Add redefinition checks for variables

Ensure that variables are not redefined within the same scope, both for
global and function-local variables. This commit modifies the `define`
method in `CompilerScope` to return a `Result` and includes checks for
existing definitions. The `Binder`'s logic for handling global variables
and function parameters is also updated to incorporate these new checks
and return appropriate errors.

src/ast/compiler/binder.rs

@@ -25,11 +25,14 @@ impl CompilerScope {
         }
     }
 
-    fn define(&mut self, name: &str, ty: StaticType) -> u32 {
+    fn define(&mut self, name: &str, ty: StaticType) -> Result<u32, String> {
+        if self.locals.contains_key(name) {
+            return Err(format!("Variable '{}' is already defined in this scope level.", name));
+        }
         let slot = self.slot_count;
         self.locals.insert(name.to_string(), LocalInfo { slot, ty });
         self.slot_count += 1;
-        slot
+        Ok(slot)
     }
 
     fn resolve(&self, name: &str) -> Option<LocalInfo> {
@@ -135,16 +138,15 @@ impl Binder {
                 // 1. Pre-declare name to support recursion
                 let slot_or_idx = if self.functions.len() == 1 {
                     let mut globals = self.globals.borrow_mut();
-                    if let Some((idx, _)) = globals.get(name.as_ref()) {
-                        *idx
-                    } else {
+                    if globals.contains_key(name.as_ref()) {
+                        return Err(format!("Global variable '{}' is already defined.", name));
+                    }
                     let idx = globals.len() as u32;
                     globals.insert(name.to_string(), (idx, StaticType::Any));
                     idx
-                    }
                 } else {
                     let current_fn = self.functions.last_mut().unwrap();
-                    current_fn.scope.define(name, StaticType::Any)
+                    current_fn.scope.define(name, StaticType::Any)?
                 };
 
                 // 2. Bind Value (now 'name' is visible)
@@ -206,8 +208,8 @@ impl Binder {
                 {
                     let current_fn = self.functions.last_mut().unwrap();
                     for param in params {
-                        // For now, lambda params are Any unless we have a way to specify them
-                        current_fn.scope.define(param, StaticType::Any);
+                        // Lambda params must also be unique in their scope
+                        current_fn.scope.define(param, StaticType::Any)?;
                         param_types.push(StaticType::Any);
                     }
                 }
@@ -411,4 +413,17 @@ mod tests {
             panic!("Root should be a Lambda");
         }
     }
+
+    #[test]
+    fn test_redefinition_error() {
+        let source = "(do (def x 1) (def x 2))";
+        let mut parser = Parser::new(source).unwrap();
+        let untyped = parser.parse_expression().unwrap();
+        
+        let globals = Rc::new(RefCell::new(HashMap::new()));
+        let result = Binder::bind_root(globals, &untyped);
+        
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("already defined"));
+    }
 }