Refactor function compiler scope handling

Introduce a `ScopeKind` enum to distinguish between root and local
scopes.
This allows the `FunctionCompiler` to correctly handle global variable
declarations in the root scope and local variables in other scopes.
The `define_variable` method is introduced to encapsulate this logic.

src/ast/compiler/binder.rs

@@ -52,16 +52,49 @@ impl CompilerScope {
     }
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum ScopeKind {
+    Root,
+    Local,
+}
+
 struct FunctionCompiler {
     scope: CompilerScope,
     upvalues: Vec<Address>,
+    kind: ScopeKind,
 }
 
 impl FunctionCompiler {
-    fn new() -> Self {
+    fn new(kind: ScopeKind) -> Self {
         Self {
             scope: CompilerScope::new(),
             upvalues: Vec::new(),
+            kind,
+        }
+    }
+
+    fn define_variable(
+        &mut self,
+        name: &Symbol,
+        globals: &Rc<RefCell<HashMap<Symbol, u32>>>,
+    ) -> Result<Address, String> {
+        match self.kind {
+            ScopeKind::Root => {
+                let mut globals_map = globals.borrow_mut();
+                if globals_map.contains_key(name) {
+                    return Err(format!(
+                        "Global variable '{}' is already defined.",
+                        name.name
+                    ));
+                }
+                let idx = globals_map.len() as u32;
+                globals_map.insert(name.clone(), idx);
+                Ok(Address::Global(idx))
+            }
+            ScopeKind::Local => {
+                let slot = self.scope.define(name)?;
+                Ok(Address::Local(slot))
+            }
         }
     }
 
@@ -97,7 +130,9 @@ impl Binder {
             globals,
             capture_map: captures,
         };
-        binder.functions.push(FunctionCompiler::new());
+        binder
+            .functions
+            .push(FunctionCompiler::new(ScopeKind::Root));
         binder
     }
 
@@ -115,22 +150,8 @@ impl Binder {
         name: &Symbol,
         _kind: crate::ast::compiler::bound_nodes::DeclarationKind,
     ) -> Result<Address, String> {
-        if self.functions.len() == 1 {
-            let mut globals = self.globals.borrow_mut();
-            if globals.contains_key(name) {
-                return Err(format!(
-                    "Global variable '{}' is already defined.",
-                    name.name
-                ));
-            }
-            let idx = globals.len() as u32;
-            globals.insert(name.clone(), idx);
-            Ok(Address::Global(idx))
-        } else {
-            let current_fn = self.functions.last_mut().unwrap();
-            let slot = current_fn.scope.define(name)?;
-            Ok(Address::Local(slot))
-        }
+        let current_fn = self.functions.last_mut().unwrap();
+        current_fn.define_variable(name, &self.globals)
     }
 
     pub fn bind(&mut self, node: &Node<UntypedKind>) -> Result<BoundNode, String> {
@@ -244,7 +265,7 @@ impl Binder {
 
             UntypedKind::Lambda { params, body } => {
                 let identity = node.identity.clone();
-                self.functions.push(FunctionCompiler::new());
+                self.functions.push(FunctionCompiler::new(ScopeKind::Local));
 
                 // 1. Bind the parameter pattern/tuple
                 let params_bound = self.bind_pattern(params, DeclarationKind::Parameter)?;