diff --git a/src/api.rs b/src/api.rs
index 874aaa4e..48052e85 100644
--- a/src/api.rs
+++ b/src/api.rs
@@ -4,9 +4,7 @@ use crate::any::{Dynamic, Variant};
 use crate::engine::{make_getter, make_setter, Engine, State, FUNC_INDEXER};
 use crate::error::ParseError;
 use crate::fn_call::FuncArgs;
-use crate::fn_native::{
-    IteratorCallback, ObjectGetCallback, ObjectIndexerCallback, ObjectSetCallback,
-};
+use crate::fn_native::{IteratorFn, ObjectGetCallback, ObjectIndexerCallback, ObjectSetCallback};
 use crate::fn_register::RegisterFn;
 use crate::optimize::{optimize_into_ast, OptimizationLevel};
 use crate::parser::{parse, parse_global_expr, AST};
@@ -123,8 +121,8 @@ impl Engine {
 
     /// Register an iterator adapter for a type with the `Engine`.
     /// This is an advanced feature.
-    pub fn register_iterator<T: Variant + Clone, F: IteratorCallback>(&mut self, f: F) {
-        self.global_module.set_iter(TypeId::of::<T>(), Box::new(f));
+    pub fn register_iterator<T: Variant + Clone>(&mut self, f: IteratorFn) {
+        self.global_module.set_iter(TypeId::of::<T>(), f);
     }
 
     /// Register a getter function for a member of a registered type with the `Engine`.
diff --git a/src/fn_native.rs b/src/fn_native.rs
index 619fb102..262bad7c 100644
--- a/src/fn_native.rs
+++ b/src/fn_native.rs
@@ -11,10 +11,7 @@ pub type FnAny = dyn Fn(&mut FnCallArgs) -> Result<Dynamic, Box<EvalAltResult>>
 #[cfg(not(feature = "sync"))]
 pub type FnAny = dyn Fn(&mut FnCallArgs) -> Result<Dynamic, Box<EvalAltResult>>;
 
-#[cfg(feature = "sync")]
-pub type IteratorFn = dyn Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + Send + Sync;
-#[cfg(not(feature = "sync"))]
-pub type IteratorFn = dyn Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>>;
+pub type IteratorFn = fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>>;
 
 #[cfg(feature = "sync")]
 pub type PrintCallback = dyn Fn(&str) + Send + Sync + 'static;
@@ -57,32 +54,11 @@ pub trait ObjectIndexerCallback<T, X, U>: Fn(&mut T, X) -> U + 'static {}
 #[cfg(not(feature = "sync"))]
 impl<F: Fn(&mut T, X) -> U + 'static, T, X, U> ObjectIndexerCallback<T, X, U> for F {}
 
-#[cfg(feature = "sync")]
-pub trait IteratorCallback:
-    Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + Send + Sync + 'static
-{
-}
-#[cfg(feature = "sync")]
-impl<F: Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + Send + Sync + 'static> IteratorCallback
-    for F
-{
-}
-
-#[cfg(not(feature = "sync"))]
-pub trait IteratorCallback: Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + 'static {}
-#[cfg(not(feature = "sync"))]
-impl<F: Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + 'static> IteratorCallback for F {}
-
 #[cfg(not(feature = "sync"))]
 pub type SharedNativeFunction = Rc<FnAny>;
 #[cfg(feature = "sync")]
 pub type SharedNativeFunction = Arc<FnAny>;
 
-#[cfg(not(feature = "sync"))]
-pub type SharedIteratorFn = Rc<IteratorFn>;
-#[cfg(feature = "sync")]
-pub type SharedIteratorFn = Arc<IteratorFn>;
-
 #[cfg(feature = "sync")]
 pub type SharedFnDef = Arc<FnDef>;
 #[cfg(not(feature = "sync"))]
@@ -97,7 +73,7 @@ pub enum CallableFunction {
     /// and the rest passed by value.
     Method(SharedNativeFunction),
     /// An iterator function.
-    Iterator(SharedIteratorFn),
+    Iterator(IteratorFn),
     /// A script-defined function.
     Script(SharedFnDef),
 }
@@ -158,9 +134,9 @@ impl CallableFunction {
     /// # Panics
     ///
     /// Panics if the `CallableFunction` is not `Iterator`.
-    pub fn get_iter_fn(&self) -> &IteratorFn {
+    pub fn get_iter_fn(&self) -> IteratorFn {
         match self {
-            Self::Iterator(f) => f.as_ref(),
+            Self::Iterator(f) => *f,
             Self::Pure(_) | Self::Method(_) | Self::Script(_) => panic!(),
         }
     }
diff --git a/src/module.rs b/src/module.rs
index 5e751df8..48e6e936 100644
--- a/src/module.rs
+++ b/src/module.rs
@@ -3,7 +3,7 @@
 use crate::any::{Dynamic, Variant};
 use crate::calc_fn_hash;
 use crate::engine::{Engine, FunctionsLib};
-use crate::fn_native::{CallableFunction as CF, FnCallArgs, IteratorFn, SharedIteratorFn};
+use crate::fn_native::{CallableFunction as CF, FnCallArgs, IteratorFn};
 use crate::parser::{
     FnAccess,
     FnAccess::{Private, Public},
@@ -53,7 +53,7 @@ pub struct Module {
     fn_lib: FunctionsLib,
 
     /// Iterator functions, keyed by the type producing the iterator.
-    type_iterators: HashMap<TypeId, SharedIteratorFn>,
+    type_iterators: HashMap<TypeId, IteratorFn>,
 
     /// Flattened collection of all external Rust functions, native or scripted,
     /// including those in sub-modules.
@@ -659,13 +659,13 @@ impl Module {
     }
 
     /// Set a type iterator into the module.
-    pub fn set_iter(&mut self, typ: TypeId, func: Box<IteratorFn>) {
-        self.type_iterators.insert(typ, func.into());
+    pub fn set_iter(&mut self, typ: TypeId, func: IteratorFn) {
+        self.type_iterators.insert(typ, func);
     }
 
     /// Get the specified type iterator.
-    pub fn get_iter(&self, id: TypeId) -> Option<&IteratorFn> {
-        self.type_iterators.get(&id).map(|v| v.as_ref())
+    pub fn get_iter(&self, id: TypeId) -> Option<IteratorFn> {
+        self.type_iterators.get(&id).cloned()
     }
 }
 
diff --git a/src/packages/array_basic.rs b/src/packages/array_basic.rs
index bdfd03aa..eb56bbf4 100644
--- a/src/packages/array_basic.rs
+++ b/src/packages/array_basic.rs
@@ -120,8 +120,6 @@ def_package!(crate:BasicArrayPackage:"Basic array utilities.", lib, {
     // Register array iterator
     lib.set_iter(
         TypeId::of::<Array>(),
-        Box::new(|arr| Box::new(
-            arr.cast::<Array>().into_iter()) as Box<dyn Iterator<Item = Dynamic>>
-        ),
+        |arr| Box::new(arr.cast::<Array>().into_iter()) as Box<dyn Iterator<Item = Dynamic>>,
     );
 });
diff --git a/src/packages/iter_basic.rs b/src/packages/iter_basic.rs
index 553873e7..ebf55992 100644
--- a/src/packages/iter_basic.rs
+++ b/src/packages/iter_basic.rs
@@ -14,13 +14,10 @@ fn reg_range<T: Variant + Clone>(lib: &mut Module)
 where
     Range<T>: Iterator<Item = T>,
 {
-    lib.set_iter(
-        TypeId::of::<Range<T>>(),
-        Box::new(|source| {
-            Box::new(source.cast::<Range<T>>().map(|x| x.into_dynamic()))
-                as Box<dyn Iterator<Item = Dynamic>>
-        }),
-    );
+    lib.set_iter(TypeId::of::<Range<T>>(), |source| {
+        Box::new(source.cast::<Range<T>>().map(|x| x.into_dynamic()))
+            as Box<dyn Iterator<Item = Dynamic>>
+    });
 }
 
 fn get_range<T: Variant + Clone>(from: T, to: T) -> FuncReturn<Range<T>> {
@@ -58,13 +55,10 @@ where
     T: Variant + Clone + PartialOrd,
     StepRange<T>: Iterator<Item = T>,
 {
-    lib.set_iter(
-        TypeId::of::<StepRange<T>>(),
-        Box::new(|source| {
-            Box::new(source.cast::<StepRange<T>>().map(|x| x.into_dynamic()))
-                as Box<dyn Iterator<Item = Dynamic>>
-        }),
-    );
+    lib.set_iter(TypeId::of::<StepRange<T>>(), |source| {
+        Box::new(source.cast::<StepRange<T>>().map(|x| x.into_dynamic()))
+            as Box<dyn Iterator<Item = Dynamic>>
+    });
 }
 
 fn get_step_range<T>(from: T, to: T, step: T) -> FuncReturn<StepRange<T>>
diff --git a/src/packages/mod.rs b/src/packages/mod.rs
index 62df57a6..54b6ecea 100644
--- a/src/packages/mod.rs
+++ b/src/packages/mod.rs
@@ -83,7 +83,7 @@ impl PackagesCollection {
         self.packages.iter().any(|p| p.contains_iter(id))
     }
     /// Get the specified TypeId iterator.
-    pub fn get_iter(&self, id: TypeId) -> Option<&IteratorFn> {
+    pub fn get_iter(&self, id: TypeId) -> Option<IteratorFn> {
         self.packages
             .iter()
             .map(|p| p.get_iter(id))