diff --git a/src/engine.rs b/src/engine.rs
index d394c31d..53cb3ddc 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -46,22 +46,22 @@ type IteratorFn = dyn Fn(&Dynamic) -> Box<dyn Iterator<Item = Dynamic>>;
 ///     }
 /// }
 /// ```
-pub struct Engine<'a> {
+pub struct Engine<'e> {
     /// A hashmap containing all compiled functions known to the engine
-    pub(crate) external_functions: HashMap<FnSpec<'a>, Arc<FnIntExt>>,
+    pub(crate) external_functions: HashMap<FnSpec<'e>, Arc<FnIntExt<'e>>>,
     /// A hashmap containing all script-defined functions
-    pub(crate) script_functions: HashMap<FnSpec<'a>, Arc<FnIntExt>>,
+    pub(crate) script_functions: HashMap<FnSpec<'e>, Arc<FnIntExt<'e>>>,
     /// A hashmap containing all iterators known to the engine
     pub(crate) type_iterators: HashMap<TypeId, Arc<IteratorFn>>,
     pub(crate) type_names: HashMap<String, String>,
 
-    pub(crate) on_print: Box<dyn FnMut(&str) + 'a>,
-    pub(crate) on_debug: Box<dyn FnMut(&str) + 'a>,
+    pub(crate) on_print: Box<dyn FnMut(&str) + 'e>,
+    pub(crate) on_debug: Box<dyn FnMut(&str) + 'e>,
 }
 
-pub enum FnIntExt {
+pub enum FnIntExt<'a> {
     Ext(Box<FnAny>),
-    Int(FnDef),
+    Int(FnDef<'a>),
 }
 
 pub type FnAny = dyn Fn(FnCallArgs, Position) -> Result<Dynamic, EvalAltResult>;
@@ -104,27 +104,27 @@ impl Engine<'_> {
 
         if let Some(f) = fn_def {
             match *f {
-                FnIntExt::Ext(ref f) => {
-                    let r = f(args, pos)?;
+                FnIntExt::Ext(ref func) => {
+                    let result = func(args, pos)?;
 
                     let callback = match spec.name.as_ref() {
                         KEYWORD_PRINT => self.on_print.as_mut(),
                         KEYWORD_DEBUG => self.on_debug.as_mut(),
-                        _ => return Ok(r),
+                        _ => return Ok(result),
                     };
 
-                    Ok(callback(
-                        &r.downcast::<String>()
-                            .map(|s| *s)
-                            .unwrap_or("error: not a string".into()),
-                    )
-                    .into_dynamic())
+                    let val = &result
+                        .downcast::<String>()
+                        .map(|s| *s)
+                        .unwrap_or("error: not a string".into());
+
+                    Ok(callback(val).into_dynamic())
                 }
-                FnIntExt::Int(ref f) => {
-                    if f.params.len() != args.len() {
+                FnIntExt::Int(ref func) => {
+                    if func.params.len() != args.len() {
                         return Err(EvalAltResult::ErrorFunctionArgsMismatch(
                             spec.name.into(),
-                            f.params.len(),
+                            func.params.len(),
                             args.len(),
                             pos,
                         ));
@@ -133,13 +133,13 @@ impl Engine<'_> {
                     let mut scope = Scope::new();
 
                     scope.extend(
-                        f.params
+                        func.params
                             .iter()
-                            .cloned()
+                            .map(|s| s.clone())
                             .zip(args.iter().map(|x| (*x).into_dynamic())),
                     );
 
-                    match self.eval_stmt(&mut scope, &*f.body) {
+                    match self.eval_stmt(&mut scope, &*func.body) {
                         Err(EvalAltResult::Return(x, _)) => Ok(x),
                         other => other,
                     }
@@ -319,12 +319,12 @@ impl Engine<'_> {
     }
 
     /// Evaluate an index expression
-    fn eval_index_expr(
+    fn eval_index_expr<'a>(
         &mut self,
         scope: &mut Scope,
-        lhs: &Expr,
+        lhs: &'a Expr,
         idx_expr: &Expr,
-    ) -> Result<(IndexSourceType, Option<(String, usize)>, usize, Dynamic), EvalAltResult> {
+    ) -> Result<(IndexSourceType, Option<(&'a str, usize)>, usize, Dynamic), EvalAltResult> {
         let idx = self.eval_index_value(scope, idx_expr)?;
 
         match lhs {
@@ -336,7 +336,7 @@ impl Engine<'_> {
                 lhs.position(),
             )
             .map(|(src_idx, (val, src_type))| {
-                (src_type, Some((id.clone(), src_idx)), idx as usize, val)
+                (src_type, Some((id.as_str(), src_idx)), idx as usize, val)
             }),
 
             // (expr)[idx_expr]
@@ -371,7 +371,8 @@ impl Engine<'_> {
             // array_id[idx] = val
             IndexSourceType::Array => {
                 let arr = scope.get_mut_by_type::<Array>(id, src_idx);
-                (arr[idx as usize] = val).into_dynamic()
+                arr[idx as usize] = val;
+                ().into_dynamic()
             }
 
             // string_id[idx] = val
@@ -381,7 +382,8 @@ impl Engine<'_> {
                 let ch = *val
                     .downcast::<char>()
                     .expect("char value expected to update an index position in a string");
-                Self::str_replace_char(s, idx as usize, ch).into_dynamic()
+                Self::str_replace_char(s, idx as usize, ch);
+                ().into_dynamic()
             }
 
             // All other variable types should be an error
@@ -419,7 +421,7 @@ impl Engine<'_> {
                 // of the above `clone`.
                 if let Some((id, src_idx)) = src {
                     Self::update_indexed_variable_in_scope(
-                        src_type, scope, &id, src_idx, idx, target,
+                        src_type, scope, id, src_idx, idx, target,
                     );
                 }
 
@@ -510,7 +512,7 @@ impl Engine<'_> {
 
                 if let Some((id, src_idx)) = src {
                     Self::update_indexed_variable_in_scope(
-                        src_type, scope, &id, src_idx, idx, target,
+                        src_type, scope, id, src_idx, idx, target,
                     );
                 }
 
@@ -525,10 +527,10 @@ impl Engine<'_> {
     /// Evaluate an expression
     fn eval_expr(&mut self, scope: &mut Scope, expr: &Expr) -> Result<Dynamic, EvalAltResult> {
         match expr {
-            Expr::IntegerConstant(i, _) => Ok((*i).into_dynamic()),
-            Expr::FloatConstant(i, _) => Ok((*i).into_dynamic()),
+            Expr::IntegerConstant(i, _) => Ok(i.into_dynamic()),
+            Expr::FloatConstant(f, _) => Ok(f.into_dynamic()),
             Expr::StringConstant(s, _) => Ok(s.into_dynamic()),
-            Expr::CharConstant(c, _) => Ok((*c).into_dynamic()),
+            Expr::CharConstant(c, _) => Ok(c.into_dynamic()),
             Expr::Identifier(id, pos) => {
                 Self::search_scope(scope, id, Ok, *pos).map(|(_, val)| val)
             }
diff --git a/src/error.rs b/src/error.rs
index 3096040e..52ca89fc 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -75,6 +75,8 @@ pub enum ParseErrorType {
     FnMissingName,
     /// A function definition is missing the parameters list. Wrapped value is the function name.
     FnMissingParams(String),
+    /// Assignment to an inappropriate LHS (left-hand-side) expression.
+    AssignmentToInvalidLHS,
 }
 
 /// Error when parsing a script.
@@ -114,6 +116,7 @@ impl Error for ParseError {
             ParseErrorType::FnMissingName => "Expecting name in function declaration",
             ParseErrorType::FnMissingParams(_) => "Expecting parameters in function declaration",
             ParseErrorType::WrongFnDefinition => "Function definitions must be at top level and cannot be inside a block or another function",
+            ParseErrorType::AssignmentToInvalidLHS => "Assignment to an unsupported left-hand side expression"
         }
     }
 
diff --git a/src/parser.rs b/src/parser.rs
index 204998b0..18052ee9 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -2,7 +2,7 @@ use crate::any::Dynamic;
 use crate::error::{LexError, ParseError, ParseErrorType};
 use std::char;
 use std::iter::Peekable;
-use std::str::Chars;
+use std::{borrow::Cow, str::Chars};
 
 type LERR = LexError;
 type PERR = ParseErrorType;
@@ -99,12 +99,12 @@ impl std::fmt::Debug for Position {
 }
 
 /// Compiled AST (abstract syntax tree) of a Rhai script.
-pub struct AST(pub(crate) Vec<Stmt>, pub(crate) Vec<FnDef>);
+pub struct AST(pub(crate) Vec<Stmt>, pub(crate) Vec<FnDef<'static>>);
 
 #[derive(Debug, Clone)]
-pub struct FnDef {
-    pub name: String,
-    pub params: Vec<String>,
+pub struct FnDef<'a> {
+    pub name: Cow<'a, str>,
+    pub params: Vec<Cow<'a, str>>,
     pub body: Box<Stmt>,
     pub pos: Position,
 }
@@ -232,14 +232,14 @@ pub enum Token {
 }
 
 impl Token {
-    pub fn syntax(&self) -> std::borrow::Cow<'static, str> {
+    pub fn syntax<'a>(&'a self) -> Cow<'a, str> {
         use self::Token::*;
 
         match *self {
-            IntegerConstant(ref s) => s.to_string().into(),
-            FloatConstant(ref s) => s.to_string().into(),
-            Identifier(ref s) => s.to_string().into(),
-            CharConstant(ref s) => s.to_string().into(),
+            IntegerConstant(ref i) => i.to_string().into(),
+            FloatConstant(ref f) => f.to_string().into(),
+            Identifier(ref s) => s.into(),
+            CharConstant(ref c) => c.to_string().into(),
             LexError(ref err) => err.to_string().into(),
 
             ref token => (match token {
@@ -301,7 +301,7 @@ impl Token {
                 PowerOfAssign => "~=",
                 For => "for",
                 In => "in",
-                _ => panic!(),
+                _ => panic!("operator should be match in outer scope"),
             })
             .into(),
         }
@@ -538,8 +538,7 @@ impl<'a> TokenIterator<'a> {
             }
         }
 
-        let out: String = result.iter().collect();
-        Ok(out)
+        Ok(result.iter().collect())
     }
 
     fn inner_next(&mut self) -> Option<(Token, Position)> {
@@ -640,20 +639,20 @@ impl<'a> TokenIterator<'a> {
                             },
                             pos,
                         ));
+                    } else {
+                        let out: String = result.iter().filter(|&&c| c != '_').collect();
+
+                        return Some((
+                            if let Ok(val) = out.parse::<i64>() {
+                                Token::IntegerConstant(val)
+                            } else if let Ok(val) = out.parse::<f64>() {
+                                Token::FloatConstant(val)
+                            } else {
+                                Token::LexError(LERR::MalformedNumber(result.iter().collect()))
+                            },
+                            pos,
+                        ));
                     }
-
-                    let out: String = result.iter().filter(|&&c| c != '_').collect();
-
-                    return Some((
-                        if let Ok(val) = out.parse::<i64>() {
-                            Token::IntegerConstant(val)
-                        } else if let Ok(val) = out.parse::<f64>() {
-                            Token::FloatConstant(val)
-                        } else {
-                            Token::LexError(LERR::MalformedNumber(result.iter().collect()))
-                        },
-                        pos,
-                    ));
                 }
                 'A'..='Z' | 'a'..='z' | '_' => {
                     let mut result = Vec::new();
@@ -687,7 +686,7 @@ impl<'a> TokenIterator<'a> {
                             "fn" => Token::Fn,
                             "for" => Token::For,
                             "in" => Token::In,
-                            x => Token::Identifier(x.into()),
+                            _ => Token::Identifier(out),
                         },
                         pos,
                     ));
@@ -1264,6 +1263,21 @@ fn parse_unary<'a>(input: &mut Peekable<TokenIterator<'a>>) -> Result<Expr, Pars
     }
 }
 
+fn parse_assignment(lhs: Expr, rhs: Expr) -> Result<Expr, ParseError> {
+    match lhs {
+        // Only assignments to a variable, and index erxpression and a dot expression is valid LHS
+        Expr::Identifier(_, _) | Expr::Index(_, _) | Expr::Dot(_, _) => {
+            Ok(Expr::Assignment(Box::new(lhs), Box::new(rhs)))
+        }
+
+        // All other LHS cannot be assigned to
+        _ => Err(ParseError::new(
+            PERR::AssignmentToInvalidLHS,
+            lhs.position(),
+        )),
+    }
+}
+
 fn parse_binary_op<'a>(
     input: &mut Peekable<TokenIterator<'a>>,
     precedence: i8,
@@ -1308,7 +1322,7 @@ fn parse_binary_op<'a>(
                 }
                 Token::Divide => Expr::FunctionCall("/".into(), vec![current_lhs, rhs], None, pos),
 
-                Token::Equals => Expr::Assignment(Box::new(current_lhs), Box::new(rhs)),
+                Token::Equals => parse_assignment(current_lhs, rhs)?,
                 Token::PlusAssign => {
                     let lhs_copy = current_lhs.clone();
                     Expr::Assignment(
@@ -1687,7 +1701,7 @@ fn parse_stmt<'a>(input: &mut Peekable<TokenIterator<'a>>) -> Result<Stmt, Parse
     }
 }
 
-fn parse_fn<'a>(input: &mut Peekable<TokenIterator<'a>>) -> Result<FnDef, ParseError> {
+fn parse_fn<'a>(input: &mut Peekable<TokenIterator<'a>>) -> Result<FnDef<'static>, ParseError> {
     let pos = match input.next() {
         Some((_, tok_pos)) => tok_pos,
         _ => return Err(ParseError::new(PERR::InputPastEndOfFile, Position::eof())),
@@ -1723,7 +1737,7 @@ fn parse_fn<'a>(input: &mut Peekable<TokenIterator<'a>>) -> Result<FnDef, ParseE
                 Some((Token::RightParen, _)) => break,
                 Some((Token::Comma, _)) => (),
                 Some((Token::Identifier(s), _)) => {
-                    params.push(s);
+                    params.push(s.into());
                 }
                 Some((_, pos)) => return Err(ParseError::new(PERR::MalformedCallExpr, pos)),
                 None => return Err(ParseError::new(PERR::MalformedCallExpr, Position::eof())),
@@ -1734,7 +1748,7 @@ fn parse_fn<'a>(input: &mut Peekable<TokenIterator<'a>>) -> Result<FnDef, ParseE
     let body = parse_block(input)?;
 
     Ok(FnDef {
-        name: name,
+        name: name.into(),
         params: params,
         body: Box::new(body),
         pos: pos,
diff --git a/src/scope.rs b/src/scope.rs
index 9e7f63dc..9a5eb815 100644
--- a/src/scope.rs
+++ b/src/scope.rs
@@ -1,4 +1,5 @@
 use crate::any::{Any, Dynamic};
+use std::borrow::Cow;
 
 /// A type containing information about current scope.
 /// Useful for keeping state between `Engine` runs
@@ -17,9 +18,9 @@ use crate::any::{Any, Dynamic};
 ///
 /// When searching for variables, newly-added variables are found before similarly-named but older variables,
 /// allowing for automatic _shadowing_ of variables.
-pub struct Scope(Vec<(String, Dynamic)>);
+pub struct Scope<'a>(Vec<(Cow<'a, str>, Dynamic)>);
 
-impl Scope {
+impl<'a> Scope<'a> {
     /// Create a new Scope.
     pub fn new() -> Self {
         Self(Vec::new())
@@ -36,18 +37,18 @@ impl Scope {
     }
 
     /// Add (push) a new variable to the Scope.
-    pub fn push<T: Any>(&mut self, key: String, value: T) {
-        self.0.push((key, Box::new(value)));
+    pub fn push<K: Into<Cow<'a, str>>, T: Any>(&mut self, key: K, value: T) {
+        self.0.push((key.into(), Box::new(value)));
     }
 
     /// Add (push) a new variable to the Scope.
-    pub(crate) fn push_dynamic(&mut self, key: String, value: Dynamic) {
-        self.0.push((key, value));
+    pub(crate) fn push_dynamic<K: Into<Cow<'a, str>>>(&mut self, key: K, value: Dynamic) {
+        self.0.push((key.into(), value));
     }
 
     /// Remove (pop) the last variable from the Scope.
     pub fn pop(&mut self) -> Option<(String, Dynamic)> {
-        self.0.pop()
+        self.0.pop().map(|(key, value)| (key.to_string(), value))
     }
 
     /// Truncate (rewind) the Scope to a previous size.
@@ -56,13 +57,13 @@ impl Scope {
     }
 
     /// Find a variable in the Scope, starting from the last.
-    pub fn get(&self, key: &str) -> Option<(usize, String, Dynamic)> {
+    pub fn get(&self, key: &str) -> Option<(usize, &str, Dynamic)> {
         self.0
             .iter()
             .enumerate()
             .rev() // Always search a Scope in reverse order
             .find(|(_, (name, _))| name == key)
-            .map(|(i, (name, value))| (i, name.clone(), value.clone()))
+            .map(|(i, (name, value))| (i, name.as_ref(), value.clone()))
     }
 
     /// Get the value of a variable in the Scope, starting from the last.
@@ -97,20 +98,26 @@ impl Scope {
         self.0
             .iter()
             .rev() // Always search a Scope in reverse order
-            .map(|(key, value)| (key.as_str(), value))
+            .map(|(key, value)| (key.as_ref(), value))
     }
 
+    /*
     /// Get a mutable iterator to variables in the Scope.
     pub fn iter_mut(&mut self) -> impl Iterator<Item = (&str, &mut Dynamic)> {
         self.0
             .iter_mut()
             .rev() // Always search a Scope in reverse order
-            .map(|(key, value)| (key.as_str(), value))
+            .map(|(key, value)| (key.as_ref(), value))
     }
+    */
 }
 
-impl std::iter::Extend<(String, Dynamic)> for Scope {
-    fn extend<T: IntoIterator<Item = (String, Dynamic)>>(&mut self, iter: T) {
-        self.0.extend(iter);
+impl<'a, K> std::iter::Extend<(K, Dynamic)> for Scope<'a>
+where
+    K: Into<Cow<'a, str>>,
+{
+    fn extend<T: IntoIterator<Item = (K, Dynamic)>>(&mut self, iter: T) {
+        self.0
+            .extend(iter.into_iter().map(|(key, value)| (key.into(), value)));
     }
 }
diff --git a/tests/var_scope.rs b/tests/var_scope.rs
index 5522ba67..d1f0c24e 100644
--- a/tests/var_scope.rs
+++ b/tests/var_scope.rs
@@ -25,8 +25,8 @@ fn test_scope_eval() -> Result<(), EvalAltResult> {
     // Then push some initialized variables into the state
     // NOTE: Remember the default numbers used by Rhai are i64 and f64.
     //       Better stick to them or it gets hard to work with other variables in the script.
-    scope.push("y".into(), 42_i64);
-    scope.push("z".into(), 999_i64);
+    scope.push("y", 42_i64);
+    scope.push("z", 999_i64);
 
     // First invocation
     engine