diff --git a/README.md b/README.md
index b8b58cf1..55acc262 100644
--- a/README.md
+++ b/README.md
@@ -68,7 +68,7 @@ Optional features
 | `only_i32`    | Set the system integer type to `i32` and disable all other integer types. `INT` is set to `i32`.                                                         |
 | `only_i64`    | Set the system integer type to `i64` and disable all other integer types. `INT` is set to `i64`.                                                         |
 | `no_std`      | Build for `no-std`. Notice that additional dependencies will be pulled in to replace `std` features.                                                     |
-| `sync`        | Restrict all values types to those that are `Send + Sync`. Under this feature, [`Scope`] and `AST` are both `Send + Sync`.                               |
+| `sync`        | Restrict all values types to those that are `Send + Sync`. Under this feature, `Engine`, [`Scope`] and `AST` are all `Send + Sync`.                      |
 
 By default, Rhai includes all the standard functionalities in a small, tight package.  Most features are here to opt-**out** of certain functionalities that are not needed.
 Excluding unneeded functionalities can result in smaller, faster builds as well as less bugs due to a more restricted language.
diff --git a/src/api.rs b/src/api.rs
index b3acfa79..6b6cf9bd 100644
--- a/src/api.rs
+++ b/src/api.rs
@@ -22,6 +22,44 @@ use crate::stdlib::{
 #[cfg(not(feature = "no_std"))]
 use crate::stdlib::{fs::File, io::prelude::*, path::PathBuf};
 
+// Define callback function types
+#[cfg(feature = "sync")]
+pub trait ObjectGetCallback<T, U>: Fn(&mut T) -> U + Send + Sync + 'static {}
+#[cfg(feature = "sync")]
+impl<F: Fn(&mut T) -> U + Send + Sync + 'static, T, U> ObjectGetCallback<T, U> for F {}
+
+#[cfg(not(feature = "sync"))]
+pub trait ObjectGetCallback<T, U>: Fn(&mut T) -> U + 'static {}
+#[cfg(not(feature = "sync"))]
+impl<F: Fn(&mut T) -> U + 'static, T, U> ObjectGetCallback<T, U> for F {}
+
+#[cfg(feature = "sync")]
+pub trait ObjectSetCallback<T, U>: Fn(&mut T, U) + Send + Sync + 'static {}
+#[cfg(feature = "sync")]
+impl<F: Fn(&mut T, U) + Send + Sync + 'static, T, U> ObjectSetCallback<T, U> for F {}
+
+#[cfg(not(feature = "sync"))]
+pub trait ObjectSetCallback<T, U>: Fn(&mut T, U) + 'static {}
+#[cfg(not(feature = "sync"))]
+impl<F: Fn(&mut T, U) + 'static, T, U> ObjectSetCallback<T, 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 {}
+
+/// Engine public API
 impl<'e> Engine<'e> {
     /// Register a custom function.
     pub(crate) fn register_fn_raw(&mut self, fn_name: &str, args: Vec<TypeId>, f: Box<FnAny>) {
@@ -126,10 +164,7 @@ impl<'e> Engine<'e> {
 
     /// Register an iterator adapter for a type with the `Engine`.
     /// This is an advanced feature.
-    pub fn register_iterator<T: Any, F>(&mut self, f: F)
-    where
-        F: Fn(&Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + 'static,
-    {
+    pub fn register_iterator<T: Any, F: IteratorCallback>(&mut self, f: F) {
         self.type_iterators.insert(TypeId::of::<T>(), Box::new(f));
     }
 
@@ -170,11 +205,12 @@ impl<'e> Engine<'e> {
     /// # }
     /// ```
     #[cfg(not(feature = "no_object"))]
-    pub fn register_get<T: Any + Clone, U: Any + Clone>(
-        &mut self,
-        name: &str,
-        callback: impl Fn(&mut T) -> U + 'static,
-    ) {
+    pub fn register_get<T, U, F>(&mut self, name: &str, callback: F)
+    where
+        T: Any + Clone,
+        U: Any + Clone,
+        F: ObjectGetCallback<T, U>,
+    {
         self.register_fn(&make_getter(name), callback);
     }
 
@@ -215,11 +251,12 @@ impl<'e> Engine<'e> {
     /// # }
     /// ```
     #[cfg(not(feature = "no_object"))]
-    pub fn register_set<T: Any + Clone, U: Any + Clone>(
-        &mut self,
-        name: &str,
-        callback: impl Fn(&mut T, U) -> () + 'static,
-    ) {
+    pub fn register_set<T, U, F>(&mut self, name: &str, callback: F)
+    where
+        T: Any + Clone,
+        U: Any + Clone,
+        F: ObjectSetCallback<T, U>,
+    {
         self.register_fn(&make_setter(name), callback);
     }
 
@@ -262,12 +299,13 @@ impl<'e> Engine<'e> {
     /// # }
     /// ```
     #[cfg(not(feature = "no_object"))]
-    pub fn register_get_set<T: Any + Clone, U: Any + Clone>(
-        &mut self,
-        name: &str,
-        get_fn: impl Fn(&mut T) -> U + 'static,
-        set_fn: impl Fn(&mut T, U) -> () + 'static,
-    ) {
+    pub fn register_get_set<T, U, G, S>(&mut self, name: &str, get_fn: G, set_fn: S)
+    where
+        T: Any + Clone,
+        U: Any + Clone,
+        G: ObjectGetCallback<T, U>,
+        S: ObjectSetCallback<T, U>,
+    {
         self.register_get(name, get_fn);
         self.register_set(name, set_fn);
     }
@@ -925,6 +963,11 @@ impl<'e> Engine<'e> {
     /// # Ok(())
     /// # }
     /// ```
+    #[cfg(feature = "sync")]
+    pub fn on_print(&mut self, callback: impl FnMut(&str) + Send + Sync + 'e) {
+        self.on_print = Box::new(callback);
+    }
+    #[cfg(not(feature = "sync"))]
     pub fn on_print(&mut self, callback: impl FnMut(&str) + 'e) {
         self.on_print = Box::new(callback);
     }
@@ -949,6 +992,11 @@ impl<'e> Engine<'e> {
     /// # Ok(())
     /// # }
     /// ```
+    #[cfg(feature = "sync")]
+    pub fn on_debug(&mut self, callback: impl FnMut(&str) + Send + Sync + 'e) {
+        self.on_debug = Box::new(callback);
+    }
+    #[cfg(not(feature = "sync"))]
     pub fn on_debug(&mut self, callback: impl FnMut(&str) + 'e) {
         self.on_debug = Box::new(callback);
     }
diff --git a/src/builtin.rs b/src/builtin.rs
index d7f5840e..db6b5345 100644
--- a/src/builtin.rs
+++ b/src/builtin.rs
@@ -1,7 +1,7 @@
 //! Helper module that allows registration of the _core library_ and
 //! _standard library_ of utility functions.
 
-use crate::any::Any;
+use crate::any::{Any, Dynamic};
 use crate::engine::{Engine, FUNC_TO_STRING, KEYWORD_DEBUG, KEYWORD_PRINT};
 use crate::fn_register::{RegisterDynamicFn, RegisterFn, RegisterResultFn};
 use crate::parser::{Position, INT};
@@ -612,8 +612,9 @@ impl Engine<'_> {
                 reg_fn1!(self, KEYWORD_DEBUG, to_debug, String, Array);
 
                 // Register array iterator
-                self.register_iterator::<Array, _>(|a| {
+                self.register_iterator::<Array, _>(|a: &Dynamic| {
                     Box::new(a.downcast_ref::<Array>().unwrap().clone().into_iter())
+                        as Box<dyn Iterator<Item = Dynamic>>
                 });
             }
 
@@ -636,13 +637,13 @@ impl Engine<'_> {
         where
             Range<T>: Iterator<Item = T>,
         {
-            engine.register_iterator::<Range<T>, _>(|a| {
+            engine.register_iterator::<Range<T>, _>(|a: &Dynamic| {
                 Box::new(
                     a.downcast_ref::<Range<T>>()
                         .unwrap()
                         .clone()
                         .map(|n| n.into_dynamic()),
-                )
+                ) as Box<dyn Iterator<Item = Dynamic>>
             });
         }
 
diff --git a/src/engine.rs b/src/engine.rs
index 5230f5b8..235068a2 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -32,8 +32,14 @@ pub type Map = HashMap<String, Dynamic>;
 
 pub type FnCallArgs<'a> = [&'a mut Variant];
 
+#[cfg(feature = "sync")]
+pub type FnAny = dyn Fn(&mut FnCallArgs, Position) -> Result<Dynamic, EvalAltResult> + Send + Sync;
+#[cfg(not(feature = "sync"))]
 pub type FnAny = dyn Fn(&mut FnCallArgs, Position) -> Result<Dynamic, EvalAltResult>;
 
+#[cfg(feature = "sync")]
+type IteratorFn = dyn Fn(&Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + Send + Sync;
+#[cfg(not(feature = "sync"))]
 type IteratorFn = dyn Fn(&Dynamic) -> Box<dyn Iterator<Item = Dynamic>>;
 
 pub const MAX_CALL_STACK_DEPTH: usize = 64;
@@ -193,8 +199,15 @@ pub struct Engine<'e> {
     pub(crate) type_names: HashMap<String, String>,
 
     /// Closure for implementing the `print` command.
+    #[cfg(feature = "sync")]
+    pub(crate) on_print: Box<dyn FnMut(&str) + Send + Sync + 'e>,
+    #[cfg(not(feature = "sync"))]
     pub(crate) on_print: Box<dyn FnMut(&str) + 'e>,
+
     /// Closure for implementing the `debug` command.
+    #[cfg(feature = "sync")]
+    pub(crate) on_debug: Box<dyn FnMut(&str) + Send + Sync + 'e>,
+    #[cfg(not(feature = "sync"))]
     pub(crate) on_debug: Box<dyn FnMut(&str) + 'e>,
 
     /// Optimize the AST after compilation.
@@ -280,6 +293,11 @@ fn extract_prop_from_setter(fn_name: &str) -> Option<&str> {
 impl Engine<'_> {
     /// Create a new `Engine`
     pub fn new() -> Self {
+        // fn abc<F: Fn() + Send + Sync>(f: F) {
+        //     f();
+        // }
+        // abc(|| ());
+
         Default::default()
     }
 
diff --git a/src/fn_register.rs b/src/fn_register.rs
index cfc1b345..7ff2cc28 100644
--- a/src/fn_register.rs
+++ b/src/fn_register.rs
@@ -138,7 +138,13 @@ macro_rules! def_register {
     //                                            ^ dereferencing function
         impl<
             $($par: Any + Clone,)*
+
+            #[cfg(feature = "sync")]
+            FN: Fn($($param),*) -> RET + Send + Sync + 'static,
+
+            #[cfg(not(feature = "sync"))]
             FN: Fn($($param),*) -> RET + 'static,
+
             RET: Any
         > RegisterFn<FN, ($($mark,)*), RET> for Engine<'_>
         {
@@ -171,6 +177,11 @@ macro_rules! def_register {
 
         impl<
             $($par: Any + Clone,)*
+
+            #[cfg(feature = "sync")]
+            FN: Fn($($param),*) -> Dynamic + Send + Sync + 'static,
+
+            #[cfg(not(feature = "sync"))]
             FN: Fn($($param),*) -> Dynamic + 'static,
         > RegisterDynamicFn<FN, ($($mark,)*)> for Engine<'_>
         {
@@ -202,7 +213,12 @@ macro_rules! def_register {
 
         impl<
             $($par: Any + Clone,)*
+
+            #[cfg(feature = "sync")]
+            FN: Fn($($param),*) -> Result<RET, EvalAltResult> + Send + Sync + 'static,
+            #[cfg(not(feature = "sync"))]
             FN: Fn($($param),*) -> Result<RET, EvalAltResult> + 'static,
+
             RET: Any
         > RegisterResultFn<FN, ($($mark,)*), RET> for Engine<'_>
         {