diff --git a/CHANGELOG.md b/CHANGELOG.md
index 01592c85..d9aecc83 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,24 @@
 Rhai Release Notes
 ==================
 
+Version 1.10.0
+==============
+
+Bug fixes
+---------
+
+* API for registering property getters/setters and indexers to an `Engine` now works with functions that take a first parameter of `NativeCallContext`.
+* Missing API function `Module::set_getter_setter_fn` is added.
+
+New features
+------------
+
+Fallible type iterators
+-----------------------
+
+* For very special needs, the ability to register fallible type iterators is added.
+
+
 Version 1.9.0
 =============
 
diff --git a/src/api/build_type.rs b/src/api/build_type.rs
index 23199b0b..7b54fadf 100644
--- a/src/api/build_type.rs
+++ b/src/api/build_type.rs
@@ -125,10 +125,10 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
 
     /// Register a custom function.
     #[inline(always)]
-    pub fn with_fn<N, A, F>(&mut self, name: N, method: F) -> &mut Self
+    pub fn with_fn<N, A, F, R, S>(&mut self, name: N, method: F) -> &mut Self
     where
         N: AsRef<str> + Into<Identifier>,
-        F: RegisterNativeFunction<A, ()>,
+        F: RegisterNativeFunction<A, R, S>,
     {
         self.engine.register_fn(name, method);
         self
@@ -136,10 +136,10 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
 
     /// Register a custom fallible function.
     #[inline(always)]
-    pub fn with_result_fn<N, A, F, R>(&mut self, name: N, method: F) -> &mut Self
+    pub fn with_result_fn<N, A, F, R, S>(&mut self, name: N, method: F) -> &mut Self
     where
         N: AsRef<str> + Into<Identifier>,
-        F: RegisterNativeFunction<A, RhaiResultOf<R>>,
+        F: RegisterNativeFunction<A, R, RhaiResultOf<S>>,
     {
         self.engine.register_result_fn(name, method);
         self
diff --git a/src/api/json.rs b/src/api/json.rs
index ae00bbed..034062bf 100644
--- a/src/api/json.rs
+++ b/src/api/json.rs
@@ -72,16 +72,13 @@ impl Engine {
                         Token::LeftBrace => Token::MapStart,
                         // Disallowed syntax
                         t @ (Token::Unit | Token::MapStart) => Token::LexError(
-                            LexError::ImproperSymbol(
-                                t.literal_syntax().to_string(),
-                                "".to_string(),
-                            )
-                            .into(),
+                            LexError::ImproperSymbol(t.literal_syntax().to_string(), String::new())
+                                .into(),
                         ),
                         Token::InterpolatedString(..) => Token::LexError(
                             LexError::ImproperSymbol(
                                 "interpolated string".to_string(),
-                                "".to_string(),
+                                String::new(),
                             )
                             .into(),
                         ),
@@ -93,7 +90,7 @@ impl Engine {
                 Some(&|token, _, _| {
                     match token {
                         Token::Reserved(s) if &*s == "null" => Token::LexError(
-                            LexError::ImproperSymbol("null".to_string(), "".to_string()).into(),
+                            LexError::ImproperSymbol("null".to_string(), String::new()).into(),
                         ),
                         // `{` => `#{`
                         Token::LeftBrace => Token::MapStart,
diff --git a/src/api/register.rs b/src/api/register.rs
index 3ebb8695..5625773d 100644
--- a/src/api/register.rs
+++ b/src/api/register.rs
@@ -1,5 +1,6 @@
 //! Module that defines the public function/module registration API of [`Engine`].
 
+use crate::func::register::Mut;
 use crate::func::{FnCallArgs, RegisterNativeFunction, SendSync};
 use crate::types::dynamic::Variant;
 use crate::{
@@ -50,10 +51,10 @@ impl Engine {
     /// # }
     /// ```
     #[inline]
-    pub fn register_fn<N, A, F>(&mut self, name: N, func: F) -> &mut Self
+    pub fn register_fn<N, A, F, R, S>(&mut self, name: N, func: F) -> &mut Self
     where
         N: AsRef<str> + Into<Identifier>,
-        F: RegisterNativeFunction<A, ()>,
+        F: RegisterNativeFunction<A, R, S>,
     {
         let param_types = F::param_types();
 
@@ -112,10 +113,10 @@ impl Engine {
     ///       .expect_err("expecting division by zero error!");
     /// ```
     #[inline]
-    pub fn register_result_fn<N, A, F, R>(&mut self, name: N, func: F) -> &mut Self
+    pub fn register_result_fn<N, A, F, R, S>(&mut self, name: N, func: F) -> &mut Self
     where
         N: AsRef<str> + Into<Identifier>,
-        F: RegisterNativeFunction<A, RhaiResultOf<R>>,
+        F: RegisterNativeFunction<A, R, RhaiResultOf<S>>,
     {
         let param_types = F::param_types();
 
@@ -291,7 +292,7 @@ impl Engine {
             .set_custom_type_raw(fully_qualified_type_path, name);
         self
     }
-    /// Register an type iterator for an iterable type with the [`Engine`].
+    /// Register a type iterator for an iterable type with the [`Engine`].
     /// This is an advanced API.
     #[inline(always)]
     pub fn register_iterator<T>(&mut self) -> &mut Self
@@ -302,6 +303,17 @@ impl Engine {
         self.global_namespace_mut().set_iterable::<T>();
         self
     }
+    /// Register a fallible type iterator for an iterable type with the [`Engine`].
+    /// This is an advanced API.
+    #[inline(always)]
+    pub fn register_iterator_result<T, X>(&mut self) -> &mut Self
+    where
+        T: Variant + Clone + IntoIterator<Item = RhaiResultOf<X>>,
+        X: Variant + Clone,
+    {
+        self.global_namespace_mut().set_iterable_result::<T, X>();
+        self
+    }
     /// Register a getter function for a member of a registered type with the [`Engine`].
     ///
     /// The function signature must start with `&mut self` and not `&self`.
@@ -344,10 +356,10 @@ impl Engine {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn register_get<T: Variant + Clone, V: Variant + Clone>(
+    pub fn register_get<T: Variant + Clone, V: Variant + Clone, S>(
         &mut self,
         name: impl AsRef<str>,
-        get_fn: impl Fn(&mut T) -> V + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>,), V, S> + SendSync + 'static,
     ) -> &mut Self {
         self.register_fn(crate::engine::make_getter(name.as_ref()).as_str(), get_fn)
     }
@@ -393,10 +405,10 @@ impl Engine {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn register_get_result<T: Variant + Clone, V: Variant + Clone>(
+    pub fn register_get_result<T: Variant + Clone, V: Variant + Clone, S>(
         &mut self,
         name: impl AsRef<str>,
-        get_fn: impl Fn(&mut T) -> RhaiResultOf<V> + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>,), V, RhaiResultOf<S>> + SendSync + 'static,
     ) -> &mut Self {
         self.register_result_fn(crate::engine::make_getter(name.as_ref()).as_str(), get_fn)
     }
@@ -443,10 +455,10 @@ impl Engine {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn register_set<T: Variant + Clone, V: Variant + Clone>(
+    pub fn register_set<T: Variant + Clone, V: Variant + Clone, S>(
         &mut self,
         name: impl AsRef<str>,
-        set_fn: impl Fn(&mut T, V) + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, V), (), S> + SendSync + 'static,
     ) -> &mut Self {
         self.register_fn(crate::engine::make_setter(name.as_ref()).as_str(), set_fn)
     }
@@ -494,10 +506,10 @@ impl Engine {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn register_set_result<T: Variant + Clone, V: Variant + Clone>(
+    pub fn register_set_result<T: Variant + Clone, V: Variant + Clone, S>(
         &mut self,
         name: impl AsRef<str>,
-        set_fn: impl Fn(&mut T, V) -> RhaiResultOf<()> + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, V), (), RhaiResultOf<S>> + SendSync + 'static,
     ) -> &mut Self {
         self.register_result_fn(crate::engine::make_setter(name.as_ref()).as_str(), set_fn)
     }
@@ -548,11 +560,11 @@ impl Engine {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn register_get_set<T: Variant + Clone, V: Variant + Clone>(
+    pub fn register_get_set<T: Variant + Clone, V: Variant + Clone, S1, S2>(
         &mut self,
         name: impl AsRef<str>,
-        get_fn: impl Fn(&mut T) -> V + SendSync + 'static,
-        set_fn: impl Fn(&mut T, V) + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>,), V, S1> + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, V), (), S2> + SendSync + 'static,
     ) -> &mut Self {
         self.register_get(&name, get_fn).register_set(&name, set_fn)
     }
@@ -607,9 +619,9 @@ impl Engine {
     /// ```
     #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
     #[inline]
-    pub fn register_indexer_get<T: Variant + Clone, X: Variant + Clone, V: Variant + Clone>(
+    pub fn register_indexer_get<T: Variant + Clone, X: Variant + Clone, V: Variant + Clone, S>(
         &mut self,
-        get_fn: impl Fn(&mut T, X) -> V + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>, X), V, S> + SendSync + 'static,
     ) -> &mut Self {
         #[cfg(not(feature = "no_index"))]
         if TypeId::of::<T>() == TypeId::of::<crate::Array>() {
@@ -686,9 +698,10 @@ impl Engine {
         T: Variant + Clone,
         X: Variant + Clone,
         V: Variant + Clone,
+        S,
     >(
         &mut self,
-        get_fn: impl Fn(&mut T, X) -> RhaiResultOf<V> + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>, X), V, RhaiResultOf<S>> + SendSync + 'static,
     ) -> &mut Self {
         #[cfg(not(feature = "no_index"))]
         if TypeId::of::<T>() == TypeId::of::<crate::Array>() {
@@ -761,9 +774,9 @@ impl Engine {
     /// ```
     #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
     #[inline]
-    pub fn register_indexer_set<T: Variant + Clone, X: Variant + Clone, V: Variant + Clone>(
+    pub fn register_indexer_set<T: Variant + Clone, X: Variant + Clone, V: Variant + Clone, S>(
         &mut self,
-        set_fn: impl Fn(&mut T, X, V) + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), (), S> + SendSync + 'static,
     ) -> &mut Self {
         #[cfg(not(feature = "no_index"))]
         if TypeId::of::<T>() == TypeId::of::<crate::Array>() {
@@ -841,9 +854,10 @@ impl Engine {
         T: Variant + Clone,
         X: Variant + Clone,
         V: Variant + Clone,
+        S,
     >(
         &mut self,
-        set_fn: impl Fn(&mut T, X, V) -> RhaiResultOf<()> + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), (), RhaiResultOf<S>> + SendSync + 'static,
     ) -> &mut Self {
         #[cfg(not(feature = "no_index"))]
         if TypeId::of::<T>() == TypeId::of::<crate::Array>() {
@@ -917,10 +931,16 @@ impl Engine {
     /// ```
     #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
     #[inline(always)]
-    pub fn register_indexer_get_set<T: Variant + Clone, X: Variant + Clone, V: Variant + Clone>(
+    pub fn register_indexer_get_set<
+        T: Variant + Clone,
+        X: Variant + Clone,
+        V: Variant + Clone,
+        S1,
+        S2,
+    >(
         &mut self,
-        get_fn: impl Fn(&mut T, X) -> V + SendSync + 'static,
-        set_fn: impl Fn(&mut T, X, V) + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>, X), V, S1> + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), (), S2> + SendSync + 'static,
     ) -> &mut Self {
         self.register_indexer_get(get_fn)
             .register_indexer_set(set_fn)
diff --git a/src/bin/rhai-dbg.rs b/src/bin/rhai-dbg.rs
index 30d521fa..a83b680c 100644
--- a/src/bin/rhai-dbg.rs
+++ b/src/bin/rhai-dbg.rs
@@ -90,12 +90,12 @@ fn print_error(input: &str, mut err: EvalAltResult) {
 
     let line_no = if lines.len() > 1 {
         if pos.is_none() {
-            "".to_string()
+            String::new()
         } else {
             format!("{}: ", pos.line().unwrap())
         }
     } else {
-        "".to_string()
+        String::new()
     };
 
     // Print error position
diff --git a/src/bin/rhai-repl.rs b/src/bin/rhai-repl.rs
index 932bcf30..00e01e8b 100644
--- a/src/bin/rhai-repl.rs
+++ b/src/bin/rhai-repl.rs
@@ -15,12 +15,12 @@ fn print_error(input: &str, mut err: EvalAltResult) {
 
     let line_no = if lines.len() > 1 {
         if pos.is_none() {
-            "".to_string()
+            String::new()
         } else {
             format!("{}: ", pos.line().unwrap())
         }
     } else {
-        "".to_string()
+        String::new()
     };
 
     // Print error position
diff --git a/src/eval/stmt.rs b/src/eval/stmt.rs
index 8e8ab47c..18d16eba 100644
--- a/src/eval/stmt.rs
+++ b/src/eval/stmt.rs
@@ -641,38 +641,21 @@ impl Engine {
                                     break;
                                 }
 
-                                let index_value = (x as INT).into();
+                                let index_value = Dynamic::from(x as INT);
 
-                                #[cfg(not(feature = "no_closure"))]
-                                {
-                                    let index_var = scope.get_mut_by_index(counter_index);
-                                    if index_var.is_shared() {
-                                        *index_var.write_lock().expect("`Dynamic`") = index_value;
-                                    } else {
-                                        *index_var = index_value;
-                                    }
-                                }
-                                #[cfg(feature = "no_closure")]
-                                {
-                                    *scope.get_mut_by_index(counter_index) = index_value;
-                                }
+                                *scope.get_mut_by_index(counter_index).write_lock().unwrap() =
+                                    index_value;
                             }
 
-                            let value = iter_value.flatten();
-
-                            #[cfg(not(feature = "no_closure"))]
-                            {
-                                let loop_var = scope.get_mut_by_index(index);
-                                if loop_var.is_shared() {
-                                    *loop_var.write_lock().expect("`Dynamic`") = value;
-                                } else {
-                                    *loop_var = value;
+                            let value = match iter_value {
+                                Ok(v) => v.flatten(),
+                                Err(err) => {
+                                    loop_result = Err(err.fill_position(expr.position()));
+                                    break;
                                 }
-                            }
-                            #[cfg(feature = "no_closure")]
-                            {
-                                *scope.get_mut_by_index(index) = value;
-                            }
+                            };
+
+                            *scope.get_mut_by_index(index).write_lock().unwrap() = value;
 
                             #[cfg(not(feature = "unchecked"))]
                             if let Err(err) = self
diff --git a/src/func/native.rs b/src/func/native.rs
index 6b3d46de..26801793 100644
--- a/src/func/native.rs
+++ b/src/func/native.rs
@@ -426,10 +426,11 @@ pub type FnBuiltin = fn(NativeCallContext, &mut FnCallArgs) -> RhaiResult;
 
 /// Function that gets an iterator from a type.
 #[cfg(not(feature = "sync"))]
-pub type IteratorFn = dyn Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>>;
+pub type IteratorFn = dyn Fn(Dynamic) -> Box<dyn Iterator<Item = RhaiResultOf<Dynamic>>>;
 /// Function that gets an iterator from a type.
 #[cfg(feature = "sync")]
-pub type IteratorFn = dyn Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + Send + Sync;
+pub type IteratorFn =
+    dyn Fn(Dynamic) -> Box<dyn Iterator<Item = RhaiResultOf<Dynamic>>> + Send + Sync;
 
 #[cfg(not(feature = "sync"))]
 pub type FnPlugin = dyn PluginFunction;
diff --git a/src/func/register.rs b/src/func/register.rs
index 41944cf6..d9e45e76 100644
--- a/src/func/register.rs
+++ b/src/func/register.rs
@@ -60,7 +60,7 @@ pub fn by_value<T: Variant + Clone>(data: &mut Dynamic) -> T {
 }
 
 /// Trait to register custom Rust functions.
-pub trait RegisterNativeFunction<Args, Result> {
+pub trait RegisterNativeFunction<ARGS, RET, RESULT> {
     /// Convert this function into a [`CallableFunction`].
     #[must_use]
     fn into_callable_function(self) -> CallableFunction;
@@ -128,7 +128,7 @@ macro_rules! def_register {
             FN: Fn($($param),*) -> RET + SendSync + 'static,
             $($par: Variant + Clone,)*
             RET: Variant + Clone
-        > RegisterNativeFunction<($($mark,)*), ()> for FN {
+        > RegisterNativeFunction<($($mark,)*), RET, ()> for FN {
             #[inline(always)] fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
             #[cfg(feature = "metadata")] #[inline(always)] fn param_names() -> Box<[&'static str]> { vec![$(std::any::type_name::<$param>()),*].into_boxed_slice() }
             #[cfg(feature = "metadata")] #[inline(always)] fn return_type() -> TypeId { TypeId::of::<RET>() }
@@ -154,7 +154,7 @@ macro_rules! def_register {
             FN: for<'a> Fn(NativeCallContext<'a>, $($param),*) -> RET + SendSync + 'static,
             $($par: Variant + Clone,)*
             RET: Variant + Clone
-        > RegisterNativeFunction<(NativeCallContext<'static>, $($mark,)*), ()> for FN {
+        > RegisterNativeFunction<($($mark,)*), RET, NativeCallContext<'static>> for FN {
             #[inline(always)] fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
             #[cfg(feature = "metadata")] #[inline(always)] fn param_names() -> Box<[&'static str]> { vec![$(std::any::type_name::<$param>()),*].into_boxed_slice() }
             #[cfg(feature = "metadata")] #[inline(always)] fn return_type() -> TypeId { TypeId::of::<RET>() }
@@ -180,7 +180,7 @@ macro_rules! def_register {
             FN: Fn($($param),*) -> RhaiResultOf<RET> + SendSync + 'static,
             $($par: Variant + Clone,)*
             RET: Variant + Clone
-        > RegisterNativeFunction<($($mark,)*), RhaiResultOf<RET>> for FN {
+        > RegisterNativeFunction<($($mark,)*), RET, RhaiResultOf<()>> for FN {
             #[inline(always)] fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
             #[cfg(feature = "metadata")] #[inline(always)] fn param_names() -> Box<[&'static str]> { vec![$(std::any::type_name::<$param>()),*].into_boxed_slice() }
             #[cfg(feature = "metadata")] #[inline(always)] fn return_type() -> TypeId { TypeId::of::<RhaiResultOf<RET>>() }
@@ -203,7 +203,7 @@ macro_rules! def_register {
             FN: for<'a> Fn(NativeCallContext<'a>, $($param),*) -> RhaiResultOf<RET> + SendSync + 'static,
             $($par: Variant + Clone,)*
             RET: Variant + Clone
-        > RegisterNativeFunction<(NativeCallContext<'static>, $($mark,)*), RhaiResultOf<RET>> for FN {
+        > RegisterNativeFunction<($($mark,)*), RET, RhaiResultOf<NativeCallContext<'static>>> for FN {
             #[inline(always)] fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
             #[cfg(feature = "metadata")] #[inline(always)] fn param_names() -> Box<[&'static str]> { vec![$(std::any::type_name::<$param>()),*].into_boxed_slice() }
             #[cfg(feature = "metadata")] #[inline(always)] fn return_type() -> TypeId { TypeId::of::<RhaiResultOf<RET>>() }
diff --git a/src/module/mod.rs b/src/module/mod.rs
index fb3e47ab..143be2a5 100644
--- a/src/module/mod.rs
+++ b/src/module/mod.rs
@@ -1,6 +1,7 @@
 //! Module defining external-loaded modules for Rhai.
 
 use crate::ast::FnAccess;
+use crate::func::register::Mut;
 use crate::func::{
     shared_take_or_clone, CallableFunction, FnCallArgs, IteratorFn, RegisterNativeFunction,
     SendSync,
@@ -1290,11 +1291,11 @@ impl Module {
     /// assert!(module.contains_fn(hash));
     /// ```
     #[inline(always)]
-    pub fn set_native_fn<ARGS, N, T, F>(&mut self, name: N, func: F) -> u64
+    pub fn set_native_fn<ARGS, N, T, F, S>(&mut self, name: N, func: F) -> u64
     where
         N: AsRef<str> + Into<Identifier>,
         T: Variant + Clone,
-        F: RegisterNativeFunction<ARGS, RhaiResultOf<T>>,
+        F: RegisterNativeFunction<ARGS, T, RhaiResultOf<S>>,
     {
         self.set_fn(
             name,
@@ -1326,14 +1327,11 @@ impl Module {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn set_getter_fn<ARGS, A, T, F>(&mut self, name: impl AsRef<str>, func: F) -> u64
+    pub fn set_getter_fn<A, T, F, S>(&mut self, name: impl AsRef<str>, func: F) -> u64
     where
         A: Variant + Clone,
         T: Variant + Clone,
-        F: RegisterNativeFunction<ARGS, RhaiResultOf<T>>
-            + Fn(&mut A) -> RhaiResultOf<T>
-            + SendSync
-            + 'static,
+        F: RegisterNativeFunction<(Mut<A>,), T, RhaiResultOf<S>> + SendSync + 'static,
     {
         self.set_fn(
             crate::engine::make_getter(name.as_ref()).as_str(),
@@ -1370,14 +1368,11 @@ impl Module {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn set_setter_fn<ARGS, A, B, F>(&mut self, name: impl AsRef<str>, func: F) -> u64
+    pub fn set_setter_fn<A, T, F, S>(&mut self, name: impl AsRef<str>, func: F) -> u64
     where
         A: Variant + Clone,
-        B: Variant + Clone,
-        F: RegisterNativeFunction<ARGS, RhaiResultOf<()>>
-            + Fn(&mut A, B) -> RhaiResultOf<()>
-            + SendSync
-            + 'static,
+        T: Variant + Clone,
+        F: RegisterNativeFunction<(Mut<A>, T), (), RhaiResultOf<S>> + SendSync + 'static,
     {
         self.set_fn(
             crate::engine::make_setter(name.as_ref()).as_str(),
@@ -1389,6 +1384,48 @@ impl Module {
         )
     }
 
+    /// Set a pair of Rust getter and setter functions into the [`Module`], returning both non-zero hash keys.
+    /// This is a short-hand for [`set_getter_fn`][Module::set_getter_fn] and [`set_setter_fn`][Module::set_setter_fn].
+    ///
+    /// These function are automatically exposed to the global namespace.
+    ///
+    /// If there are similar existing Rust functions, they are replaced.
+    ///
+    /// # Function Metadata
+    ///
+    /// No metadata for the function is registered.
+    /// Use [`update_fn_metadata`][Module::update_fn_metadata] to add metadata.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use rhai::{Module, ImmutableString};
+    ///
+    /// let mut module = Module::new();
+    /// let (hash_get, hash_set) = module.set_getter_setter_fn("value",
+    ///                                 |x: &mut i64| { Ok(x.to_string().into()) },
+    ///                                 |x: &mut i64, y: ImmutableString| {
+    ///                                     *x = y.len() as i64;
+    ///                                     Ok(())
+    ///                                 }
+    /// );
+    /// assert!(module.contains_fn(hash_get));
+    /// assert!(module.contains_fn(hash_set));
+    /// ```
+    #[cfg(not(feature = "no_object"))]
+    #[inline(always)]
+    pub fn set_getter_setter_fn<A: Variant + Clone, T: Variant + Clone, S1, S2>(
+        &mut self,
+        name: impl AsRef<str>,
+        getter: impl RegisterNativeFunction<(Mut<A>,), T, RhaiResultOf<S1>> + SendSync + 'static,
+        setter: impl RegisterNativeFunction<(Mut<A>, T), (), RhaiResultOf<S2>> + SendSync + 'static,
+    ) -> (u64, u64) {
+        (
+            self.set_getter_fn(name.as_ref(), getter),
+            self.set_setter_fn(name.as_ref(), setter),
+        )
+    }
+
     /// Set a Rust index getter taking two parameters (the first one mutable) into the [`Module`],
     /// returning a non-zero hash key.
     /// This function is automatically exposed to the global namespace.
@@ -1418,15 +1455,12 @@ impl Module {
     /// ```
     #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
     #[inline]
-    pub fn set_indexer_get_fn<ARGS, A, B, T, F>(&mut self, func: F) -> u64
+    pub fn set_indexer_get_fn<A, B, T, F, S>(&mut self, func: F) -> u64
     where
         A: Variant + Clone,
         B: Variant + Clone,
         T: Variant + Clone,
-        F: RegisterNativeFunction<ARGS, RhaiResultOf<T>>
-            + Fn(&mut A, B) -> RhaiResultOf<T>
-            + SendSync
-            + 'static,
+        F: RegisterNativeFunction<(Mut<A>, B), T, RhaiResultOf<S>> + SendSync + 'static,
     {
         #[cfg(not(feature = "no_index"))]
         if TypeId::of::<A>() == TypeId::of::<crate::Array>() {
@@ -1482,15 +1516,12 @@ impl Module {
     /// ```
     #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
     #[inline]
-    pub fn set_indexer_set_fn<ARGS, A, B, C, F>(&mut self, func: F) -> u64
+    pub fn set_indexer_set_fn<A, B, T, F, S>(&mut self, func: F) -> u64
     where
         A: Variant + Clone,
         B: Variant + Clone,
-        C: Variant + Clone,
-        F: RegisterNativeFunction<ARGS, RhaiResultOf<()>>
-            + Fn(&mut A, B, C) -> RhaiResultOf<()>
-            + SendSync
-            + 'static,
+        T: Variant + Clone,
+        F: RegisterNativeFunction<(Mut<A>, B, T), (), RhaiResultOf<S>> + SendSync + 'static,
     {
         #[cfg(not(feature = "no_index"))]
         if TypeId::of::<A>() == TypeId::of::<crate::Array>() {
@@ -1517,10 +1548,12 @@ impl Module {
         )
     }
 
-    /// Set a pair of Rust index getter and setter functions, returning both non-zero hash keys.
+    /// Set a pair of Rust index getter and setter functions into the [`Module`], returning both non-zero hash keys.
     /// This is a short-hand for [`set_indexer_get_fn`][Module::set_indexer_get_fn] and
     /// [`set_indexer_set_fn`][Module::set_indexer_set_fn].
     ///
+    /// These functions are automatically exposed to the global namespace.
+    ///
     /// If there are similar existing Rust functions, they are replaced.
     ///
     /// # Panics
@@ -1552,10 +1585,10 @@ impl Module {
     /// ```
     #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
     #[inline(always)]
-    pub fn set_indexer_get_set_fn<A, B, T>(
+    pub fn set_indexer_get_set_fn<A, B, T, S1, S2>(
         &mut self,
-        get_fn: impl Fn(&mut A, B) -> RhaiResultOf<T> + SendSync + 'static,
-        set_fn: impl Fn(&mut A, B, T) -> RhaiResultOf<()> + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<A>, B), T, RhaiResultOf<S1>> + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<A>, B, T), (), RhaiResultOf<S2>> + SendSync + 'static,
     ) -> (u64, u64)
     where
         A: Variant + Clone,
@@ -2016,7 +2049,7 @@ impl Module {
             if let Some(fn_ptr) = value.downcast_ref::<crate::FnPtr>() {
                 if ast.iter_fn_def().any(|f| f.name == fn_ptr.fn_name()) {
                     return Err(crate::ERR::ErrorMismatchDataType(
-                        "".to_string(),
+                        String::new(),
                         if fn_ptr.is_anonymous() {
                             format!("cannot export closure in variable {_name}")
                         } else {
@@ -2217,29 +2250,23 @@ impl Module {
     }
 
     /// Set a type iterator into the [`Module`].
-    #[cfg(not(feature = "sync"))]
-    #[inline]
-    pub fn set_iter(
-        &mut self,
-        type_id: TypeId,
-        func: impl Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + 'static,
-    ) -> &mut Self {
-        let func = Shared::new(func);
-        if self.indexed {
-            self.all_type_iterators.insert(type_id, func.clone());
-            self.contains_indexed_global_functions = true;
-        }
-        self.type_iterators.insert(type_id, func);
-        self
-    }
-
-    /// Set a type iterator into the [`Module`].
-    #[cfg(feature = "sync")]
-    #[inline]
+    #[inline(always)]
     pub fn set_iter(
         &mut self,
         type_id: TypeId,
         func: impl Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + SendSync + 'static,
+    ) -> &mut Self {
+        self.set_iter_result(type_id, move |x| {
+            Box::new(func(x).map(Ok)) as Box<dyn Iterator<Item = RhaiResultOf<Dynamic>>>
+        })
+    }
+
+    /// Set a fallible type iterator into the [`Module`].
+    #[inline]
+    pub fn set_iter_result(
+        &mut self,
+        type_id: TypeId,
+        func: impl Fn(Dynamic) -> Box<dyn Iterator<Item = RhaiResultOf<Dynamic>>> + SendSync + 'static,
     ) -> &mut Self {
         let func = Shared::new(func);
         if self.indexed {
@@ -2251,7 +2278,7 @@ impl Module {
     }
 
     /// Set a type iterator into the [`Module`].
-    #[inline]
+    #[inline(always)]
     pub fn set_iterable<T>(&mut self) -> &mut Self
     where
         T: Variant + Clone + IntoIterator,
@@ -2262,8 +2289,20 @@ impl Module {
         })
     }
 
+    /// Set a fallible type iterator into the [`Module`].
+    #[inline(always)]
+    pub fn set_iterable_result<T, X>(&mut self) -> &mut Self
+    where
+        T: Variant + Clone + IntoIterator<Item = RhaiResultOf<X>>,
+        X: Variant + Clone,
+    {
+        self.set_iter_result(TypeId::of::<T>(), |obj: Dynamic| {
+            Box::new(obj.cast::<T>().into_iter().map(|v| v.map(Dynamic::from)))
+        })
+    }
+
     /// Set an iterator type into the [`Module`] as a type iterator.
-    #[inline]
+    #[inline(always)]
     pub fn set_iterator<T>(&mut self) -> &mut Self
     where
         T: Variant + Clone + Iterator,
@@ -2274,6 +2313,18 @@ impl Module {
         })
     }
 
+    /// Set a iterator type into the [`Module`] as a fallible type iterator.
+    #[inline(always)]
+    pub fn set_iterator_result<T, X>(&mut self) -> &mut Self
+    where
+        T: Variant + Clone + Iterator<Item = RhaiResultOf<X>>,
+        X: Variant + Clone,
+    {
+        self.set_iter_result(TypeId::of::<T>(), |obj: Dynamic| {
+            Box::new(obj.cast::<T>().map(|v| v.map(Dynamic::from)))
+        })
+    }
+
     /// Get the specified type iterator.
     #[cfg(not(feature = "no_module"))]
     #[inline]
diff --git a/src/parser.rs b/src/parser.rs
index 754513c8..48a8a97b 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -2051,12 +2051,12 @@ impl Engine {
                                 Ok(Stmt::Assignment((op_info, (lhs, rhs).into()).into()))
                             }
                             // expr[???] = rhs, expr.??? = rhs
-                            ref expr => Err(PERR::AssignmentToInvalidLHS("".to_string())
+                            ref expr => Err(PERR::AssignmentToInvalidLHS(String::new())
                                 .into_err(expr.position())),
                         }
                     }
                     Some(err_pos) => {
-                        Err(PERR::AssignmentToInvalidLHS("".to_string()).into_err(err_pos))
+                        Err(PERR::AssignmentToInvalidLHS(String::new()).into_err(err_pos))
                     }
                 }
             }
@@ -2067,7 +2067,7 @@ impl Engine {
             )
             .into_err(op_pos)),
             // expr = rhs
-            _ => Err(PERR::AssignmentToInvalidLHS("".to_string()).into_err(lhs.position())),
+            _ => Err(PERR::AssignmentToInvalidLHS(String::new()).into_err(lhs.position())),
         }
     }
 
@@ -3665,8 +3665,9 @@ impl Engine {
                     (Token::Pipe, ..) => break,
                     (Token::Identifier(s), pos) => {
                         if params_list.iter().any(|p| p.as_str() == &*s) {
-                            return Err(PERR::FnDuplicatedParam("".to_string(), s.to_string())
-                                .into_err(pos));
+                            return Err(
+                                PERR::FnDuplicatedParam(String::new(), s.to_string()).into_err(pos)
+                            );
                         }
                         let s = state.get_interned_string(s);
                         state.stack.push(s.clone(), ());
diff --git a/src/types/parse_error.rs b/src/types/parse_error.rs
index b1cbf12a..a80255f0 100644
--- a/src/types/parse_error.rs
+++ b/src/types/parse_error.rs
@@ -51,8 +51,7 @@ impl fmt::Display for LexError {
             Self::ImproperSymbol(s, d) if d.is_empty() => {
                 write!(f, "Invalid symbol encountered: '{}'", s)
             }
-            Self::ImproperSymbol(s, d) if s.is_empty() => f.write_str(d),
-            Self::ImproperSymbol(s, d) => write!(f, "{}: '{}'", d, s),
+            Self::ImproperSymbol(.., d) => f.write_str(d),
         }
     }
 }
diff --git a/tests/assignments.rs b/tests/assignments.rs
index 1be3901b..e2a90d4f 100644
--- a/tests/assignments.rs
+++ b/tests/assignments.rs
@@ -25,28 +25,28 @@ fn test_assignments_bad_lhs() -> Result<(), Box<EvalAltResult>> {
             .compile("(x+y) = 42;")
             .expect_err("should error")
             .err_type(),
-        ParseErrorType::AssignmentToInvalidLHS("".to_string())
+        ParseErrorType::AssignmentToInvalidLHS(String::new())
     );
     assert_eq!(
         *engine
             .compile("foo(x) = 42;")
             .expect_err("should error")
             .err_type(),
-        ParseErrorType::AssignmentToInvalidLHS("".to_string())
+        ParseErrorType::AssignmentToInvalidLHS(String::new())
     );
     assert_eq!(
         *engine
             .compile("true = 42;")
             .expect_err("should error")
             .err_type(),
-        ParseErrorType::AssignmentToConstant("".to_string())
+        ParseErrorType::AssignmentToConstant(String::new())
     );
     assert_eq!(
         *engine
             .compile("123 = 42;")
             .expect_err("should error")
             .err_type(),
-        ParseErrorType::AssignmentToConstant("".to_string())
+        ParseErrorType::AssignmentToConstant(String::new())
     );
 
     #[cfg(not(feature = "no_object"))]
@@ -56,21 +56,21 @@ fn test_assignments_bad_lhs() -> Result<(), Box<EvalAltResult>> {
                 .compile("x.foo() = 42;")
                 .expect_err("should error")
                 .err_type(),
-            ParseErrorType::AssignmentToInvalidLHS("".to_string())
+            ParseErrorType::AssignmentToInvalidLHS(String::new())
         );
         assert_eq!(
             *engine
                 .compile("x.foo().x.y = 42;")
                 .expect_err("should error")
                 .err_type(),
-            ParseErrorType::AssignmentToInvalidLHS("".to_string())
+            ParseErrorType::AssignmentToInvalidLHS(String::new())
         );
         assert_eq!(
             *engine
                 .compile("x.y.z.foo() = 42;")
                 .expect_err("should error")
                 .err_type(),
-            ParseErrorType::AssignmentToInvalidLHS("".to_string())
+            ParseErrorType::AssignmentToInvalidLHS(String::new())
         );
         #[cfg(not(feature = "no_index"))]
         assert_eq!(
@@ -78,7 +78,7 @@ fn test_assignments_bad_lhs() -> Result<(), Box<EvalAltResult>> {
                 .compile("x.foo()[0] = 42;")
                 .expect_err("should error")
                 .err_type(),
-            ParseErrorType::AssignmentToInvalidLHS("".to_string())
+            ParseErrorType::AssignmentToInvalidLHS(String::new())
         );
         #[cfg(not(feature = "no_index"))]
         assert_eq!(
@@ -86,7 +86,7 @@ fn test_assignments_bad_lhs() -> Result<(), Box<EvalAltResult>> {
                 .compile("x[y].z.foo() = 42;")
                 .expect_err("should error")
                 .err_type(),
-            ParseErrorType::AssignmentToInvalidLHS("".to_string())
+            ParseErrorType::AssignmentToInvalidLHS(String::new())
         );
     }
 
diff --git a/tests/custom_syntax.rs b/tests/custom_syntax.rs
index 0f7992f4..6c70dcd8 100644
--- a/tests/custom_syntax.rs
+++ b/tests/custom_syntax.rs
@@ -303,7 +303,7 @@ fn test_custom_syntax_raw() -> Result<(), Box<EvalAltResult>> {
             .compile("hello hey")
             .expect_err("should error")
             .err_type(),
-        ParseErrorType::BadInput(LexError::ImproperSymbol("hey".to_string(), "".to_string()))
+        ParseErrorType::BadInput(LexError::ImproperSymbol("hey".to_string(), String::new()))
     );
 
     Ok(())
diff --git a/tests/get_set.rs b/tests/get_set.rs
index 2879d56b..d9065952 100644
--- a/tests/get_set.rs
+++ b/tests/get_set.rs
@@ -1,6 +1,6 @@
 #![cfg(not(feature = "no_object"))]
 
-use rhai::{Engine, EvalAltResult, Scope, INT};
+use rhai::{Engine, EvalAltResult, NativeCallContext, Scope, INT};
 
 #[test]
 fn test_get_set() -> Result<(), Box<EvalAltResult>> {
@@ -217,12 +217,14 @@ fn test_get_set_chain_without_write_back() -> Result<(), Box<EvalAltResult>> {
         .register_get_set(
             "value",
             |t: &mut Inner| t.value,
-            |_: &mut Inner, new: INT| panic!("Inner::value setter called with {}", new),
+            |_: NativeCallContext, _: &mut Inner, new: INT| {
+                panic!("Inner::value setter called with {}", new)
+            },
         )
         .register_type::<Outer>()
         .register_get_set(
             "inner",
-            |t: &mut Outer| t.inner.clone(),
+            |_: NativeCallContext, t: &mut Outer| t.inner.clone(),
             |_: &mut Outer, new: Inner| panic!("Outer::inner setter called with {:?}", new),
         );