diff --git a/src/engine.rs b/src/engine.rs
index 85d5a031..9c0ed804 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -189,7 +189,7 @@ impl StaticVec {
 }
 
 /// A type that holds all the current states of the Engine.
-#[derive(Debug, Clone, Hash, Copy)]
+#[derive(Debug, Clone, Copy)]
 pub struct State {
     /// Normally, access to variables are parsed with a relative offset into the scope to avoid a lookup.
     /// In some situation, e.g. after running an `eval` statement, subsequent offsets may become mis-aligned.
@@ -198,6 +198,7 @@ pub struct State {
 }
 
 impl State {
+    /// Create a new `State`.
     pub fn new() -> Self {
         Self {
             always_search: false,
@@ -914,10 +915,9 @@ impl Engine {
         match dot_lhs {
             // id.??? or id[???]
             Expr::Variable(id, index, pos) => {
-                let (target, typ) = if !state.always_search && *index > 0 {
-                    scope.get_mut(scope.len() - *index)
-                } else {
-                    search_scope(scope, id, *pos)?
+                let (target, typ) = match index {
+                    Some(i) if !state.always_search => scope.get_mut(scope.len() - i.get()),
+                    _ => search_scope(scope, id, *pos)?,
                 };
 
                 // Constants cannot be modified
@@ -1156,8 +1156,8 @@ impl Engine {
             Expr::FloatConstant(f, _) => Ok((*f).into()),
             Expr::StringConstant(s, _) => Ok(s.to_string().into()),
             Expr::CharConstant(c, _) => Ok((*c).into()),
-            Expr::Variable(_, index, _) if !state.always_search && *index > 0 => {
-                Ok(scope.get_mut(scope.len() - *index).0.clone())
+            Expr::Variable(_, Some(index), _) if !state.always_search => {
+                Ok(scope.get_mut(scope.len() - index.get()).0.clone())
             }
             Expr::Variable(id, _, pos) => search_scope(scope, id, *pos).map(|(v, _)| v.clone()),
             Expr::Property(_, _) => panic!("unexpected property."),
@@ -1272,13 +1272,17 @@ impl Engine {
                     && args.len() == 1
                     && !self.has_override(fn_lib, KEYWORD_EVAL)
                 {
+                    let prev_len = scope.len();
+
                     // Evaluate the text string as a script
                     let result =
                         self.eval_script_expr(scope, fn_lib, args[0], arg_exprs[0].position());
 
-                    // IMPORTANT! The eval may define new variables in the current scope!
-                    //            We can no longer trust the variable offsets.
-                    state.always_search = true;
+                    if scope.len() != prev_len {
+                        // IMPORTANT! If the eval defines new variables in the current scope,
+                        //            all variable offsets from this point on will be mis-aligned.
+                        state.always_search = true;
+                    }
 
                     return result;
                 }
diff --git a/src/optimize.rs b/src/optimize.rs
index 1514b36c..a63075a3 100644
--- a/src/optimize.rs
+++ b/src/optimize.rs
@@ -729,10 +729,10 @@ pub fn optimize_into_ast(
 
                     // Optimize the function body
                     let mut body =
-                        optimize(vec![fn_def.body], engine, &Scope::new(), &fn_lib, level);
+                        optimize(vec![*fn_def.body], engine, &Scope::new(), &fn_lib, level);
 
                     // {} -> Noop
-                    fn_def.body = match body.pop().unwrap_or_else(|| Stmt::Noop(pos)) {
+                    fn_def.body = Box::new(match body.pop().unwrap_or_else(|| Stmt::Noop(pos)) {
                         // { return val; } -> val
                         Stmt::ReturnWithVal(Some(val), ReturnType::Return, _) => Stmt::Expr(val),
                         // { return; } -> ()
@@ -741,7 +741,7 @@ pub fn optimize_into_ast(
                         }
                         // All others
                         stmt => stmt,
-                    };
+                    });
                 }
                 fn_def
             })
diff --git a/src/parser.rs b/src/parser.rs
index 5fe913b3..dccad46e 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -14,6 +14,7 @@ use crate::stdlib::{
     collections::HashMap,
     format,
     iter::Peekable,
+    num::NonZeroUsize,
     ops::Add,
     rc::Rc,
     string::{String, ToString},
@@ -169,7 +170,7 @@ pub struct FnDef {
     /// Names of function parameters.
     pub params: Vec<String>,
     /// Function body.
-    pub body: Stmt,
+    pub body: Box<Stmt>,
     /// Position of the function definition.
     pub pos: Position,
 }
@@ -184,29 +185,37 @@ pub enum ReturnType {
 }
 
 /// A type that encapsulates a local stack with variable names to simulate an actual runtime scope.
+#[derive(Debug, Clone)]
 struct Stack(Vec<String>);
 
 impl Stack {
+    /// Create a new `Stack`.
     pub fn new() -> Self {
         Self(Vec::new())
     }
+    /// Get the number of variables in the `Stack`.
     pub fn len(&self) -> usize {
         self.0.len()
     }
+    /// Push (add) a new variable onto the `Stack`.
     pub fn push(&mut self, name: String) {
         self.0.push(name);
     }
+    /// Rewind the stack to a previous size.
     pub fn rewind(&mut self, len: usize) {
         self.0.truncate(len);
     }
-    pub fn find(&self, name: &str) -> usize {
+    /// Find a variable by name in the `Stack`, searching in reverse.
+    /// The return value is the offset to be deducted from `Stack::len`,
+    /// i.e. the top element of the `Stack` is offset 1.
+    /// Return zero when the variable name is not found in the `Stack`.
+    pub fn find(&self, name: &str) -> Option<NonZeroUsize> {
         self.0
             .iter()
             .rev()
             .enumerate()
             .find(|(_, n)| *n == name)
-            .map(|(i, _)| i + 1)
-            .unwrap_or(0)
+            .and_then(|(i, _)| NonZeroUsize::new(i + 1))
     }
 }
 
@@ -308,7 +317,7 @@ pub enum Expr {
     /// String constant.
     StringConstant(String, Position),
     /// Variable access.
-    Variable(String, usize, Position),
+    Variable(String, Option<NonZeroUsize>, Position),
     /// Property access.
     Property(String, Position),
     /// { stmt }
@@ -1836,10 +1845,10 @@ fn parse_fn<'a>(
         })?;
 
     // Parse function body
-    let body = match input.peek().unwrap() {
+    let body = Box::new(match input.peek().unwrap() {
         (Token::LeftBrace, _) => parse_block(input, stack, false, allow_stmt_expr)?,
         (_, pos) => return Err(PERR::FnMissingBody(name).into_err(*pos)),
-    };
+    });
 
     let params = params.into_iter().map(|(p, _)| p).collect();