diff --git a/src/api/build_type.rs b/src/api/build_type.rs
index 8fa940e7..08b483d0 100644
--- a/src/api/build_type.rs
+++ b/src/api/build_type.rs
@@ -117,10 +117,10 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
 
     /// Register a custom function.
     #[inline(always)]
-    pub fn with_fn<A, const N: usize, R, S>(
+    pub fn with_fn<A, const N: usize, const C: bool, R: Variant + Clone, const L: bool>(
         &mut self,
         name: impl AsRef<str> + Into<Identifier>,
-        method: impl RegisterNativeFunction<A, N, R, S>,
+        method: impl RegisterNativeFunction<A, N, C, R, L>,
     ) -> &mut Self {
         self.engine.register_fn(name, method);
         self
@@ -149,10 +149,10 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     ///
     /// Not available under `no_object`.
     #[inline(always)]
-    pub fn with_get<V: Variant + Clone, S>(
+    pub fn with_get<const C: bool, V: Variant + Clone, const L: bool>(
         &mut self,
         name: impl AsRef<str>,
-        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, V, S> + crate::func::SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, C, V, L> + crate::func::SendSync + 'static,
     ) -> &mut Self {
         self.engine.register_get(name, get_fn);
         self
@@ -162,10 +162,10 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     ///
     /// Not available under `no_object`.
     #[inline(always)]
-    pub fn with_set<V: Variant + Clone, S>(
+    pub fn with_set<const C: bool, V: Variant + Clone, const L: bool>(
         &mut self,
         name: impl AsRef<str>,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, (), S> + crate::func::SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, C, (), L> + crate::func::SendSync + 'static,
     ) -> &mut Self {
         self.engine.register_set(name, set_fn);
         self
@@ -177,11 +177,19 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     ///
     /// Not available under `no_object`.
     #[inline(always)]
-    pub fn with_get_set<V: Variant + Clone, S1, S2>(
+    pub fn with_get_set<
+        const C1: bool,
+        const C2: bool,
+        V: Variant + Clone,
+        const L1: bool,
+        const L2: bool,
+    >(
         &mut self,
         name: impl AsRef<str>,
-        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, V, S1> + crate::func::SendSync + 'static,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, (), S2> + crate::func::SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, C1, V, L1> + crate::func::SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, C2, (), L2>
+            + crate::func::SendSync
+            + 'static,
     ) -> &mut Self {
         self.engine.register_get_set(name, get_fn, set_fn);
         self
@@ -196,9 +204,14 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     ///
     /// Not available under both `no_index` and `no_object`.
     #[inline(always)]
-    pub fn with_indexer_get<X: Variant + Clone, V: Variant + Clone, S>(
+    pub fn with_indexer_get<
+        X: Variant + Clone,
+        const C: bool,
+        V: Variant + Clone,
+        const L: bool,
+    >(
         &mut self,
-        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, V, S> + crate::func::SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, C, V, L> + crate::func::SendSync + 'static,
     ) -> &mut Self {
         self.engine.register_indexer_get(get_fn);
         self
@@ -208,9 +221,16 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     ///
     /// Not available under both `no_index` and `no_object`.
     #[inline(always)]
-    pub fn with_indexer_set<X: Variant + Clone, V: Variant + Clone, S>(
+    pub fn with_indexer_set<
+        X: Variant + Clone,
+        const C: bool,
+        V: Variant + Clone,
+        const L: bool,
+    >(
         &mut self,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, (), S> + crate::func::SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, C, (), L>
+            + crate::func::SendSync
+            + 'static,
     ) -> &mut Self {
         self.engine.register_indexer_set(set_fn);
         self
@@ -220,10 +240,19 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     ///
     /// Not available under both `no_index` and `no_object`.
     #[inline(always)]
-    pub fn with_indexer_get_set<X: Variant + Clone, V: Variant + Clone, S1, S2>(
+    pub fn with_indexer_get_set<
+        X: Variant + Clone,
+        const C1: bool,
+        const C2: bool,
+        V: Variant + Clone,
+        const L1: bool,
+        const L2: bool,
+    >(
         &mut self,
-        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, V, S1> + crate::func::SendSync + 'static,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, (), S2> + crate::func::SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, C1, V, L1> + crate::func::SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, C2, (), L2>
+            + crate::func::SendSync
+            + 'static,
     ) -> &mut Self {
         self.engine.register_indexer_get_set(get_fn, set_fn);
         self
diff --git a/src/api/deprecated.rs b/src/api/deprecated.rs
index d090d6d4..b40d936a 100644
--- a/src/api/deprecated.rs
+++ b/src/api/deprecated.rs
@@ -185,10 +185,10 @@ impl Engine {
     /// This method will be removed in the next major version.
     #[deprecated(since = "1.9.1", note = "use `register_fn` instead")]
     #[inline(always)]
-    pub fn register_result_fn<A, const N: usize, R, S>(
+    pub fn register_result_fn<A, const N: usize, const C: bool, R: Variant + Clone>(
         &mut self,
         name: impl AsRef<str> + Into<Identifier>,
-        func: impl RegisterNativeFunction<A, N, R, RhaiResultOf<S>>,
+        func: impl RegisterNativeFunction<A, N, C, R, true>,
     ) -> &mut Self {
         self.register_fn(name, func)
     }
@@ -206,12 +206,10 @@ impl Engine {
     #[deprecated(since = "1.9.1", note = "use `register_get` instead")]
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn register_get_result<T: Variant + Clone, V: Variant + Clone, S>(
+    pub fn register_get_result<T: Variant + Clone, const C: bool, V: Variant + Clone>(
         &mut self,
         name: impl AsRef<str>,
-        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, V, RhaiResultOf<S>>
-            + crate::func::SendSync
-            + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, C, V, true> + crate::func::SendSync + 'static,
     ) -> &mut Self {
         self.register_get(name, get_fn)
     }
@@ -227,10 +225,10 @@ impl Engine {
     #[deprecated(since = "1.9.1", note = "use `register_set` instead")]
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn register_set_result<T: Variant + Clone, V: Variant + Clone, S>(
+    pub fn register_set_result<T: Variant + Clone, V: Variant + Clone, const C: bool, S>(
         &mut self,
         name: impl AsRef<str>,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, (), RhaiResultOf<S>>
+        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, C, (), true>
             + crate::func::SendSync
             + 'static,
     ) -> &mut Self {
@@ -254,10 +252,10 @@ impl Engine {
         T: Variant + Clone,
         X: Variant + Clone,
         V: Variant + Clone,
-        S,
+        const C: bool,
     >(
         &mut self,
-        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, V, RhaiResultOf<S>>
+        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, C, V, true>
             + crate::func::SendSync
             + 'static,
     ) -> &mut Self {
@@ -279,10 +277,10 @@ impl Engine {
         T: Variant + Clone,
         X: Variant + Clone,
         V: Variant + Clone,
-        S,
+        const C: bool,
     >(
         &mut self,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, (), RhaiResultOf<S>>
+        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, C, (), true>
             + crate::func::SendSync
             + 'static,
     ) -> &mut Self {
@@ -516,10 +514,15 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     /// This method will be removed in the next major version.
     #[deprecated(since = "1.9.1", note = "use `with_fn` instead")]
     #[inline(always)]
-    pub fn with_result_fn<X, A, const N: usize, F, R, S>(&mut self, name: X, method: F) -> &mut Self
+    pub fn with_result_fn<S, A, const N: usize, const C: bool, R, F>(
+        &mut self,
+        name: S,
+        method: F,
+    ) -> &mut Self
     where
-        X: AsRef<str> + Into<Identifier>,
-        F: RegisterNativeFunction<A, N, R, RhaiResultOf<S>>,
+        S: AsRef<str> + Into<Identifier>,
+        R: Variant + Clone,
+        F: RegisterNativeFunction<A, N, C, R, true>,
     {
         self.with_fn(name, method)
     }
@@ -538,12 +541,10 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     #[deprecated(since = "1.9.1", note = "use `with_get` instead")]
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn with_get_result<V: Variant + Clone, S>(
+    pub fn with_get_result<V: Variant + Clone, const C: bool>(
         &mut self,
         name: impl AsRef<str>,
-        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, V, RhaiResultOf<S>>
-            + crate::func::SendSync
-            + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, C, V, true> + crate::func::SendSync + 'static,
     ) -> &mut Self {
         self.with_get(name, get_fn)
     }
@@ -560,10 +561,10 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     #[deprecated(since = "1.9.1", note = "use `with_set` instead")]
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn with_set_result<V: Variant + Clone, S>(
+    pub fn with_set_result<V: Variant + Clone, const C: bool>(
         &mut self,
         name: impl AsRef<str>,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, (), RhaiResultOf<S>>
+        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, C, (), true>
             + crate::func::SendSync
             + 'static,
     ) -> &mut Self {
@@ -584,9 +585,9 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     #[deprecated(since = "1.9.1", note = "use `with_indexer_get` instead")]
     #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
     #[inline(always)]
-    pub fn with_indexer_get_result<X: Variant + Clone, V: Variant + Clone, S>(
+    pub fn with_indexer_get_result<X: Variant + Clone, V: Variant + Clone, const C: bool>(
         &mut self,
-        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, V, RhaiResultOf<S>>
+        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, C, V, true>
             + crate::func::SendSync
             + 'static,
     ) -> &mut Self {
@@ -605,9 +606,9 @@ impl<'a, T: Variant + Clone> TypeBuilder<'a, T> {
     #[deprecated(since = "1.9.1", note = "use `with_indexer_set` instead")]
     #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
     #[inline(always)]
-    pub fn with_indexer_set_result<X: Variant + Clone, V: Variant + Clone, S>(
+    pub fn with_indexer_set_result<X: Variant + Clone, V: Variant + Clone, const C: bool>(
         &mut self,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, (), RhaiResultOf<S>>
+        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, C, (), true>
             + crate::func::SendSync
             + 'static,
     ) -> &mut Self {
diff --git a/src/api/register.rs b/src/api/register.rs
index 37a7179d..eeb50fda 100644
--- a/src/api/register.rs
+++ b/src/api/register.rs
@@ -56,7 +56,14 @@ impl Engine {
     /// # }
     /// ```
     #[inline]
-    pub fn register_fn<A, const N: usize, R, S, F: RegisterNativeFunction<A, N, R, S>>(
+    pub fn register_fn<
+        A,
+        const N: usize,
+        const C: bool,
+        R: Variant + Clone,
+        const L: bool,
+        F: RegisterNativeFunction<A, N, C, R, L>,
+    >(
         &mut self,
         name: impl AsRef<str> + Into<Identifier>,
         func: F,
@@ -299,10 +306,10 @@ impl Engine {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn register_get<T: Variant + Clone, V: Variant + Clone, S>(
+    pub fn register_get<T: Variant + Clone, const C: bool, V: Variant + Clone, const L: bool>(
         &mut self,
         name: impl AsRef<str>,
-        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, V, S> + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, C, V, L> + SendSync + 'static,
     ) -> &mut Self {
         self.register_fn(crate::engine::make_getter(name.as_ref()).as_str(), get_fn)
     }
@@ -349,10 +356,10 @@ impl Engine {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn register_set<T: Variant + Clone, V: Variant + Clone, S>(
+    pub fn register_set<T: Variant + Clone, const C: bool, V: Variant + Clone, const L: bool>(
         &mut self,
         name: impl AsRef<str>,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, (), S> + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, C, (), L> + SendSync + 'static,
     ) -> &mut Self {
         self.register_fn(crate::engine::make_setter(name.as_ref()).as_str(), set_fn)
     }
@@ -403,11 +410,18 @@ impl Engine {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn register_get_set<T: Variant + Clone, V: Variant + Clone, S1, S2>(
+    pub fn register_get_set<
+        T: Variant + Clone,
+        const C1: bool,
+        const C2: bool,
+        V: Variant + Clone,
+        const L1: bool,
+        const L2: bool,
+    >(
         &mut self,
         name: impl AsRef<str>,
-        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, V, S1> + SendSync + 'static,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, (), S2> + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>,), 1, C1, V, L1> + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, V), 2, C2, (), L2> + SendSync + 'static,
     ) -> &mut Self {
         self.register_get(&name, get_fn).register_set(&name, set_fn)
     }
@@ -462,9 +476,15 @@ 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, S>(
+    pub fn register_indexer_get<
+        T: Variant + Clone,
+        X: Variant + Clone,
+        const C: bool,
+        V: Variant + Clone,
+        const L: bool,
+    >(
         &mut self,
-        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, V, S> + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, C, V, L> + SendSync + 'static,
     ) -> &mut Self {
         #[cfg(not(feature = "no_index"))]
         if TypeId::of::<T>() == TypeId::of::<crate::Array>() {
@@ -537,9 +557,15 @@ 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, S>(
+    pub fn register_indexer_set<
+        T: Variant + Clone,
+        X: Variant + Clone,
+        const C: bool,
+        V: Variant + Clone,
+        const L: bool,
+    >(
         &mut self,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, (), S> + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, C, (), L> + SendSync + 'static,
     ) -> &mut Self {
         #[cfg(not(feature = "no_index"))]
         if TypeId::of::<T>() == TypeId::of::<crate::Array>() {
@@ -616,13 +642,15 @@ impl Engine {
     pub fn register_indexer_get_set<
         T: Variant + Clone,
         X: Variant + Clone,
+        const C1: bool,
+        const C2: bool,
         V: Variant + Clone,
-        S1,
-        S2,
+        const L1: bool,
+        const L2: bool,
     >(
         &mut self,
-        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, V, S1> + SendSync + 'static,
-        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, (), S2> + SendSync + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<T>, X), 2, C1, V, L1> + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<T>, X, V), 3, C2, (), L2> + SendSync + 'static,
     ) -> &mut Self {
         self.register_indexer_get(get_fn)
             .register_indexer_set(set_fn)
diff --git a/src/func/register.rs b/src/func/register.rs
index 0144eeba..9f31aad6 100644
--- a/src/func/register.rs
+++ b/src/func/register.rs
@@ -26,13 +26,13 @@ use std::{
 ///
 /// # Examples
 ///
-/// `RegisterNativeFunction<(Mut<A>, B, Ref<C>), 3, R, ()>` = `Fn(&mut A, B, &C) -> R`
+/// `RegisterNativeFunction<(Mut<A>, B, Ref<C>), 3, false, R, false>` = `Fn(&mut A, B, &C) -> R`
 ///
-/// `RegisterNativeFunction<(Mut<A>, B, Ref<C>), 3, R, NativeCallContext>` = `Fn(NativeCallContext, &mut A, B, &C) -> R`
+/// `RegisterNativeFunction<(Mut<A>, B, Ref<C>), 3, true,  R, false>` = `Fn(NativeCallContext, &mut A, B, &C) -> R`
 ///
-/// `RegisterNativeFunction<(Mut<A>, B, Ref<C>), 3, R, RhaiResultOf<()>>` = `Fn(&mut A, B, &C) -> Result<R, Box<EvalAltResult>>`
+/// `RegisterNativeFunction<(Mut<A>, B, Ref<C>), 3, false, R, true>`  = `Fn(&mut A, B, &C) -> Result<R, Box<EvalAltResult>>`
 ///
-/// `RegisterNativeFunction<(Mut<A>, B, Ref<C>), 3, R, RhaiResultOf<NativeCallContext>>` = `Fn(NativeCallContext, &mut A, B, &C) -> Result<R, Box<EvalAltResult>>`
+/// `RegisterNativeFunction<(Mut<A>, B, Ref<C>), 3, true,  R, true>`  = `Fn(NativeCallContext, &mut A, B, &C) -> Result<R, Box<EvalAltResult>>`
 ///
 /// These types are not actually used anywhere.
 pub struct Mut<T>(T);
@@ -73,9 +73,10 @@ pub fn by_value<T: Variant + Clone>(data: &mut Dynamic) -> T {
 ///
 /// * `ARGS` - a tuple containing parameter types, with `&mut T` represented by `Mut<T>`.
 /// * `NUM` - a constant generic containing the number of parameters, must be consistent with `ARGS`.
+/// * `CTX` - a constant boolean generic indicating whether there is a `NativeCallContext` parameter.
 /// * `RET` - return type of the function; if the function returns `Result`, it is the unwrapped inner value type.
-/// * `RESULT` - a type to distinguish between multiple forms.  Must match the function signature.
-pub trait RegisterNativeFunction<ARGS, const NUM: usize, RET, RESULT> {
+/// * `FALL` - a constant boolean generic indicating whether the function is fallible (i.e. returns `Result<T, Box<EvalAltResult>>`).
+pub trait RegisterNativeFunction<ARGS, const NUM: usize, const CTX: bool, RET, const FALL: bool> {
     /// Convert this function into a [`CallableFunction`].
     #[must_use]
     fn into_callable_function(self) -> CallableFunction;
@@ -105,8 +106,6 @@ pub trait RegisterNativeFunction<ARGS, const NUM: usize, RET, RESULT> {
     }
 }
 
-const EXPECT_ARGS: &str = "arguments";
-
 macro_rules! check_constant {
     ($abi:ident, $n:expr, $ctx:ident, $args:ident) => {
         #[cfg(any(not(feature = "no_object"), not(feature = "no_index")))]
@@ -150,7 +149,7 @@ macro_rules! def_register {
             FN: Fn($($param),*) -> RET + SendSync + 'static,
             $($par: Variant + Clone,)*
             RET: Variant + Clone
-        > RegisterNativeFunction<($($mark,)*), $n, RET, ()> for FN {
+        > RegisterNativeFunction<($($mark,)*), $n, false, RET, false> for FN {
             #[inline(always)] fn param_types() -> [TypeId;$n] { [$(TypeId::of::<$par>()),*] }
             #[inline(always)] fn num_params() -> usize { $n }
             #[cfg(feature = "metadata")] #[inline(always)] fn param_names() -> [&'static str;$n] { [$(type_name::<$param>()),*] }
@@ -161,7 +160,7 @@ macro_rules! def_register {
                     check_constant!($abi, $n, ctx, args);
 
                     let mut drain = args.iter_mut();
-                    $(let mut $par = $clone(drain.next().expect(EXPECT_ARGS)); )*
+                    $(let mut $par = $clone(drain.next().unwrap()); )*
 
                     // Call the function with each argument value
                     let r = self($($arg),*);
@@ -176,7 +175,7 @@ macro_rules! def_register {
             FN: for<'a> Fn(NativeCallContext<'a>, $($param),*) -> RET + SendSync + 'static,
             $($par: Variant + Clone,)*
             RET: Variant + Clone
-        > RegisterNativeFunction<($($mark,)*), $n, RET, NativeCallContext<'static>> for FN {
+        > RegisterNativeFunction<($($mark,)*), $n, true, RET, false> for FN {
             #[inline(always)] fn param_types() -> [TypeId;$n] { [$(TypeId::of::<$par>()),*] }
             #[inline(always)] fn num_params() -> usize { $n }
             #[cfg(feature = "metadata")] #[inline(always)] fn param_names() -> [&'static str;$n] { [$(type_name::<$param>()),*] }
@@ -187,7 +186,7 @@ macro_rules! def_register {
                     check_constant!($abi, $n, ctx, args);
 
                     let mut drain = args.iter_mut();
-                    $(let mut $par = $clone(drain.next().expect(EXPECT_ARGS)); )*
+                    $(let mut $par = $clone(drain.next().unwrap()); )*
 
                     // Call the function with each argument value
                     let r = self(ctx, $($arg),*);
@@ -202,7 +201,7 @@ macro_rules! def_register {
             FN: Fn($($param),*) -> RhaiResultOf<RET> + SendSync + 'static,
             $($par: Variant + Clone,)*
             RET: Variant + Clone
-        > RegisterNativeFunction<($($mark,)*), $n, RET, RhaiResultOf<()>> for FN {
+        > RegisterNativeFunction<($($mark,)*), $n, false, RET, true> for FN {
             #[inline(always)] fn param_types() -> [TypeId;$n] { [$(TypeId::of::<$par>()),*] }
             #[inline(always)] fn num_params() -> usize { $n }
             #[cfg(feature = "metadata")] #[inline(always)] fn param_names() -> [&'static str;$n] { [$(type_name::<$param>()),*] }
@@ -214,7 +213,7 @@ macro_rules! def_register {
                     check_constant!($abi, $n, ctx, args);
 
                     let mut drain = args.iter_mut();
-                    $(let mut $par = $clone(drain.next().expect(EXPECT_ARGS)); )*
+                    $(let mut $par = $clone(drain.next().unwrap()); )*
 
                     // Call the function with each argument value
                     self($($arg),*).map(Dynamic::from)
@@ -226,7 +225,7 @@ macro_rules! def_register {
             FN: for<'a> Fn(NativeCallContext<'a>, $($param),*) -> RhaiResultOf<RET> + SendSync + 'static,
             $($par: Variant + Clone,)*
             RET: Variant + Clone
-        > RegisterNativeFunction<($($mark,)*), $n, RET, RhaiResultOf<NativeCallContext<'static>>> for FN {
+        > RegisterNativeFunction<($($mark,)*), $n, true, RET, true> for FN {
             #[inline(always)] fn param_types() -> [TypeId;$n] { [$(TypeId::of::<$par>()),*] }
             #[inline(always)] fn num_params() -> usize { $n }
             #[cfg(feature = "metadata")] #[inline(always)] fn param_names() -> [&'static str;$n] { [$(type_name::<$param>()),*] }
@@ -238,7 +237,7 @@ macro_rules! def_register {
                     check_constant!($abi, $n, ctx, args);
 
                     let mut drain = args.iter_mut();
-                    $(let mut $par = $clone(drain.next().expect(EXPECT_ARGS)); )*
+                    $(let mut $par = $clone(drain.next().unwrap()); )*
 
                     // Call the function with each argument value
                     self(ctx, $($arg),*).map(Dynamic::from)
diff --git a/src/module/mod.rs b/src/module/mod.rs
index af70abb7..67c4038b 100644
--- a/src/module/mod.rs
+++ b/src/module/mod.rs
@@ -1237,14 +1237,14 @@ impl Module {
     /// assert!(module.contains_fn(hash));
     /// ```
     #[inline(always)]
-    pub fn set_native_fn<A, const N: usize, T, F, S>(
+    pub fn set_native_fn<A, const N: usize, const C: bool, T, F>(
         &mut self,
         name: impl AsRef<str> + Into<Identifier>,
         func: F,
     ) -> u64
     where
         T: Variant + Clone,
-        F: RegisterNativeFunction<A, N, T, RhaiResultOf<S>>,
+        F: RegisterNativeFunction<A, N, C, T, true>,
     {
         self.set_fn(
             name,
@@ -1276,11 +1276,11 @@ impl Module {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn set_getter_fn<A, T, F, S>(&mut self, name: impl AsRef<str>, func: F) -> u64
+    pub fn set_getter_fn<A, const C: bool, T, F>(&mut self, name: impl AsRef<str>, func: F) -> u64
     where
         A: Variant + Clone,
         T: Variant + Clone,
-        F: RegisterNativeFunction<(Mut<A>,), 1, T, RhaiResultOf<S>> + SendSync + 'static,
+        F: RegisterNativeFunction<(Mut<A>,), 1, C, T, true> + SendSync + 'static,
     {
         self.set_fn(
             crate::engine::make_getter(name.as_ref()).as_str(),
@@ -1317,11 +1317,11 @@ impl Module {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn set_setter_fn<A, T, F, S>(&mut self, name: impl AsRef<str>, func: F) -> u64
+    pub fn set_setter_fn<A, const C: bool, T, F>(&mut self, name: impl AsRef<str>, func: F) -> u64
     where
         A: Variant + Clone,
         T: Variant + Clone,
-        F: RegisterNativeFunction<(Mut<A>, T), 2, (), RhaiResultOf<S>> + SendSync + 'static,
+        F: RegisterNativeFunction<(Mut<A>, T), 2, C, (), true> + SendSync + 'static,
     {
         self.set_fn(
             crate::engine::make_setter(name.as_ref()).as_str(),
@@ -1363,11 +1363,16 @@ impl Module {
     /// ```
     #[cfg(not(feature = "no_object"))]
     #[inline(always)]
-    pub fn set_getter_setter_fn<A: Variant + Clone, T: Variant + Clone, S1, S2>(
+    pub fn set_getter_setter_fn<
+        A: Variant + Clone,
+        const C1: bool,
+        const C2: bool,
+        T: Variant + Clone,
+    >(
         &mut self,
         name: impl AsRef<str>,
-        getter: impl RegisterNativeFunction<(Mut<A>,), 1, T, RhaiResultOf<S1>> + SendSync + 'static,
-        setter: impl RegisterNativeFunction<(Mut<A>, T), 2, (), RhaiResultOf<S2>> + SendSync + 'static,
+        getter: impl RegisterNativeFunction<(Mut<A>,), 1, C1, T, true> + SendSync + 'static,
+        setter: impl RegisterNativeFunction<(Mut<A>, T), 2, C2, (), true> + SendSync + 'static,
     ) -> (u64, u64) {
         (
             self.set_getter_fn(name.as_ref(), getter),
@@ -1404,12 +1409,12 @@ impl Module {
     /// ```
     #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
     #[inline]
-    pub fn set_indexer_get_fn<A, B, T, F, S>(&mut self, func: F) -> u64
+    pub fn set_indexer_get_fn<A, B, const C: bool, T, F>(&mut self, func: F) -> u64
     where
         A: Variant + Clone,
         B: Variant + Clone,
         T: Variant + Clone,
-        F: RegisterNativeFunction<(Mut<A>, B), 2, T, RhaiResultOf<S>> + SendSync + 'static,
+        F: RegisterNativeFunction<(Mut<A>, B), 2, C, T, true> + SendSync + 'static,
     {
         #[cfg(not(feature = "no_index"))]
         if TypeId::of::<A>() == TypeId::of::<crate::Array>() {
@@ -1465,12 +1470,12 @@ impl Module {
     /// ```
     #[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
     #[inline]
-    pub fn set_indexer_set_fn<A, B, T, F, S>(&mut self, func: F) -> u64
+    pub fn set_indexer_set_fn<A, B, const C: bool, T, F>(&mut self, func: F) -> u64
     where
         A: Variant + Clone,
         B: Variant + Clone,
         T: Variant + Clone,
-        F: RegisterNativeFunction<(Mut<A>, B, T), 3, (), RhaiResultOf<S>> + SendSync + 'static,
+        F: RegisterNativeFunction<(Mut<A>, B, T), 3, C, (), true> + SendSync + 'static,
     {
         #[cfg(not(feature = "no_index"))]
         if TypeId::of::<A>() == TypeId::of::<crate::Array>() {
@@ -1537,15 +1542,13 @@ impl Module {
     pub fn set_indexer_get_set_fn<
         A: Variant + Clone,
         B: Variant + Clone,
+        const C1: bool,
+        const C2: bool,
         T: Variant + Clone,
-        S1,
-        S2,
     >(
         &mut self,
-        get_fn: impl RegisterNativeFunction<(Mut<A>, B), 2, T, RhaiResultOf<S1>> + SendSync + 'static,
-        set_fn: impl RegisterNativeFunction<(Mut<A>, B, T), 3, (), RhaiResultOf<S2>>
-            + SendSync
-            + 'static,
+        get_fn: impl RegisterNativeFunction<(Mut<A>, B), 2, C1, T, true> + SendSync + 'static,
+        set_fn: impl RegisterNativeFunction<(Mut<A>, B, T), 3, C2, (), true> + SendSync + 'static,
     ) -> (u64, u64) {
         (
             self.set_indexer_get_fn(get_fn),