diff --git a/src/bin/rhai-repl.rs b/src/bin/rhai-repl.rs
index 43662ef3..a51dc96f 100644
--- a/src/bin/rhai-repl.rs
+++ b/src/bin/rhai-repl.rs
@@ -141,9 +141,12 @@ fn main() {
     engine.set_optimization_level(rhai::OptimizationLevel::None);
 
     // Set a file module resolver without caching
-    let mut resolver = FileModuleResolver::new();
-    resolver.enable_cache(false);
-    engine.set_module_resolver(resolver);
+    #[cfg(not(feature = "no_module"))]
+    {
+        let mut resolver = FileModuleResolver::new();
+        resolver.enable_cache(false);
+        engine.set_module_resolver(resolver);
+    }
 
     // Make Engine immutable
     let engine = engine;
diff --git a/src/engine.rs b/src/engine.rs
index 69a87931..fa8d8713 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -712,12 +712,12 @@ pub struct Engine {
     pub(crate) module_resolver: Box<dyn crate::ModuleResolver>,
 
     /// A map mapping type names to pretty-print names.
-    pub(crate) type_names: BTreeMap<String, String>,
+    pub(crate) type_names: BTreeMap<Identifier, Identifier>,
 
     /// A set of symbols to disable.
-    pub(crate) disabled_symbols: BTreeSet<String>,
+    pub(crate) disabled_symbols: BTreeSet<Identifier>,
     /// A map containing custom keywords and precedence to recognize.
-    pub(crate) custom_keywords: BTreeMap<String, Option<Precedence>>,
+    pub(crate) custom_keywords: BTreeMap<Identifier, Option<Precedence>>,
     /// Custom syntax.
     pub(crate) custom_syntax: BTreeMap<Identifier, CustomSyntax>,
     /// Callback closure for resolving variable access.
@@ -2653,7 +2653,7 @@ impl Engine {
     pub fn map_type_name<'a>(&'a self, name: &'a str) -> &'a str {
         self.type_names
             .get(name)
-            .map(String::as_str)
+            .map(|s| s.as_str())
             .unwrap_or_else(|| map_std_type_name(name))
     }
 
diff --git a/src/engine_settings.rs b/src/engine_settings.rs
index b666714e..017d19b0 100644
--- a/src/engine_settings.rs
+++ b/src/engine_settings.rs
@@ -1,9 +1,9 @@
 //! Configuration settings for [`Engine`].
 
-use crate::engine::Precedence;
 use crate::stdlib::{format, string::String};
 use crate::token::Token;
 use crate::Engine;
+use crate::{engine::Precedence, Identifier};
 
 #[cfg(not(feature = "unchecked"))]
 use crate::stdlib::num::{NonZeroU64, NonZeroUsize};
@@ -236,7 +236,7 @@ impl Engine {
     /// # }
     /// ```
     #[inline(always)]
-    pub fn disable_symbol(&mut self, symbol: impl Into<String>) -> &mut Self {
+    pub fn disable_symbol(&mut self, symbol: impl Into<Identifier>) -> &mut Self {
         self.disabled_symbols.insert(symbol.into());
         self
     }
@@ -270,7 +270,7 @@ impl Engine {
     /// ```
     pub fn register_custom_operator(
         &mut self,
-        keyword: impl AsRef<str> + Into<String>,
+        keyword: impl AsRef<str> + Into<Identifier>,
         precedence: u8,
     ) -> Result<&mut Self, String> {
         let precedence = Precedence::new(precedence);
diff --git a/src/module/resolvers/file.rs b/src/module/resolvers/file.rs
index 081a8692..2915eb08 100644
--- a/src/module/resolvers/file.rs
+++ b/src/module/resolvers/file.rs
@@ -3,9 +3,8 @@ use crate::stdlib::{
     collections::BTreeMap,
     io::Error as IoError,
     path::{Path, PathBuf},
-    string::String,
 };
-use crate::{Engine, EvalAltResult, Module, ModuleResolver, Position, Shared};
+use crate::{Engine, EvalAltResult, Identifier, Module, ModuleResolver, Position, Shared};
 
 pub const RHAI_SCRIPT_EXTENSION: &str = "rhai";
 
@@ -42,7 +41,7 @@ pub const RHAI_SCRIPT_EXTENSION: &str = "rhai";
 #[derive(Debug)]
 pub struct FileModuleResolver {
     base_path: Option<PathBuf>,
-    extension: String,
+    extension: Identifier,
     cache_enabled: bool,
 
     #[cfg(not(feature = "sync"))]
@@ -118,7 +117,7 @@ impl FileModuleResolver {
     /// engine.set_module_resolver(resolver);
     /// ```
     #[inline(always)]
-    pub fn new_with_extension(extension: impl Into<String>) -> Self {
+    pub fn new_with_extension(extension: impl Into<Identifier>) -> Self {
         Self {
             base_path: None,
             extension: extension.into(),
@@ -145,7 +144,7 @@ impl FileModuleResolver {
     #[inline(always)]
     pub fn new_with_path_and_extension(
         path: impl Into<PathBuf>,
-        extension: impl Into<String>,
+        extension: impl Into<Identifier>,
     ) -> Self {
         Self {
             base_path: Some(path.into()),
@@ -175,7 +174,7 @@ impl FileModuleResolver {
 
     /// Set the script file extension.
     #[inline(always)]
-    pub fn set_extension(&mut self, extension: impl Into<String>) -> &mut Self {
+    pub fn set_extension(&mut self, extension: impl Into<Identifier>) -> &mut Self {
         self.extension = extension.into();
         self
     }
@@ -256,7 +255,7 @@ impl FileModuleResolver {
             file_path = path.into();
         }
 
-        file_path.set_extension(&self.extension); // Force extension
+        file_path.set_extension(self.extension.as_str()); // Force extension
         file_path
     }
 }
diff --git a/src/module/resolvers/stat.rs b/src/module/resolvers/stat.rs
index e8a0c705..8a697e20 100644
--- a/src/module/resolvers/stat.rs
+++ b/src/module/resolvers/stat.rs
@@ -1,5 +1,5 @@
 use crate::stdlib::{boxed::Box, collections::BTreeMap, ops::AddAssign, string::String};
-use crate::{Engine, EvalAltResult, Module, ModuleResolver, Position, Shared};
+use crate::{Engine, EvalAltResult, Identifier, Module, ModuleResolver, Position, Shared};
 
 /// A static [module][Module] resolution service that serves [modules][Module] added into it.
 ///
@@ -19,7 +19,7 @@ use crate::{Engine, EvalAltResult, Module, ModuleResolver, Position, Shared};
 /// engine.set_module_resolver(resolver);
 /// ```
 #[derive(Debug, Clone, Default)]
-pub struct StaticModuleResolver(BTreeMap<String, Shared<Module>>);
+pub struct StaticModuleResolver(BTreeMap<Identifier, Shared<Module>>);
 
 impl StaticModuleResolver {
     /// Create a new [`StaticModuleResolver`].
@@ -44,7 +44,7 @@ impl StaticModuleResolver {
     }
     /// Add a [module][Module] keyed by its path.
     #[inline(always)]
-    pub fn insert(&mut self, path: impl Into<String>, mut module: Module) {
+    pub fn insert(&mut self, path: impl Into<Identifier>, mut module: Module) {
         module.build_index();
         self.0.insert(path.into(), module.into());
     }
@@ -70,13 +70,13 @@ impl StaticModuleResolver {
     }
     /// Get a mutable iterator of all the modules.
     #[inline(always)]
-    pub fn into_iter(self) -> impl Iterator<Item = (String, Shared<Module>)> {
+    pub fn into_iter(self) -> impl Iterator<Item = (Identifier, Shared<Module>)> {
         self.0.into_iter()
     }
     /// Get an iterator of all the [module][Module] paths.
     #[inline(always)]
     pub fn paths(&self) -> impl Iterator<Item = &str> {
-        self.0.keys().map(String::as_str)
+        self.0.keys().map(|s| s.as_str())
     }
     /// Get an iterator of all the [modules][Module].
     #[inline(always)]
diff --git a/src/packages/string_basic.rs b/src/packages/string_basic.rs
index 69f1e392..2cceda30 100644
--- a/src/packages/string_basic.rs
+++ b/src/packages/string_basic.rs
@@ -10,8 +10,8 @@ use crate::Array;
 #[cfg(not(feature = "no_object"))]
 use crate::Map;
 
-#[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
-const FUNC_TO_DEBUG: &'static str = "to_debug";
+pub const FUNC_TO_STRING: &'static str = "to_string";
+pub const FUNC_TO_DEBUG: &'static str = "to_debug";
 
 def_package!(crate:BasicStringPackage:"Basic string utilities, including printing.", lib, {
     combine_with_exported_module!(lib, "print_debug", print_debug_functions);
@@ -19,9 +19,8 @@ def_package!(crate:BasicStringPackage:"Basic string utilities, including printin
 
 // Register print and debug
 
-#[cfg(any(not(feature = "no_index"), not(feature = "no_object")))]
 #[inline(always)]
-fn print_with_func(
+pub fn print_with_func(
     fn_name: &str,
     ctx: &NativeCallContext,
     value: &mut Dynamic,
@@ -39,17 +38,25 @@ fn print_with_func(
 mod print_debug_functions {
     use crate::ImmutableString;
 
-    #[rhai_fn(name = "print", name = "to_string", pure)]
-    pub fn print_generic(item: &mut Dynamic) -> ImmutableString {
-        item.to_string().into()
+    #[rhai_fn(name = "print", pure)]
+    pub fn print_generic(ctx: NativeCallContext, item: &mut Dynamic) -> ImmutableString {
+        print_with_func(FUNC_TO_STRING, &ctx, item)
     }
-    #[rhai_fn(name = "debug", name = "to_debug", pure)]
-    pub fn debug_generic(item: &mut Dynamic) -> ImmutableString {
-        format!("{:?}", item).into()
+    #[rhai_fn(name = "to_string", pure)]
+    pub fn to_string_generic(ctx: NativeCallContext, item: &mut Dynamic) -> ImmutableString {
+        ctx.engine().map_type_name(&item.to_string()).into()
+    }
+    #[rhai_fn(name = "debug", pure)]
+    pub fn debug_generic(ctx: NativeCallContext, item: &mut Dynamic) -> ImmutableString {
+        print_with_func(FUNC_TO_DEBUG, &ctx, item)
+    }
+    #[rhai_fn(name = "to_debug", pure)]
+    pub fn to_debug_generic(ctx: NativeCallContext, item: &mut Dynamic) -> ImmutableString {
+        ctx.engine().map_type_name(&format!("{:?}", item)).into()
     }
     #[rhai_fn(name = "print", name = "debug")]
     pub fn print_empty_string() -> ImmutableString {
-        "".to_string().into()
+        Default::default()
     }
     #[rhai_fn(name = "print", name = "to_string")]
     pub fn print_string(s: ImmutableString) -> ImmutableString {
diff --git a/src/packages/string_more.rs b/src/packages/string_more.rs
index 3477c7c2..817a43bb 100644
--- a/src/packages/string_more.rs
+++ b/src/packages/string_more.rs
@@ -6,6 +6,8 @@ use crate::stdlib::{
 };
 use crate::{def_package, Dynamic, ImmutableString, StaticVec, INT};
 
+use super::string_basic::{print_with_func, FUNC_TO_STRING};
+
 def_package!(crate:MoreStringPackage:"Additional string utilities, including string building.", lib, {
     combine_with_exported_module!(lib, "string", string_functions);
 
@@ -21,22 +23,30 @@ mod string_functions {
     use crate::ImmutableString;
 
     #[rhai_fn(name = "+", name = "append")]
-    pub fn add_append(string: &str, item: Dynamic) -> ImmutableString {
-        format!("{}{}", string, item).into()
+    pub fn add_append(ctx: NativeCallContext, string: &str, mut item: Dynamic) -> ImmutableString {
+        let s = print_with_func(FUNC_TO_STRING, &ctx, &mut item);
+        format!("{}{}", string, s).into()
     }
     #[rhai_fn(name = "+", pure)]
-    pub fn add_prepend(item: &mut Dynamic, string: &str) -> ImmutableString {
-        format!("{}{}", item, string).into()
+    pub fn add_prepend(
+        ctx: NativeCallContext,
+        item: &mut Dynamic,
+        string: &str,
+    ) -> ImmutableString {
+        let s = print_with_func(FUNC_TO_STRING, &ctx, item);
+        format!("{}{}", s, string).into()
     }
 
     #[rhai_fn(name = "+")]
-    pub fn add_append_unit(string: ImmutableString, _x: ()) -> ImmutableString {
+    pub fn add_append_unit(string: ImmutableString, _item: ()) -> ImmutableString {
         string
     }
     #[rhai_fn(name = "+")]
-    pub fn add_prepend_unit(_x: (), string: ImmutableString) -> ImmutableString {
+    pub fn add_prepend_unit(_item: (), string: ImmutableString) -> ImmutableString {
         string
     }
+    #[rhai_fn(name = "+=")]
+    pub fn add_append_assign_unit(_string: &mut ImmutableString, _item: ()) {}
 
     #[rhai_fn(name = "len", get = "len")]
     pub fn len(string: &str) -> INT {
diff --git a/src/parser.rs b/src/parser.rs
index 0925cc38..8fb73c08 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -1621,7 +1621,7 @@ fn parse_binary_op(
             Token::Custom(c) => state
                 .engine
                 .custom_keywords
-                .get(c)
+                .get(c.as_str())
                 .cloned()
                 .ok_or_else(|| PERR::Reserved(c.clone()).into_err(*current_pos))?,
             Token::Reserved(c) if !is_valid_identifier(c.chars()) => {
@@ -1646,7 +1646,7 @@ fn parse_binary_op(
             Token::Custom(c) => state
                 .engine
                 .custom_keywords
-                .get(c)
+                .get(c.as_str())
                 .cloned()
                 .ok_or_else(|| PERR::Reserved(c.clone()).into_err(*next_pos))?,
             Token::Reserved(c) if !is_valid_identifier(c.chars()) => {
@@ -1753,7 +1753,7 @@ fn parse_binary_op(
                 if state
                     .engine
                     .custom_keywords
-                    .get(&s)
+                    .get(s.as_str())
                     .map_or(false, Option::is_some) =>
             {
                 let hash = calc_fn_hash(empty(), &s, 2);
diff --git a/src/token.rs b/src/token.rs
index 0aea77b5..20990833 100644
--- a/src/token.rs
+++ b/src/token.rs
@@ -1828,7 +1828,7 @@ impl<'a> Iterator for TokenIterator<'a> {
             None => return None,
             // Reserved keyword/symbol
             Some((Token::Reserved(s), pos)) => (match
-                (s.as_str(), self.engine.custom_keywords.contains_key(&s))
+                (s.as_str(), self.engine.custom_keywords.contains_key(s.as_str()))
             {
                 ("===", false) => Token::LexError(LERR::ImproperSymbol(s,
                     "'===' is not a valid operator. This is not JavaScript! Should it be '=='?".to_string(),
@@ -1869,7 +1869,7 @@ impl<'a> Iterator for TokenIterator<'a> {
                 (_, false) => Token::Reserved(s),
             }, pos),
             // Custom keyword
-            Some((Token::Identifier(s), pos)) if self.engine.custom_keywords.contains_key(&s) => {
+            Some((Token::Identifier(s), pos)) if self.engine.custom_keywords.contains_key(s.as_str()) => {
                 (Token::Custom(s), pos)
             }
             // Custom standard keyword/symbol - must be disabled
diff --git a/tests/string.rs b/tests/string.rs
index 2e2d74a7..236c5d12 100644
--- a/tests/string.rs
+++ b/tests/string.rs
@@ -222,6 +222,40 @@ fn test_string_substring() -> Result<(), Box<EvalAltResult>> {
     Ok(())
 }
 
+#[test]
+fn test_string_format() -> Result<(), Box<EvalAltResult>> {
+    #[derive(Debug, Clone)]
+    struct TestStruct {
+        field: i64,
+    }
+
+    let mut engine = Engine::new();
+
+    engine
+        .register_type_with_name::<TestStruct>("TestStruct")
+        .register_fn("new_ts", || TestStruct { field: 42 })
+        .register_fn("to_string", |ts: TestStruct| format!("TS={}", ts.field))
+        .register_fn("to_debug", |ts: TestStruct| {
+            format!("!!!TS={}!!!", ts.field)
+        });
+
+    assert_eq!(
+        engine.eval::<String>(r#"let x = new_ts(); "foo" + x"#)?,
+        "fooTS=42"
+    );
+    assert_eq!(
+        engine.eval::<String>(r#"let x = new_ts(); x + "foo""#)?,
+        "TS=42foo"
+    );
+    #[cfg(not(feature = "no_index"))]
+    assert_eq!(
+        engine.eval::<String>(r#"let x = [new_ts()]; "foo" + x"#)?,
+        "foo[!!!TS=42!!!]"
+    );
+
+    Ok(())
+}
+
 #[test]
 fn test_string_fn() -> Result<(), Box<EvalAltResult>> {
     let mut engine = Engine::new();