loaded under the _global namespace_ (i.e. they're available without module qualifiers).
 Once a package is created (e.g. via `new`), it can be _shared_ (via `get`) among multiple instances of [`Engine`],
-even across threads (under the [`sync`] feature). Therefore, a package only has to be created _once_.
+even across threads (under [`sync`]). Therefore, a package only has to be created _once_.
 
 Packages are actually implemented as [modules], so they share a lot of behavior and characteristics.
 The main difference is that a package loads under the _global_ namespace, while a module loads under its own
@@ -572,7 +572,7 @@ if type_of(x) == "string" {
 
 [`Dynamic`]: #dynamic-values
 
-A `Dynamic` value can be _any_ type. However, under the [`sync`] feature, all types must be `Send + Sync`.
+A `Dynamic` value can be _any_ type. However, under [`sync`], all types must be `Send + Sync`.
 
 Because [`type_of()`] a `Dynamic` value returns the type of the actual value, it is usually used to perform type-specific
 actions based on the actual value's type.
@@ -975,8 +975,8 @@ let result = engine.eval::<i64>(
 println!("result: {}", result);                     // prints 1
 ```
 
-Under the [`no_object`] feature, however, the _method_ style of function calls
+Under [`no_object`], however, the _method_ style of function calls (i.e. calling a function as an object-method)
-(i.e. calling a function as an object-method) is no longer supported.
+is no longer supported.
 
 ```rust
 // Below is a syntax error under 'no_object' because 'clear' cannot be called in method style.
@@ -999,8 +999,7 @@ let x = new_ts();
 print(x.type_of());                                 // prints "Hello"
 ```
 
-Getters and setters
+### Getters and setters
--------------------
 
 Similarly, custom types can expose members by registering a `get` and/or `set` function.
 
@@ -1040,12 +1039,10 @@ let result = engine.eval::<String>(r#"let a = new_ts(); a.xyz = "42"; a.xyz"#)?;
 println!("Answer: {}", result);                     // prints 42
 ```
 
-Indexers
+### Indexers
---------
 
 Custom types can also expose an _indexer_ by registering an indexer function.
 A custom type with an indexer function defined can use the bracket '`[]`' notation to get a property value
-(but not update it - indexers are read-only).
 
 ```rust
 #[derive(Clone)]
@@ -1057,9 +1054,12 @@ impl TestStruct {
     fn get_field(&mut self, index: i64) -> i64 {
         self.fields[index as usize]
     }
+    fn set_field(&mut self, index: i64, value: i64) {
+        self.fields[index as usize] = value
+    }
 
     fn new() -> Self {
-        TestStruct { fields: vec![1, 2, 42, 4, 5] }
+        TestStruct { fields: vec![1, 2, 3, 4, 5] }
     }
 }
 
@@ -1068,22 +1068,31 @@ let engine = Engine::new();
 engine.register_type::<TestStruct>();
 
 engine.register_fn("new_ts", TestStruct::new);
-engine.register_indexer(TestStruct::get_field);
 
-let result = engine.eval::<i64>("let a = new_ts(); a[2]")?;
+// Shorthand: engine.register_indexer_get_set(TestStruct::get_field, TestStruct::set_field);
+engine.register_indexer_get(TestStruct::get_field);
+engine.register_indexer_set(TestStruct::set_field);
+
+let result = engine.eval::<i64>("let a = new_ts(); a[2] = 42; a[2]")?;
 
 println!("Answer: {}", result);                     // prints 42
 ```
 
-Needless to say, `register_type`, `register_type_with_name`, `register_get`, `register_set`, `register_get_set`
+For efficiency reasons, indexers **cannot** be used to overload (i.e. override) built-in indexing operations for
-and `register_indexer` are not available under the [`no_object`] feature.
+[arrays] and [object maps].
-`register_indexer` is also not available under the [`no_index`] feature.
 
-Printing for custom types
+### Disabling custom types
--------------------------
 
-To use custom types for `print` and `debug`, or format its value into a [string], it is necessary that the following
+The custom types API `register_type`, `register_type_with_name`, `register_get`, `register_set`, `register_get_set`,
-functions be registered (assuming the custom type is `T` and it is `Display + Debug`):
+`register_indexer_get`, `register_indexer_set` and `register_indexer_get_set` are not available under [`no_object`].
+
+The indexers API `register_indexer_get`, `register_indexer_set` and `register_indexer_get_set` are also
+not available under [`no_index`].
+
+### Printing for custom types
+
+To use custom types for `print` and `debug`, or convert its value into a [string], it is necessary that the following
+functions be registered (assuming the custom type is `T : Display + Debug`):
 
 | Function    | Signature                                        | Typical implementation         | Usage                                                                                   |
 | ----------- | ------------------------------------------------ | ------------------------------ | --------------------------------------------------------------------------------------- |
@@ -1103,7 +1112,7 @@ By default, Rhai treats each [`Engine`] invocation as a fresh one, persisting on
 but no global state. This gives each evaluation a clean starting slate. In order to continue using the same global state
 from one invocation to the next, such a state must be manually created and passed in.
 
-All `Scope` variables are [`Dynamic`], meaning they can store values of any type.  Under the [`sync`] feature, however,
+All `Scope` variables are [`Dynamic`], meaning they can store values of any type.  Under [`sync`], however,
 only types that are `Send + Sync` are supported, and the entire `Scope` itself will also be `Send + Sync`.
 This is extremely useful in multi-threaded applications.
 
@@ -1205,7 +1214,7 @@ The following are reserved keywords in Rhai:
 | `import`, `export`, `as`                          | Modules               |        [`no_module`]        |
 
 Keywords cannot be the name of a [function] or [variable], unless the relevant exclusive feature is enabled.
-For example, `fn` is a valid variable name under the [`no_function`] feature.
+For example, `fn` is a valid variable name under [`no_function`].
 
 Statements
 ----------
@@ -2376,9 +2385,9 @@ which simply loads a script file based on the path (with `.rhai` extension attac
 Built-in module resolvers are grouped under the `rhai::module_resolvers` module namespace.
 
 | Module Resolver        | Description                                                                                                                                                                                                                                                                                                                                        |
-| ---------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| ---------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
-| `FileModuleResolver`   | The default module resolution service, not available under the [`no_std`] feature. Loads a script file (based off the current directory) with `.rhai` extension.<br/>The base directory can be changed via the `FileModuleResolver::new_with_path()` constructor function.<br/>`FileModuleResolver::create_module()` loads a script file and returns a module. |
+| `FileModuleResolver`   | The default module resolution service, not available under [`no_std`]. Loads a script file (based off the current directory) with `.rhai` extension.<br/>The base directory can be changed via the `FileModuleResolver::new_with_path()` constructor function.<br/>`FileModuleResolver::create_module()` loads a script file and returns a module. |
-| `StaticModuleResolver` | Loads modules that are statically added. This can be used under the [`no_std`] feature.                                                                                                                                                                                                                                                                        |
+| `StaticModuleResolver` | Loads modules that are statically added. This can be used under [`no_std`].                                                                                                                                                                                                                                                                        |
 
 An [`Engine`]'s module resolver is set via a call to `Engine::set_module_resolver`:
 

RELEASES.md

@@ -8,6 +8,13 @@ Breaking changes
 ----------------
 
 * Callback closure passed to `Engine::on_progress` now takes `&u64` instead of `u64` to be consistent with other callback signatures.
+* `Engine::register_indexer` is renamed to `Engine::register_indexer_get`.
+* `Module::set_indexer_fn` is renamed to `Module::set_indexer_get_fn`.
+
+New features
+------------
+
+* Indexers are now split into getters ans setters (which now support updates).  The API is split into `Engine::register_indexer_get` and `Engine::register_indexer_set` with `Engine::register_indexer_get_set` being a shorthand.  Similarly, `Module::set_indexer_get_fn` and `Module::set_indexer_set_fn` are added.
 
 
 Version 0.15.0

src/api.rs

@@ -2,7 +2,8 @@
 
 use crate::any::{Dynamic, Variant};
 use crate::engine::{
-    get_script_function_by_signature, make_getter, make_setter, Engine, State, FUNC_INDEXER,
+    get_script_function_by_signature, make_getter, make_setter, Engine, State, FUNC_INDEXER_GET,
+    FUNC_INDEXER_SET,
 };
 use crate::error::ParseError;
 use crate::fn_call::FuncArgs;
@@ -275,7 +276,7 @@ impl Engine {
         self.register_set(name, set_fn);
     }
 
-    /// Register an indexer function for a registered type with the `Engine`.
+    /// Register an index getter for a registered type with the `Engine`.
     ///
     /// The function signature must start with `&mut self` and not `&self`.
     ///
@@ -305,7 +306,7 @@ impl Engine {
     /// engine.register_fn("new_ts", TestStruct::new);
     ///
     /// // Register an indexer.
-    /// engine.register_indexer(TestStruct::get_field);
+    /// engine.register_indexer_get(TestStruct::get_field);
     ///
     /// assert_eq!(engine.eval::<i64>("let a = new_ts(); a[2]")?, 3);
     /// # Ok(())
@@ -313,7 +314,7 @@ impl Engine {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[cfg(not(feature = "no_index"))]
-    pub fn register_indexer<T, X, U>(
+    pub fn register_indexer_get<T, X, U>(
         &mut self,
         callback: impl Fn(&mut T, X) -> U + SendSync + 'static,
     ) where
@@ -321,7 +322,103 @@ impl Engine {
         U: Variant + Clone,
         X: Variant + Clone,
     {
-        self.register_fn(FUNC_INDEXER, callback);
+        self.register_fn(FUNC_INDEXER_GET, callback);
+    }
+
+    /// Register an index setter for a registered type with the `Engine`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// #[derive(Clone)]
+    /// struct TestStruct {
+    ///     fields: Vec<i64>
+    /// }
+    ///
+    /// impl TestStruct {
+    ///     fn new() -> Self                { TestStruct { fields: vec![1, 2, 3, 4, 5] } }
+    ///     fn set_field(&mut self, index: i64, value: i64) { self.fields[index as usize] = value; }
+    /// }
+    ///
+    /// # fn main() -> Result<(), Box<rhai::EvalAltResult>> {
+    /// use rhai::{Engine, RegisterFn};
+    ///
+    /// let mut engine = Engine::new();
+    ///
+    /// // Register the custom type.
+    /// engine.register_type::<TestStruct>();
+    ///
+    /// engine.register_fn("new_ts", TestStruct::new);
+    ///
+    /// // Register an indexer.
+    /// engine.register_indexer_set(TestStruct::set_field);
+    ///
+    /// assert_eq!(
+    ///     engine.eval::<TestStruct>("let a = new_ts(); a[2] = 42; a")?.fields[2],
+    ///     42
+    /// );
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[cfg(not(feature = "no_object"))]
+    #[cfg(not(feature = "no_index"))]
+    pub fn register_indexer_set<T, X, U>(
+        &mut self,
+        callback: impl Fn(&mut T, X, U) -> () + SendSync + 'static,
+    ) where
+        T: Variant + Clone,
+        U: Variant + Clone,
+        X: Variant + Clone,
+    {
+        self.register_fn(FUNC_INDEXER_SET, callback);
+    }
+
+    /// Shorthand for register both index getter and setter functions for a registered type with the `Engine`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// #[derive(Clone)]
+    /// struct TestStruct {
+    ///     fields: Vec<i64>
+    /// }
+    ///
+    /// impl TestStruct {
+    ///     fn new() -> Self                { TestStruct { fields: vec![1, 2, 3, 4, 5] } }
+    ///     fn get_field(&mut self, index: i64) -> i64      { self.fields[index as usize] }
+    ///     fn set_field(&mut self, index: i64, value: i64) { self.fields[index as usize] = value; }
+    /// }
+    ///
+    /// # fn main() -> Result<(), Box<rhai::EvalAltResult>> {
+    /// use rhai::{Engine, RegisterFn};
+    ///
+    /// let mut engine = Engine::new();
+    ///
+    /// // Register the custom type.
+    /// engine.register_type::<TestStruct>();
+    ///
+    /// engine.register_fn("new_ts", TestStruct::new);
+    ///
+    /// // Register an indexer.
+    /// engine.register_indexer_get_set(TestStruct::get_field, TestStruct::set_field);
+    ///
+    /// assert_eq!(engine.eval::<i64>("let a = new_ts(); a[2] = 42; a[2]")?, 42);
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[cfg(not(feature = "no_object"))]
+    #[cfg(not(feature = "no_index"))]
+    pub fn register_indexer_get_set<T, X, U>(
+        &mut self,
+        getter: impl Fn(&mut T, X) -> U + SendSync + 'static,
+        setter: impl Fn(&mut T, X, U) -> () + SendSync + 'static,
+    ) where
+        T: Variant + Clone,
+        U: Variant + Clone,
+        X: Variant + Clone,
+    {
+        self.register_indexer_get(getter);
+        self.register_indexer_set(setter);
     }
 
     /// Compile a string into an `AST`, which can be used later for evaluation.

src/engine.rs

@@ -74,9 +74,11 @@ pub const KEYWORD_EVAL: &str = "eval";
 pub const FUNC_TO_STRING: &str = "to_string";
 pub const FUNC_GETTER: &str = "get$";
 pub const FUNC_SETTER: &str = "set$";
-pub const FUNC_INDEXER: &str = "$index$";
+pub const FUNC_INDEXER_GET: &str = "$index$get$";
+pub const FUNC_INDEXER_SET: &str = "$index$set$";
 
 /// A type that encapsulates a mutation target for an expression with side effects.
+#[derive(Debug)]
 enum Target<'a> {
     /// The target is a mutable reference to a `Dynamic` value somewhere.
     Ref(&'a mut Dynamic),
@@ -91,40 +93,45 @@ impl Target<'_> {
     /// Is the `Target` a reference pointing to other data?
     pub fn is_ref(&self) -> bool {
         match self {
-            Target::Ref(_) => true,
+            Self::Ref(_) => true,
-            Target::Value(_) | Target::StringChar(_, _, _) => false,
+            Self::Value(_) | Self::StringChar(_, _, _) => false,
+        }
+    }
+    /// Is the `Target` an owned value?
+    pub fn is_value(&self) -> bool {
+        match self {
+            Self::Ref(_) => false,
+            Self::Value(_) => true,
+            Self::StringChar(_, _, _) => false,
         }
     }
-
     /// Get the value of the `Target` as a `Dynamic`, cloning a referenced value if necessary.
     pub fn clone_into_dynamic(self) -> Dynamic {
         match self {
-            Target::Ref(r) => r.clone(),        // Referenced value is cloned
+            Self::Ref(r) => r.clone(),        // Referenced value is cloned
-            Target::Value(v) => v,              // Owned value is simply taken
+            Self::Value(v) => v,              // Owned value is simply taken
-            Target::StringChar(_, _, ch) => ch, // Character is taken
+            Self::StringChar(_, _, ch) => ch, // Character is taken
         }
     }
-
     /// Get a mutable reference from the `Target`.
     pub fn as_mut(&mut self) -> &mut Dynamic {
         match self {
-            Target::Ref(r) => *r,
+            Self::Ref(r) => *r,
-            Target::Value(ref mut r) => r,
+            Self::Value(ref mut r) => r,
-            Target::StringChar(_, _, ref mut r) => r,
+            Self::StringChar(_, _, ref mut r) => r,
         }
     }
-
     /// Update the value of the `Target`.
     /// Position in `EvalAltResult` is None and must be set afterwards.
     pub fn set_value(&mut self, new_val: Dynamic) -> Result<(), Box<EvalAltResult>> {
         match self {
-            Target::Ref(r) => **r = new_val,
+            Self::Ref(r) => **r = new_val,
-            Target::Value(_) => {
+            Self::Value(_) => {
                 return Err(Box::new(EvalAltResult::ErrorAssignmentToUnknownLHS(
                     Position::none(),
                 )))
             }
-            Target::StringChar(Dynamic(Union::Str(ref mut s)), index, _) => {
+            Self::StringChar(Dynamic(Union::Str(ref mut s)), index, _) => {
                 // Replace the character at the specified index position
                 let new_ch = new_val
                     .as_char()
@@ -163,20 +170,17 @@ impl<T: Into<Dynamic>> From<T> for Target<'_> {
 ///
 /// This type uses some unsafe code, mainly for avoiding cloning of local variable names via
 /// direct lifetime casting.
-#[derive(Debug, Clone, Default)]
+#[derive(Debug, Eq, PartialEq, Hash, Clone, Default)]
 pub struct State {
     /// Normally, access to variables are parsed with a relative offset into the scope to avoid a lookup.
     /// In some situation, e.g. after running an `eval` statement, subsequent offsets become mis-aligned.
     /// When that happens, this flag is turned on to force a scope lookup by name.
     pub always_search: bool,
-
     /// Level of the current scope.  The global (root) level is zero, a new block (or function call)
     /// is one level higher, and so on.
     pub scope_level: usize,
-
     /// Number of operations performed.
     pub operations: u64,
-
     /// Number of modules loaded.
     pub modules: u64,
 }
@@ -361,14 +365,23 @@ fn search_scope<'s, 'a>(
         _ => unreachable!(),
     };
 
+    // Check if it is qualified
     if let Some(modules) = modules.as_ref() {
-        let module = if let Some(index) = modules.index() {
+        // Qualified - check if the root module is directly indexed
+        let index = if state.always_search {
+            None
+        } else {
+            modules.index()
+        };
+
+        let module = if let Some(index) = index {
             scope
                 .get_mut(scope.len() - index.get())
                 .0
                 .downcast_mut::<Module>()
                 .unwrap()
         } else {
+            // Find the root module in the scope
             let (id, root_pos) = modules.get(0);
 
             scope
@@ -376,20 +389,18 @@ fn search_scope<'s, 'a>(
                 .ok_or_else(|| Box::new(EvalAltResult::ErrorModuleNotFound(id.into(), *root_pos)))?
         };
 
-        return Ok((
+        let target = module.get_qualified_var_mut(name, *hash_var, *pos)?;
-            module.get_qualified_var_mut(name, *hash_var, *pos)?,
-            name,
-            // Module variables are constant
-            ScopeEntryType::Constant,
-            *pos,
-        ));
-    }
 
+        // Module variables are constant
+        Ok((target, name, ScopeEntryType::Constant, *pos))
+    } else {
+        // Unqualified - check if it is directly indexed
         let index = if state.always_search { None } else { *index };
 
         let index = if let Some(index) = index {
             scope.len() - index.get()
         } else {
+            // Find the variable in the scope
             scope
                 .get_index(name)
                 .ok_or_else(|| Box::new(EvalAltResult::ErrorVariableNotFound(name.into(), *pos)))?
@@ -398,6 +409,7 @@ fn search_scope<'s, 'a>(
 
         let (val, typ) = scope.get_mut(index);
         Ok((val, name, typ, *pos))
+    }
 }
 
 impl Engine {
@@ -585,10 +597,16 @@ impl Engine {
         .or_else(|| self.packages.get_fn(hashes.0));
 
         if let Some(func) = func {
+            // Calling pure function in method-call?
+            normalize_first_arg(
+                (func.is_pure() || func.is_script()) && is_ref,
+                &mut this_copy,
+                &mut old_this_ptr,
+                args,
+            );
+
             if func.is_script() {
                 // Run scripted function
-                normalize_first_arg(is_ref, &mut this_copy, &mut old_this_ptr, args);
-
                 let fn_def = func.get_fn_def();
                 let result =
                     self.call_script_fn(scope, state, lib, fn_name, fn_def, args, level)?;
@@ -598,14 +616,6 @@ impl Engine {
 
                 return Ok((result, false));
             } else {
-                // Calling pure function in method-call?
-                normalize_first_arg(
-                    func.is_pure() && is_ref,
-                    &mut this_copy,
-                    &mut old_this_ptr,
-                    args,
-                );
-
                 // Run external function
                 let result = func.get_native_fn()(args)?;
 
@@ -668,22 +678,40 @@ impl Engine {
             )));
         }
 
-        let types_list: Vec<_> = args
+        // index getter function not found?
-            .iter()
+        if fn_name == FUNC_INDEXER_GET && args.len() == 2 {
-            .map(|name| self.map_type_name(name.type_name()))
-            .collect();
-
-        // Getter function not found?
-        if fn_name == FUNC_INDEXER {
             return Err(Box::new(EvalAltResult::ErrorFunctionNotFound(
-                format!("[]({})", types_list.join(", ")),
+                format!(
+                    "{} [{}]",
+                    self.map_type_name(args[0].type_name()),
+                    self.map_type_name(args[1].type_name()),
+                ),
+                Position::none(),
+            )));
+        }
+
+        // index setter function not found?
+        if fn_name == FUNC_INDEXER_SET {
+            return Err(Box::new(EvalAltResult::ErrorFunctionNotFound(
+                format!(
+                    "{} [{}]=",
+                    self.map_type_name(args[0].type_name()),
+                    self.map_type_name(args[1].type_name()),
+                ),
                 Position::none(),
             )));
         }
 
         // Raise error
         Err(Box::new(EvalAltResult::ErrorFunctionNotFound(
-            format!("{} ({})", fn_name, types_list.join(", ")),
+            format!(
+                "{} ({})",
+                fn_name,
+                args.iter()
+                    .map(|name| self.map_type_name(name.type_name()))
+                    .collect::<Vec<_>>()
+                    .join(", ")
+            ),
             Position::none(),
         )))
     }
@@ -787,12 +815,8 @@ impl Engine {
         level: usize,
     ) -> Result<(Dynamic, bool), Box<EvalAltResult>> {
         // Qualifiers (none) + function name + number of arguments + argument `TypeId`'s.
-        let hash_fn = calc_fn_hash(
+        let arg_types = args.iter().map(|a| a.type_id());
-            empty(),
+        let hash_fn = calc_fn_hash(empty(), fn_name, args.len(), arg_types);
-            fn_name,
-            args.len(),
-            args.iter().map(|a| a.type_id()),
-        );
         let hashes = (hash_fn, if native_only { 0 } else { hash_fn_def });
 
         match fn_name {
@@ -890,8 +914,8 @@ impl Engine {
                     let (idx, expr, pos) = x.as_ref();
                     let is_idx = matches!(rhs, Expr::Index(_));
                     let idx_pos = idx.position();
-                    let this_ptr = &mut self
+                    let this_ptr =
-                        .get_indexed_mut(state, lib, obj, is_ref, idx_val, idx_pos, false)?;
+                        &mut self.get_indexed_mut(state, lib, target, idx_val, idx_pos, false)?;
 
                     self.eval_dot_index_chain_helper(
                         state, lib, this_ptr, expr, idx_values, is_idx, level, new_val,
@@ -900,17 +924,52 @@ impl Engine {
                 }
                 // xxx[rhs] = new_val
                 _ if new_val.is_some() => {
-                    let this_ptr =
+                    let mut idx_val2 = idx_val.clone();
-                        &mut self.get_indexed_mut(state, lib, obj, is_ref, idx_val, pos, true)?;
 
+                    match self.get_indexed_mut(state, lib, target, idx_val, pos, true) {
+                        // Indexed value is an owned value - the only possibility is an indexer
+                        // Try to call an index setter
+                        Ok(this_ptr) if this_ptr.is_value() => {
+                            let fn_name = FUNC_INDEXER_SET;
+                            let args = &mut [target.as_mut(), &mut idx_val2, &mut new_val.unwrap()];
+
+                            self.exec_fn_call(state, lib, fn_name, true, 0, args, is_ref, None, 0)
+                                .or_else(|err| match *err {
+                                    // If there is no index setter, no need to set it back because the indexer is read-only
+                                    EvalAltResult::ErrorFunctionNotFound(s, _)
+                                        if s == FUNC_INDEXER_SET =>
+                                    {
+                                        Ok(Default::default())
+                                    }
+                                    _ => Err(err),
+                                })?;
+                        }
+                        // Indexed value is a reference - update directly
+                        Ok(ref mut this_ptr) => {
                             this_ptr
                                 .set_value(new_val.unwrap())
                                 .map_err(|err| EvalAltResult::new_position(err, rhs.position()))?;
-                    Ok((Default::default(), true))
+                        }
+                        Err(err) => match *err {
+                            // No index getter - try to call an index setter
+                            EvalAltResult::ErrorIndexingType(_, _) => {
+                                let fn_name = FUNC_INDEXER_SET;
+                                let args =
+                                    &mut [target.as_mut(), &mut idx_val2, &mut new_val.unwrap()];
+
+                                self.exec_fn_call(
+                                    state, lib, fn_name, true, 0, args, is_ref, None, 0,
+                                )?;
+                            }
+                            // Error
+                            err => return Err(Box::new(err)),
+                        },
+                    }
+                    Ok(Default::default())
                 }
                 // xxx[rhs]
                 _ => self
-                    .get_indexed_mut(state, lib, obj, is_ref, idx_val, pos, false)
+                    .get_indexed_mut(state, lib, target, idx_val, pos, false)
                     .map(|v| (v.clone_into_dynamic(), false)),
             }
         } else {
@@ -940,8 +999,7 @@ impl Engine {
                 Expr::Property(x) if obj.is::<Map>() && new_val.is_some() => {
                     let ((prop, _, _), pos) = x.as_ref();
                     let index = prop.clone().into();
-                    let mut val =
+                    let mut val = self.get_indexed_mut(state, lib, target, index, *pos, true)?;
-                        self.get_indexed_mut(state, lib, obj, is_ref, index, *pos, true)?;
 
                     val.set_value(new_val.unwrap())
                         .map_err(|err| EvalAltResult::new_position(err, rhs.position()))?;
@@ -952,7 +1010,7 @@ impl Engine {
                 Expr::Property(x) if obj.is::<Map>() => {
                     let ((prop, _, _), pos) = x.as_ref();
                     let index = prop.clone().into();
-                    let val = self.get_indexed_mut(state, lib, obj, is_ref, index, *pos, false)?;
+                    let val = self.get_indexed_mut(state, lib, target, index, *pos, false)?;
 
                     Ok((val.clone_into_dynamic(), false))
                 }
@@ -981,7 +1039,7 @@ impl Engine {
                     let mut val = if let Expr::Property(p) = prop {
                         let ((prop, _, _), _) = p.as_ref();
                         let index = prop.clone().into();
-                        self.get_indexed_mut(state, lib, obj, is_ref, index, *pos, false)?
+                        self.get_indexed_mut(state, lib, target, index, *pos, false)?
                     } else {
                         unreachable!();
                     };
@@ -1160,14 +1218,16 @@ impl Engine {
         &self,
         state: &mut State,
         lib: &Module,
-        val: &'a mut Dynamic,
+        target: &'a mut Target,
-        is_ref: bool,
         mut idx: Dynamic,
         idx_pos: Position,
         create: bool,
     ) -> Result<Target<'a>, Box<EvalAltResult>> {
         self.inc_operations(state)?;
 
+        let is_ref = target.is_ref();
+        let val = target.as_mut();
+
         match val {
             #[cfg(not(feature = "no_index"))]
             Dynamic(Union::Array(arr)) => {
@@ -1233,7 +1293,7 @@ impl Engine {
             }
 
             _ => {
-                let fn_name = FUNC_INDEXER;
+                let fn_name = FUNC_INDEXER_GET;
                 let type_name = self.map_type_name(val.type_name());
                 let args = &mut [val, &mut idx];
                 self.exec_fn_call(state, lib, fn_name, true, 0, args, is_ref, None, 0)

src/module.rs

@@ -2,7 +2,7 @@
 
 use crate::any::{Dynamic, Variant};
 use crate::calc_fn_hash;
-use crate::engine::{make_getter, make_setter, Engine, FUNC_INDEXER};
+use crate::engine::{make_getter, make_setter, Engine, FUNC_INDEXER_GET, FUNC_INDEXER_SET};
 use crate::fn_native::{CallableFunction, FnCallArgs, IteratorFn, SendSync};
 use crate::parser::{
     FnAccess,
@@ -518,7 +518,7 @@ impl Module {
         self.set_fn_2_mut(make_setter(&name.into()), func)
     }
 
-    /// Set a Rust indexer function taking two parameters (the first one mutable) into the module,
+    /// Set a Rust index getter taking two parameters (the first one mutable) into the module,
     /// returning a hash key.
     ///
     /// If there is a similar existing setter Rust function, it is replaced.
@@ -529,19 +529,19 @@ impl Module {
     /// use rhai::{Module, ImmutableString};
     ///
     /// let mut module = Module::new();
-    /// let hash = module.set_indexer_fn(|x: &mut i64, y: ImmutableString| {
+    /// let hash = module.set_indexer_get_fn(|x: &mut i64, y: ImmutableString| {
     ///     Ok(*x + y.len() as i64)
     /// });
     /// assert!(module.get_fn(hash).is_some());
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[cfg(not(feature = "no_index"))]
-    pub fn set_indexer_fn<A: Variant + Clone, B: Variant + Clone, T: Variant + Clone>(
+    pub fn set_indexer_get_fn<A: Variant + Clone, B: Variant + Clone, T: Variant + Clone>(
         &mut self,
         #[cfg(not(feature = "sync"))] func: impl Fn(&mut A, B) -> FuncReturn<T> + 'static,
         #[cfg(feature = "sync")] func: impl Fn(&mut A, B) -> FuncReturn<T> + Send + Sync + 'static,
     ) -> u64 {
-        self.set_fn_2_mut(FUNC_INDEXER, func)
+        self.set_fn_2_mut(FUNC_INDEXER_GET, func)
     }
 
     /// Set a Rust function taking three parameters into the module, returning a hash key.
@@ -629,6 +629,44 @@ impl Module {
         )
     }
 
+    /// Set a Rust index setter taking three parameters (the first one mutable) into the module,
+    /// returning a hash key.
+    ///
+    /// If there is a similar existing Rust function, it is replaced.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use rhai::{Module, ImmutableString};
+    ///
+    /// let mut module = Module::new();
+    /// let hash = module.set_indexer_set_fn(|x: &mut i64, y: ImmutableString, value: i64| {
+    ///     *x = y.len() as i64 + value;
+    ///     Ok(())
+    /// });
+    /// assert!(module.get_fn(hash).is_some());
+    /// ```
+    pub fn set_indexer_set_fn<A: Variant + Clone, B: Variant + Clone>(
+        &mut self,
+        #[cfg(not(feature = "sync"))] func: impl Fn(&mut A, B, A) -> FuncReturn<()> + 'static,
+        #[cfg(feature = "sync")] func: impl Fn(&mut A, B, A) -> FuncReturn<()> + Send + Sync + 'static,
+    ) -> u64 {
+        let f = move |args: &mut FnCallArgs| {
+            let b = mem::take(args[1]).cast::<B>();
+            let c = mem::take(args[2]).cast::<A>();
+            let a = args[0].downcast_mut::<A>().unwrap();
+
+            func(a, b, c).map(Dynamic::from)
+        };
+        let args = [TypeId::of::<A>(), TypeId::of::<B>(), TypeId::of::<A>()];
+        self.set_fn(
+            FUNC_INDEXER_SET,
+            Public,
+            &args,
+            CallableFunction::from_method(Box::new(f)),
+        )
+    }
+
     /// Set a Rust function taking four parameters into the module, returning a hash key.
     ///
     /// If there is a similar existing Rust function, it is replaced.

tests/get_set.rs

@@ -42,18 +42,25 @@ fn test_get_set() -> Result<(), Box<EvalAltResult>> {
     engine.register_fn("add", |value: &mut INT| *value += 41);
     engine.register_fn("new_ts", TestStruct::new);
 
-    #[cfg(not(feature = "no_index"))]
-    engine.register_indexer(|value: &mut TestStruct, index: ImmutableString| {
-        value.array[index.len()]
-    });
-
     assert_eq!(engine.eval::<INT>("let a = new_ts(); a.x = 500; a.x")?, 500);
     assert_eq!(engine.eval::<INT>("let a = new_ts(); a.x.add(); a.x")?, 42);
     assert_eq!(engine.eval::<INT>("let a = new_ts(); a.y.add(); a.y")?, 0);
 
     #[cfg(not(feature = "no_index"))]
-    assert_eq!(engine.eval::<INT>(r#"let a = new_ts(); a["abc"]"#)?, 4);
+    {
+        engine.register_indexer_get_set(
+            |value: &mut TestStruct, index: ImmutableString| value.array[index.len()],
+            |value: &mut TestStruct, index: ImmutableString, new_val: INT| {
+                value.array[index.len()] = new_val
+            },
+        );
 
+        assert_eq!(engine.eval::<INT>(r#"let a = new_ts(); a["abc"]"#)?, 4);
+        assert_eq!(
+            engine.eval::<INT>(r#"let a = new_ts(); a["abc"] = 42; a["abc"]"#)?,
+            42
+        );
+    }
     Ok(())
 }