diff --git a/src/bin/rhai-repl.rs b/src/bin/rhai-repl.rs
index fedd1dc0..8a03d85b 100644
--- a/src/bin/rhai-repl.rs
+++ b/src/bin/rhai-repl.rs
@@ -218,7 +218,7 @@ fn main() {
             //     println!(
             //         "{}",
             //         engine
-            //             .gen_fn_metadata_to_json(Some(&main_ast), false)
+            //             .gen_fn_metadata_with_ast_to_json(&main_ast, true)
             //             .unwrap()
             //     );
             //     continue;
diff --git a/src/engine_api.rs b/src/engine_api.rs
index eb6b97f5..3ccf26bc 100644
--- a/src/engine_api.rs
+++ b/src/engine_api.rs
@@ -14,7 +14,7 @@ use crate::stdlib::{
 };
 use crate::{
     scope::Scope, Dynamic, Engine, EvalAltResult, FnAccess, FnNamespace, Module, NativeCallContext,
-    ParseError, Position, RhaiResult, Shared, AST,
+    ParseError, Position, RhaiResult, Shared, StaticVec, AST,
 };
 
 #[cfg(not(feature = "no_index"))]
@@ -51,16 +51,28 @@ impl Engine {
     /// # Ok(())
     /// # }
     /// ```
+    #[inline]
     pub fn register_fn<A, F>(&mut self, name: &str, func: F) -> &mut Self
     where
         F: RegisterNativeFunction<A, ()>,
     {
+        let param_types = F::param_types();
+        let mut param_type_names: StaticVec<_> = F::param_names()
+            .iter()
+            .map(|ty| format!("_: {}", self.map_type_name(ty)))
+            .collect();
+        if F::return_type() != TypeId::of::<()>() {
+            param_type_names.push(self.map_type_name(F::return_type_name()).to_string());
+        }
+        let param_type_names: StaticVec<_> =
+            param_type_names.iter().map(|ty| ty.as_str()).collect();
+
         self.global_namespace.set_fn(
             name,
             FnNamespace::Global,
             FnAccess::Public,
-            None,
-            &F::param_types(),
+            Some(&param_type_names),
+            &param_types,
             func.into_callable_function(),
         );
         self
@@ -89,16 +101,26 @@ impl Engine {
     /// engine.eval::<i64>("div(42, 0)")
     ///       .expect_err("expecting division by zero error!");
     /// ```
+    #[inline]
     pub fn register_result_fn<A, F>(&mut self, name: &str, func: F) -> &mut Self
     where
         F: RegisterNativeFunction<A, RhaiResult>,
     {
+        let param_types = F::param_types();
+        let mut param_type_names: StaticVec<_> = F::param_names()
+            .iter()
+            .map(|ty| format!("_: {}", self.map_type_name(ty)))
+            .collect();
+        param_type_names.push(self.map_type_name(F::return_type_name()).to_string());
+        let param_type_names: StaticVec<&str> =
+            param_type_names.iter().map(|ty| ty.as_str()).collect();
+
         self.global_namespace.set_fn(
             name,
             FnNamespace::Global,
             FnAccess::Public,
-            None,
-            &F::param_types(),
+            Some(&param_type_names),
+            &param_types,
             func.into_callable_function(),
         );
         self
diff --git a/src/fn_register.rs b/src/fn_register.rs
index f53157a0..8a773424 100644
--- a/src/fn_register.rs
+++ b/src/fn_register.rs
@@ -5,7 +5,13 @@
 use crate::dynamic::{DynamicWriteLock, Variant};
 use crate::fn_native::{CallableFunction, FnAny, FnCallArgs, SendSync};
 use crate::r#unsafe::unsafe_try_cast;
-use crate::stdlib::{any::TypeId, boxed::Box, mem, string::String, vec};
+use crate::stdlib::{
+    any::{type_name, TypeId},
+    boxed::Box,
+    mem,
+    string::String,
+    vec,
+};
 use crate::{Dynamic, EvalAltResult, NativeCallContext};
 
 // These types are used to build a unique _marker_ tuple type for each combination
@@ -55,8 +61,14 @@ pub fn by_value<T: Variant + Clone>(data: &mut Dynamic) -> T {
 
 /// Trait to register custom Rust functions.
 pub trait RegisterNativeFunction<Args, Result> {
-    /// Get the types of this function's parameters.
+    /// Get the type ID's of this function's parameters.
     fn param_types() -> Box<[TypeId]>;
+    /// Get the type names of this function's parameters.
+    fn param_names() -> Box<[&'static str]>;
+    /// Get the type ID of this function's return value.
+    fn return_type() -> TypeId;
+    /// Get the type name of this function's return value.
+    fn return_type_name() -> &'static str;
     /// Convert this function into a [`CallableFunction`].
     fn into_callable_function(self) -> CallableFunction;
 }
@@ -78,8 +90,10 @@ macro_rules! def_register {
             $($par: Variant + Clone,)*
             RET: Variant + Clone
         > RegisterNativeFunction<($($mark,)*), ()> for FN {
-            #[inline(always)]
-            fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn param_names() -> Box<[&'static str]> { vec![$(type_name::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn return_type() -> TypeId { TypeId::of::<RET>() }
+            #[inline(always)] fn return_type_name() -> &'static str { type_name::<RET>() }
             #[inline(always)]
             fn into_callable_function(self) -> CallableFunction {
                 CallableFunction::$abi(Box::new(move |_: NativeCallContext, args: &mut FnCallArgs| {
@@ -101,8 +115,10 @@ macro_rules! def_register {
             $($par: Variant + Clone,)*
             RET: Variant + Clone
         > RegisterNativeFunction<(NativeCallContext<'static>, $($mark,)*), ()> for FN {
-            #[inline(always)]
-            fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn param_names() -> Box<[&'static str]> { vec![$(type_name::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn return_type() -> TypeId { TypeId::of::<RET>() }
+            #[inline(always)] fn return_type_name() -> &'static str { type_name::<RET>() }
             #[inline(always)]
             fn into_callable_function(self) -> CallableFunction {
                 CallableFunction::$abi(Box::new(move |ctx: NativeCallContext, args: &mut FnCallArgs| {
@@ -124,8 +140,10 @@ macro_rules! def_register {
             $($par: Variant + Clone,)*
             RET: Variant + Clone
         > RegisterNativeFunction<($($mark,)*), Result<RET, Box<EvalAltResult>>> for FN {
-            #[inline(always)]
-            fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn param_names() -> Box<[&'static str]> { vec![$(type_name::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn return_type() -> TypeId { TypeId::of::<Result<RET, Box<EvalAltResult>>>() }
+            #[inline(always)] fn return_type_name() -> &'static str { type_name::<Result<RET, Box<EvalAltResult>>>() }
             #[inline(always)]
             fn into_callable_function(self) -> CallableFunction {
                 CallableFunction::$abi(Box::new(move |_: NativeCallContext, args: &mut FnCallArgs| {
@@ -144,8 +162,10 @@ macro_rules! def_register {
             $($par: Variant + Clone,)*
             RET: Variant + Clone
         > RegisterNativeFunction<(NativeCallContext<'static>, $($mark,)*), Result<RET, Box<EvalAltResult>>> for FN {
-            #[inline(always)]
-            fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn param_types() -> Box<[TypeId]> { vec![$(TypeId::of::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn param_names() -> Box<[&'static str]> { vec![$(type_name::<$par>()),*].into_boxed_slice() }
+            #[inline(always)] fn return_type() -> TypeId { TypeId::of::<Result<RET, Box<EvalAltResult>>>() }
+            #[inline(always)] fn return_type_name() -> &'static str { type_name::<Result<RET, Box<EvalAltResult>>>() }
             #[inline(always)]
             fn into_callable_function(self) -> CallableFunction {
                 CallableFunction::$abi(Box::new(move |ctx: NativeCallContext, args: &mut FnCallArgs| {