diff --git a/Cargo.toml b/Cargo.toml
index 5bae3961..c1ea251a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -22,6 +22,7 @@ num-traits = { version = "0.2.11", default-features = false }
 [features]
 #default = ["unchecked", "sync", "no_optimize", "no_float", "only_i32", "no_index", "no_object", "no_function", "no_module"]
 default = []
+plugins = []
 unchecked = []      # unchecked arithmetic
 sync = []           # restrict to only types that implement Send + Sync
 no_optimize = []    # no script optimizer
diff --git a/README.md b/README.md
index 78fbae94..401b49fc 100644
--- a/README.md
+++ b/README.md
@@ -1499,6 +1499,11 @@ record == "Bob X. Davis: age 42 ❤\n";
 "Davis" in record == true;
 'X' in record == true;
 'C' in record == false;
+
+// Strings can be iterated with a 'for' statement, yielding characters
+for ch in record {
+    print(ch);
+}
 ```
 
 The maximum allowed length of a string can be controlled via `Engine::set_max_string_size`
@@ -2011,9 +2016,18 @@ loop {
 Iterating through a range or an [array] is provided by the `for` ... `in` loop.
 
 ```rust
-let array = [1, 3, 5, 7, 9, 42];
+// Iterate through string, yielding characters
+let s = "hello, world!";
+
+for ch in s {
+    if ch > 'z' { continue; } // skip to the next iteration
+    print(ch);
+    if x == '@' { break; }   // break out of for loop
+}
 
 // Iterate through array
+let array = [1, 3, 5, 7, 9, 42];
+
 for x in array {
     if x > 10 { continue; } // skip to the next iteration
     print(x);
diff --git a/examples/repl.rs b/examples/repl.rs
index 4a3bd561..f18a56e5 100644
--- a/examples/repl.rs
+++ b/examples/repl.rs
@@ -125,7 +125,7 @@ fn main() {
 
         match engine
             .compile_with_scope(&scope, &script)
-            .map_err(|err| err.into())
+            .map_err(Into::into)
             .and_then(|r| {
                 ast_u = r.clone();
 
diff --git a/src/engine.rs b/src/engine.rs
index 8ca14356..c94d1423 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -230,6 +230,9 @@ pub fn get_script_function_by_signature<'a>(
 ///
 /// Currently, `Engine` is neither `Send` nor `Sync`. Use the `sync` feature to make it `Send + Sync`.
 pub struct Engine {
+    /// A unique ID identifying this scripting `Engine`.
+    pub id: Option<String>,
+
     /// A module containing all functions directly loaded into the Engine.
     pub(crate) global_module: Module,
     /// A collection of all library packages loaded into the Engine.
@@ -274,6 +277,8 @@ impl Default for Engine {
     fn default() -> Self {
         // Create the new scripting Engine
         let mut engine = Self {
+            id: None,
+
             packages: Default::default(),
             global_module: Default::default(),
 
@@ -431,6 +436,8 @@ impl Engine {
     /// Use the `load_package` method to load additional packages of functions.
     pub fn new_raw() -> Self {
         Self {
+            id: None,
+
             packages: Default::default(),
             global_module: Default::default(),
             module_resolver: None,
@@ -483,6 +490,15 @@ impl Engine {
         self.optimization_level = optimization_level
     }
 
+    /// The current optimization level.
+    /// It controls whether and how the `Engine` will optimize an AST after compilation.
+    ///
+    /// Not available under the `no_optimize` feature.
+    #[cfg(not(feature = "no_optimize"))]
+    pub fn optimization_level(&self) -> OptimizationLevel {
+        self.optimization_level
+    }
+
     /// Set the maximum levels of function calls allowed for a script in order to avoid
     /// infinite recursion and stack overflows.
     #[cfg(not(feature = "unchecked"))]
@@ -490,11 +506,27 @@ impl Engine {
         self.max_call_stack_depth = levels
     }
 
+    /// The maximum levels of function calls allowed for a script.
+    #[cfg(not(feature = "unchecked"))]
+    pub fn max_call_levels(&self) -> usize {
+        self.max_call_stack_depth
+    }
+
     /// Set the maximum number of operations allowed for a script to run to avoid
     /// consuming too much resources (0 for unlimited).
     #[cfg(not(feature = "unchecked"))]
     pub fn set_max_operations(&mut self, operations: u64) {
-        self.max_operations = operations;
+        self.max_operations = if operations == u64::MAX {
+            0
+        } else {
+            operations
+        };
+    }
+
+    /// The maximum number of operations allowed for a script to run (0 for unlimited).
+    #[cfg(not(feature = "unchecked"))]
+    pub fn max_operations(&self) -> u64 {
+        self.max_operations
     }
 
     /// Set the maximum number of imported modules allowed for a script.
@@ -503,31 +535,77 @@ impl Engine {
         self.max_modules = modules;
     }
 
-    /// Set the depth limits for expressions/statements (0 for unlimited).
+    /// The maximum number of imported modules allowed for a script.
+    #[cfg(not(feature = "unchecked"))]
+    pub fn max_modules(&self) -> usize {
+        self.max_modules
+    }
+
+    /// Set the depth limits for expressions (0 for unlimited).
     #[cfg(not(feature = "unchecked"))]
     pub fn set_max_expr_depths(&mut self, max_expr_depth: usize, max_function_expr_depth: usize) {
-        self.max_expr_depth = max_expr_depth;
-        self.max_function_expr_depth = max_function_expr_depth;
+        self.max_expr_depth = if max_expr_depth == usize::MAX {
+            0
+        } else {
+            max_expr_depth
+        };
+        self.max_function_expr_depth = if max_function_expr_depth == usize::MAX {
+            0
+        } else {
+            max_function_expr_depth
+        };
+    }
+
+    /// The depth limit for expressions (0 for unlimited).
+    #[cfg(not(feature = "unchecked"))]
+    pub fn max_expr_depth(&self) -> usize {
+        self.max_expr_depth
+    }
+
+    /// The depth limit for expressions in functions (0 for unlimited).
+    #[cfg(not(feature = "unchecked"))]
+    pub fn max_function_expr_depth(&self) -> usize {
+        self.max_function_expr_depth
     }
 
     /// Set the maximum length of strings (0 for unlimited).
     #[cfg(not(feature = "unchecked"))]
     pub fn set_max_string_size(&mut self, max_size: usize) {
-        self.max_string_size = max_size;
+        self.max_string_size = if max_size == usize::MAX { 0 } else { max_size };
+    }
+
+    /// The maximum length of strings (0 for unlimited).
+    #[cfg(not(feature = "unchecked"))]
+    pub fn max_string_size(&self) -> usize {
+        self.max_string_size
     }
 
     /// Set the maximum length of arrays (0 for unlimited).
     #[cfg(not(feature = "unchecked"))]
     #[cfg(not(feature = "no_index"))]
     pub fn set_max_array_size(&mut self, max_size: usize) {
-        self.max_array_size = max_size;
+        self.max_array_size = if max_size == usize::MAX { 0 } else { max_size };
+    }
+
+    /// The maximum length of arrays (0 for unlimited).
+    #[cfg(not(feature = "unchecked"))]
+    #[cfg(not(feature = "no_index"))]
+    pub fn max_array_size(&self) -> usize {
+        self.max_array_size
     }
 
     /// Set the maximum length of object maps (0 for unlimited).
     #[cfg(not(feature = "unchecked"))]
     #[cfg(not(feature = "no_object"))]
     pub fn set_max_map_size(&mut self, max_size: usize) {
-        self.max_map_size = max_size;
+        self.max_map_size = if max_size == usize::MAX { 0 } else { max_size };
+    }
+
+    /// The maximum length of object maps (0 for unlimited).
+    #[cfg(not(feature = "unchecked"))]
+    #[cfg(not(feature = "no_object"))]
+    pub fn max_map_size(&self) -> usize {
+        self.max_map_size
     }
 
     /// Set the module resolution service used by the `Engine`.
@@ -1705,6 +1783,7 @@ impl Engine {
                         self.call_script_fn(&mut scope, state, lib, name, fn_def, args, level)
                             .map_err(|err| EvalAltResult::new_position(err, *pos))
                     }
+                    Ok(f) if f.is_plugin_fn() => f.get_plugin_fn().call(args.as_mut(), *pos),
                     Ok(f) => {
                         f.get_native_fn()(self, args.as_mut()).map_err(|err| err.new_position(*pos))
                     }
@@ -2186,14 +2265,14 @@ fn run_builtin_binary_op(
 
         #[cfg(not(feature = "unchecked"))]
         match op {
-            "+" => return add(x, y).map(Into::<Dynamic>::into).map(Some),
-            "-" => return sub(x, y).map(Into::<Dynamic>::into).map(Some),
-            "*" => return mul(x, y).map(Into::<Dynamic>::into).map(Some),
-            "/" => return div(x, y).map(Into::<Dynamic>::into).map(Some),
-            "%" => return modulo(x, y).map(Into::<Dynamic>::into).map(Some),
-            "~" => return pow_i_i(x, y).map(Into::<Dynamic>::into).map(Some),
-            ">>" => return shr(x, y).map(Into::<Dynamic>::into).map(Some),
-            "<<" => return shl(x, y).map(Into::<Dynamic>::into).map(Some),
+            "+" => return add(x, y).map(Into::into).map(Some),
+            "-" => return sub(x, y).map(Into::into).map(Some),
+            "*" => return mul(x, y).map(Into::into).map(Some),
+            "/" => return div(x, y).map(Into::into).map(Some),
+            "%" => return modulo(x, y).map(Into::into).map(Some),
+            "~" => return pow_i_i(x, y).map(Into::into).map(Some),
+            ">>" => return shr(x, y).map(Into::into).map(Some),
+            "<<" => return shl(x, y).map(Into::into).map(Some),
             _ => (),
         }
 
@@ -2204,9 +2283,9 @@ fn run_builtin_binary_op(
             "*" => return Ok(Some((x * y).into())),
             "/" => return Ok(Some((x / y).into())),
             "%" => return Ok(Some((x % y).into())),
-            "~" => return pow_i_i_u(x, y).map(Into::<Dynamic>::into).map(Some),
-            ">>" => return shr_u(x, y).map(Into::<Dynamic>::into).map(Some),
-            "<<" => return shl_u(x, y).map(Into::<Dynamic>::into).map(Some),
+            "~" => return pow_i_i_u(x, y).map(Into::into).map(Some),
+            ">>" => return shr_u(x, y).map(Into::into).map(Some),
+            "<<" => return shl_u(x, y).map(Into::into).map(Some),
             _ => (),
         }
 
@@ -2280,7 +2359,7 @@ fn run_builtin_binary_op(
                 "*" => return Ok(Some((x * y).into())),
                 "/" => return Ok(Some((x / y).into())),
                 "%" => return Ok(Some((x % y).into())),
-                "~" => return pow_f_f(x, y).map(Into::<Dynamic>::into).map(Some),
+                "~" => return pow_f_f(x, y).map(Into::into).map(Some),
                 "==" => return Ok(Some((x == y).into())),
                 "!=" => return Ok(Some((x != y).into())),
                 ">" => return Ok(Some((x > y).into())),
diff --git a/src/fn_native.rs b/src/fn_native.rs
index b7805ff5..fc6e7673 100644
--- a/src/fn_native.rs
+++ b/src/fn_native.rs
@@ -1,6 +1,7 @@
 use crate::any::Dynamic;
 use crate::engine::Engine;
 use crate::parser::ScriptFnDef;
+use crate::plugin::PluginFunction;
 use crate::result::EvalAltResult;
 
 use crate::stdlib::{boxed::Box, fmt, rc::Rc, sync::Arc};
@@ -59,6 +60,11 @@ pub type FnAny =
 
 pub type IteratorFn = fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>>;
 
+#[cfg(feature = "sync")]
+pub type SharedPluginFunction = Arc<dyn PluginFunction + Send + Sync>;
+#[cfg(not(feature = "sync"))]
+pub type SharedPluginFunction = Rc<dyn PluginFunction>;
+
 #[cfg(not(feature = "sync"))]
 pub type Callback<T, R> = Box<dyn Fn(&T) -> R + 'static>;
 #[cfg(feature = "sync")]
@@ -74,6 +80,8 @@ pub enum CallableFunction {
     Method(Shared<FnAny>),
     /// An iterator function.
     Iterator(IteratorFn),
+    /// A plugin-defined function,
+    Plugin(SharedPluginFunction),
     /// A script-defined function.
     Script(Shared<ScriptFnDef>),
 }
@@ -84,6 +92,7 @@ impl fmt::Debug for CallableFunction {
             Self::Pure(_) => write!(f, "NativePureFunction"),
             Self::Method(_) => write!(f, "NativeMethod"),
             Self::Iterator(_) => write!(f, "NativeIterator"),
+            Self::Plugin(_) => write!(f, "PluginFunction"),
             Self::Script(fn_def) => fmt::Debug::fmt(fn_def, f),
         }
     }
@@ -95,6 +104,7 @@ impl CallableFunction {
         match self {
             Self::Pure(_) => true,
             Self::Method(_) | Self::Iterator(_) | Self::Script(_) => false,
+            Self::Plugin(_) => false,
         }
     }
     /// Is this a pure native Rust method-call?
@@ -102,6 +112,7 @@ impl CallableFunction {
         match self {
             Self::Method(_) => true,
             Self::Pure(_) | Self::Iterator(_) | Self::Script(_) => false,
+            Self::Plugin(_) => false,
         }
     }
     /// Is this an iterator function?
@@ -109,6 +120,7 @@ impl CallableFunction {
         match self {
             Self::Iterator(_) => true,
             Self::Pure(_) | Self::Method(_) | Self::Script(_) => false,
+            Self::Plugin(_) => false,
         }
     }
     /// Is this a Rhai-scripted function?
@@ -116,6 +128,14 @@ impl CallableFunction {
         match self {
             Self::Script(_) => true,
             Self::Pure(_) | Self::Method(_) | Self::Iterator(_) => false,
+            Self::Plugin(_) => false,
+        }
+    }
+    /// Is this a plugin-defined function?
+    pub fn is_plugin_fn(&self) -> bool {
+        match self {
+            Self::Plugin(_) => true,
+            Self::Pure(_) | Self::Method(_) | Self::Iterator(_) | Self::Script(_) => false,
         }
     }
     /// Get a reference to a native Rust function.
@@ -127,6 +147,7 @@ impl CallableFunction {
         match self {
             Self::Pure(f) | Self::Method(f) => f.as_ref(),
             Self::Iterator(_) | Self::Script(_) => panic!(),
+            Self::Plugin(_) => panic!(),
         }
     }
     /// Get a shared reference to a script-defined function definition.
@@ -136,7 +157,7 @@ impl CallableFunction {
     /// Panics if the `CallableFunction` is not `Script`.
     pub fn get_shared_fn_def(&self) -> Shared<ScriptFnDef> {
         match self {
-            Self::Pure(_) | Self::Method(_) | Self::Iterator(_) => panic!(),
+            Self::Pure(_) | Self::Method(_) | Self::Plugin(_) | Self::Iterator(_) => panic!(),
             Self::Script(f) => f.clone(),
         }
     }
@@ -149,6 +170,7 @@ impl CallableFunction {
         match self {
             Self::Pure(_) | Self::Method(_) | Self::Iterator(_) => panic!(),
             Self::Script(f) => f,
+            Self::Plugin(_) => panic!(),
         }
     }
     /// Get a reference to an iterator function.
@@ -160,6 +182,18 @@ impl CallableFunction {
         match self {
             Self::Iterator(f) => *f,
             Self::Pure(_) | Self::Method(_) | Self::Script(_) => panic!(),
+            Self::Plugin(_) => panic!(),
+        }
+    }
+    /// Get a reference to a plugin function.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the `CallableFunction` is not `Plugin`.
+    pub fn get_plugin_fn<'s>(&'s self) -> SharedPluginFunction {
+        match self {
+            Self::Plugin(f) => f.clone(),
+            Self::Pure(_) | Self::Method(_) | Self::Script(_) | Self::Iterator(_) => panic!(),
         }
     }
     /// Create a new `CallableFunction::Pure`.
@@ -170,6 +204,18 @@ impl CallableFunction {
     pub fn from_method(func: Box<FnAny>) -> Self {
         Self::Method(func.into())
     }
+
+    #[cfg(feature = "sync")]
+    /// Create a new `CallableFunction::Plugin`.
+    pub fn from_plugin(plugin: impl PluginFunction + 'static + Send + Sync) -> Self {
+        Self::Plugin(Arc::new(plugin))
+    }
+
+    #[cfg(not(feature = "sync"))]
+    /// Create a new `CallableFunction::Plugin`.
+    pub fn from_plugin(plugin: impl PluginFunction + 'static) -> Self {
+        Self::Plugin(Rc::new(plugin))
+    }
 }
 
 impl From<IteratorFn> for CallableFunction {
diff --git a/src/fn_register.rs b/src/fn_register.rs
index 1b07cbe8..cf83ae14 100644
--- a/src/fn_register.rs
+++ b/src/fn_register.rs
@@ -5,11 +5,91 @@ use crate::any::{Dynamic, Variant};
 use crate::engine::Engine;
 use crate::fn_native::{CallableFunction, FnAny, FnCallArgs, SendSync};
 use crate::parser::FnAccess;
+use crate::plugin::Plugin;
 use crate::result::EvalAltResult;
 use crate::utils::ImmutableString;
 
 use crate::stdlib::{any::TypeId, boxed::Box, mem};
 
+/// A trait to register custom plugins with the `Engine`.
+///
+/// A plugin consists of a number of functions. All functions will be registered with the engine.
+pub trait RegisterPlugin<PL: crate::plugin::Plugin> {
+    /// Allow extensions of the engine's behavior.
+    ///
+    /// This can include importing modules, registering functions to the global name space, and
+    /// more.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use rhai::{FLOAT, INT, Module, ModuleResolver, RegisterFn, RegisterPlugin};
+    /// use rhai::plugin::*;
+    /// use rhai::module_resolvers::*;
+    ///
+    /// // A function we want to expose to Rhai.
+    /// #[derive(Copy, Clone)]
+    /// struct DistanceFunction();
+    ///
+    /// impl PluginFunction for DistanceFunction {
+    ///     fn is_method_call(&self) -> bool { false }
+    ///     fn is_varadic(&self) -> bool { false }
+    ///
+    ///     fn call(&self, args: &[&mut Dynamic], pos: Position) -> Result<Dynamic, Box<EvalAltResult>> {
+    ///         let x1: &FLOAT = args[0].downcast_ref::<FLOAT>().unwrap();
+    ///         let y1: &FLOAT = args[1].downcast_ref::<FLOAT>().unwrap();
+    ///         let x2: &FLOAT = args[2].downcast_ref::<FLOAT>().unwrap();
+    ///         let y2: &FLOAT = args[3].downcast_ref::<FLOAT>().unwrap();
+    ///         let square_sum = (y2 - y1).abs().powf(2.0) + (x2 -x1).abs().powf(2.0);
+    ///         Ok(Dynamic::from(square_sum.sqrt()))
+    ///     }
+    ///
+    ///     fn clone_boxed(&self) -> Box<dyn PluginFunction> {
+    ///         Box::new(DistanceFunction())
+    ///     }
+    /// }
+    ///
+    /// // A simple custom plugin. This should not usually be done with hand-written code.
+    /// #[derive(Copy, Clone)]
+    /// pub struct AdvancedMathPlugin();
+    ///
+    /// impl Plugin for AdvancedMathPlugin {
+    ///     fn register_contents(self, engine: &mut Engine) {
+    ///         // Plugins are allowed to have side-effects on the engine.
+    ///         engine.register_fn("get_mystic_number", || { 42 as FLOAT });
+    ///
+    ///         // Main purpose: create a module to expose the functions to Rhai.
+    ///         //
+    ///         // This is currently a hack. There needs to be a better API here for "plugin"
+    ///         // modules.
+    ///         let mut m = Module::new();
+    ///         m.set_fn("euclidean_distance".to_string(), FnAccess::Public,
+    ///                  &[std::any::TypeId::of::<FLOAT>(),
+    ///                    std::any::TypeId::of::<FLOAT>(),
+    ///                    std::any::TypeId::of::<FLOAT>(),
+    ///                    std::any::TypeId::of::<FLOAT>()],
+    ///                  CallableFunction::from_plugin(DistanceFunction()));
+    ///         let mut r = StaticModuleResolver::new();
+    ///         r.insert("Math::Advanced".to_string(), m);
+    ///         engine.set_module_resolver(Some(r));
+    ///     }
+    /// }
+    ///
+    ///
+    /// # fn main() -> Result<(), Box<rhai::EvalAltResult>> {
+    ///
+    /// let mut engine = Engine::new();
+    /// engine.register_plugin(AdvancedMathPlugin());
+    ///
+    /// assert_eq!(engine.eval::<FLOAT>(
+    ///     r#"import "Math::Advanced" as math;
+    ///        let x = math::euclidean_distance(0.0, 1.0, 0.0, get_mystic_number()); x"#)?, 41.0);
+    /// # Ok(())
+    /// # }
+    /// ```
+    fn register_plugin(&mut self, plugin: PL);
+}
+
 /// Trait to register custom functions with the `Engine`.
 pub trait RegisterFn<FN, ARGS, RET> {
     /// Register a custom function with the `Engine`.
@@ -111,6 +191,12 @@ pub fn by_value<T: Variant + Clone>(data: &mut Dynamic) -> T {
     }
 }
 
+impl<PL: Plugin> RegisterPlugin<PL> for Engine {
+    fn register_plugin(&mut self, plugin: PL) {
+        plugin.register_contents(self);
+    }
+}
+
 /// This macro creates a closure wrapping a registered function.
 macro_rules! make_func {
 	($fn:ident : $map:expr ; $($par:ident => $convert:expr),*) => {
diff --git a/src/lib.rs b/src/lib.rs
index 106e83d4..e0f2245c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -75,10 +75,14 @@ mod engine;
 mod error;
 mod fn_call;
 mod fn_func;
-mod fn_native;
+pub mod fn_native;
 mod fn_register;
 mod module;
 mod optimize;
+#[cfg(not(feature = "no_module"))]
+pub mod plugin;
+#[cfg(feature = "no_module")]
+mod plugin;
 pub mod packages;
 mod parser;
 mod result;
@@ -92,6 +96,7 @@ pub use any::Dynamic;
 pub use engine::Engine;
 pub use error::{ParseError, ParseErrorType};
 pub use fn_register::{RegisterFn, RegisterResultFn};
+pub use fn_register::RegisterPlugin;
 pub use module::Module;
 pub use parser::{ImmutableString, AST, INT};
 pub use result::EvalAltResult;
diff --git a/src/packages/string_more.rs b/src/packages/string_more.rs
index 804af8ef..8f8d1731 100644
--- a/src/packages/string_more.rs
+++ b/src/packages/string_more.rs
@@ -15,6 +15,7 @@ use crate::stdlib::{
     fmt::Display,
     format,
     string::{String, ToString},
+    vec::Vec,
 };
 
 fn prepend<T: Display>(x: T, y: ImmutableString) -> FuncReturn<ImmutableString> {
@@ -293,4 +294,12 @@ def_package!(crate:MoreStringPackage:"Additional string utilities, including str
             Ok(())
         },
     );
+
+    // Register string iterator
+    lib.set_iter(
+        TypeId::of::<ImmutableString>(),
+        |arr| Box::new(
+            arr.cast::<ImmutableString>().chars().collect::<Vec<_>>().into_iter().map(Into::into)
+        ) as Box<dyn Iterator<Item = Dynamic>>,
+    );
 });
diff --git a/src/plugin.rs b/src/plugin.rs
new file mode 100644
index 00000000..8159d3c8
--- /dev/null
+++ b/src/plugin.rs
@@ -0,0 +1,42 @@
+//! Module defining plugins in Rhai. Is exported for use by plugin authors.
+
+pub use crate::Engine;
+pub use crate::any::{Dynamic, Variant};
+pub use crate::fn_native::{CallableFunction, FnCallArgs, IteratorFn};
+pub use crate::parser::{
+    FnAccess,
+    FnAccess::{Private, Public},
+    AST,
+};
+pub use crate::result::EvalAltResult;
+pub use crate::scope::{Entry as ScopeEntry, EntryType as ScopeEntryType, Scope};
+pub use crate::token::{Position, Token};
+pub use crate::utils::StaticVec;
+
+#[cfg(features = "sync")]
+/// Represents an externally-written plugin for the Rhai interpreter.
+///
+/// This trait should not be used directly. Use the `#[plugin]` procedural attribute instead.
+pub trait Plugin: Send {
+    fn register_contents(self, engine: &mut Engine);
+}
+
+#[cfg(not(features = "sync"))]
+/// Represents an externally-written plugin for the Rhai interpreter.
+///
+/// This trait should not be used directly. Use the `#[plugin]` procedural attribute instead.
+pub trait Plugin: Send + Sync {
+    fn register_contents(self, engine: &mut Engine);
+}
+
+/// Represents a function that is statically defined within a plugin.
+///
+/// This trait should not be used directly. Use the `#[plugin]` procedural attribute instead.
+pub trait PluginFunction {
+    fn is_method_call(&self) -> bool;
+    fn is_varadic(&self) -> bool;
+
+    fn call(&self, args: &[&mut Dynamic], pos: Position) -> Result<Dynamic, Box<EvalAltResult>>;
+
+    fn clone_boxed(&self) -> Box<dyn PluginFunction>;
+}
diff --git a/tests/for.rs b/tests/for.rs
index cddff6ed..4f99c06f 100644
--- a/tests/for.rs
+++ b/tests/for.rs
@@ -30,6 +30,26 @@ fn test_for_array() -> Result<(), Box<EvalAltResult>> {
     Ok(())
 }
 
+#[test]
+fn test_for_string() -> Result<(), Box<EvalAltResult>> {
+    let engine = Engine::new();
+
+    let script = r#"
+        let s = "hello";
+        let sum = 0;
+
+        for ch in s {
+            sum += ch.to_int();
+        }
+
+        sum
+    "#;
+
+    assert_eq!(engine.eval::<INT>(script)?, 532);
+
+    Ok(())
+}
+
 #[cfg(not(feature = "no_object"))]
 #[cfg(not(feature = "no_index"))]
 #[test]