feat: add Ref cells for mutable state (Ref.new, Ref.get, Ref.set, Ref.update)

Implements WISH-013 mutable state primitives. Ref<T> is a mutable container
using existing module call syntax. Supported across interpreter, JS, and C backends.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -776,7 +776,7 @@ dependencies = [
 
 [[package]]
 name = "lux"
-version = "0.1.8"
+version = "0.1.11"
 dependencies = [
  "glob",
  "lsp-server",

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "lux"
-version = "0.1.9"
+version = "0.1.11"
 edition = "2021"
 description = "A functional programming language with first-class effects, schema evolution, and behavioral types"
 license = "MIT"

flake.nix

@@ -44,7 +44,7 @@
             printf "\n"
             printf "  \033[1;35m╦  ╦ ╦╦ ╦\033[0m\n"
             printf "  \033[1;35m║  ║ ║╔╣\033[0m\n"
-            printf "  \033[1;35m╩═╝╚═╝╩ ╩\033[0m  v0.1.9\n"
+            printf "  \033[1;35m╩═╝╚═╝╩ ╩\033[0m  v0.1.11\n"
             printf "\n"
             printf "  Functional language with first-class effects\n"
             printf "\n"
@@ -62,7 +62,7 @@
 
         packages.default = pkgs.rustPlatform.buildRustPackage {
           pname = "lux";
-          version = "0.1.9";
+          version = "0.1.11";
           src = ./.;
           cargoLock.lockFile = ./Cargo.lock;
 
@@ -79,7 +79,7 @@
           };
         in muslPkgs.rustPlatform.buildRustPackage {
           pname = "lux";
-          version = "0.1.9";
+          version = "0.1.11";
           src = ./.;
           cargoLock.lockFile = ./Cargo.lock;
 

src/ast.rs

@@ -223,6 +223,8 @@ pub enum Declaration {
     Impl(ImplDecl),
     /// Extern function declaration (FFI): extern fn name(params): ReturnType
     ExternFn(ExternFnDecl),
+    /// Extern let declaration (FFI): extern let name: Type
+    ExternLet(ExternLetDecl),
 }
 
 /// Function declaration
@@ -445,6 +447,19 @@ pub struct ExternFnDecl {
     pub span: Span,
 }
 
+/// Extern let declaration (FFI)
+#[derive(Debug, Clone)]
+pub struct ExternLetDecl {
+    pub visibility: Visibility,
+    /// Documentation comment
+    pub doc: Option<String>,
+    pub name: Ident,
+    pub typ: TypeExpr,
+    /// Optional JS name override: extern let foo: T = "window.foo"
+    pub js_name: Option<String>,
+    pub span: Span,
+}
+
 /// Type expressions
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum TypeExpr {

src/codegen/c_backend.rs

@@ -3285,6 +3285,9 @@ impl CBackend {
                         if module_name.name == "Map" {
                             return self.emit_map_operation(&field.name, args);
                         }
+                        if module_name.name == "Ref" {
+                            return self.emit_ref_operation(&field.name, args);
+                        }
                         // Int module
                         if module_name.name == "Int" && field.name == "toString" {
                             let arg = self.emit_expr(&args[0])?;
@@ -3701,6 +3704,11 @@ impl CBackend {
                     return self.emit_map_operation(&operation.name, args);
                 }
 
+                // Ref module
+                if effect.name == "Ref" {
+                    return self.emit_ref_operation(&operation.name, args);
+                }
+
                 // Built-in Console effect
                 if effect.name == "Console" {
                     if operation.name == "print" {
@@ -5185,6 +5193,42 @@ impl CBackend {
         }
     }
 
+    fn emit_ref_operation(&mut self, op: &str, args: &[Expr]) -> Result<String, CGenError> {
+        match op {
+            "new" => {
+                let val = self.emit_expr(&args[0])?;
+                let boxed = self.box_value(&val, None);
+                let temp = format!("_ref_new_{}", self.fresh_name());
+                self.writeln(&format!("void** {} = (void**)malloc(sizeof(void*));", temp));
+                self.writeln(&format!("*{} = {};", temp, boxed));
+                Ok(temp)
+            }
+            "get" => {
+                let r = self.emit_expr(&args[0])?;
+                Ok(format!("(*({}))  /* Ref.get */", r))
+            }
+            "set" => {
+                let r = self.emit_expr(&args[0])?;
+                let val = self.emit_expr(&args[1])?;
+                let boxed = self.box_value(&val, None);
+                self.writeln(&format!("*{} = {};", r, boxed));
+                Ok("0 /* Unit */".to_string())
+            }
+            "update" => {
+                let r = self.emit_expr(&args[0])?;
+                let f = self.emit_expr(&args[1])?;
+                let temp = format!("_ref_upd_{}", self.fresh_name());
+                self.writeln(&format!("void* {} = ((void*(*)(void*)){})(*({}));", temp, f, r));
+                self.writeln(&format!("*{} = {};", r, temp));
+                Ok("0 /* Unit */".to_string())
+            }
+            _ => Err(CGenError {
+                message: format!("Unsupported Ref operation: {}", op),
+                span: None,
+            }),
+        }
+    }
+
     fn emit_expr_with_substitution(&mut self, expr: &Expr, from: &str, to: &str) -> Result<String, CGenError> {
         // Simple substitution - in a real implementation, this would be more sophisticated
         match expr {

src/codegen/js_backend.rs

@@ -73,6 +73,8 @@ pub struct JsBackend {
     used_effects: HashSet<String>,
     /// Extern function names mapped to their JS names
     extern_fns: HashMap<String, String>,
+    /// Extern let names mapped to their JS names
+    extern_lets: HashMap<String, String>,
 }
 
 impl JsBackend {
@@ -96,6 +98,7 @@ impl JsBackend {
             var_substitutions: HashMap::new(),
             used_effects: HashSet::new(),
             extern_fns: HashMap::new(),
+            extern_lets: HashMap::new(),
         }
     }
 
@@ -123,6 +126,13 @@ impl JsBackend {
                     self.extern_fns.insert(ext.name.name.clone(), js_name);
                     self.functions.insert(ext.name.name.clone());
                 }
+                Declaration::ExternLet(ext) => {
+                    let js_name = ext
+                        .js_name
+                        .clone()
+                        .unwrap_or_else(|| ext.name.name.clone());
+                    self.extern_lets.insert(ext.name.name.clone(), js_name);
+                }
                 _ => {}
             }
         }
@@ -1097,6 +1107,9 @@ impl JsBackend {
                 } else if self.functions.contains(&ident.name) {
                     // Function reference (used as value)
                     Ok(self.mangle_name(&ident.name))
+                } else if let Some(js_name) = self.extern_lets.get(&ident.name) {
+                    // Extern let: use JS name directly (no mangling)
+                    Ok(js_name.clone())
                 } else {
                     Ok(self.escape_js_keyword(&ident.name))
                 }
@@ -1229,6 +1242,9 @@ impl JsBackend {
                         if module_name.name == "Map" {
                             return self.emit_map_operation(&field.name, args);
                         }
+                        if module_name.name == "Ref" {
+                            return self.emit_ref_operation(&field.name, args);
+                        }
                     }
                 }
 
@@ -1375,6 +1391,11 @@ impl JsBackend {
                     return self.emit_map_operation(&operation.name, args);
                 }
 
+                // Special case: Ref module operations (not an effect)
+                if effect.name == "Ref" {
+                    return self.emit_ref_operation(&operation.name, args);
+                }
+
                 // Special case: Html module operations (not an effect)
                 if effect.name == "Html" {
                     return self.emit_html_operation(&operation.name, args);
@@ -1835,6 +1856,67 @@ impl JsBackend {
                 let func = self.emit_expr(&args[1])?;
                 Ok(format!("[...{}].sort({})", list, func))
             }
+            "find" => {
+                let list = self.emit_expr(&args[0])?;
+                let pred = self.emit_expr(&args[1])?;
+                Ok(format!(
+                    "((__l) => {{ const __r = __l.find({}); return __r !== undefined ? Lux.Some(__r) : Lux.None(); }})({})",
+                    pred, list
+                ))
+            }
+            "findIndex" => {
+                let list = self.emit_expr(&args[0])?;
+                let pred = self.emit_expr(&args[1])?;
+                Ok(format!(
+                    "((__l) => {{ const __i = __l.findIndex({}); return __i !== -1 ? Lux.Some(__i) : Lux.None(); }})({})",
+                    pred, list
+                ))
+            }
+            "any" => {
+                let list = self.emit_expr(&args[0])?;
+                let pred = self.emit_expr(&args[1])?;
+                Ok(format!("{}.some({})", list, pred))
+            }
+            "all" => {
+                let list = self.emit_expr(&args[0])?;
+                let pred = self.emit_expr(&args[1])?;
+                Ok(format!("{}.every({})", list, pred))
+            }
+            "zip" => {
+                let list1 = self.emit_expr(&args[0])?;
+                let list2 = self.emit_expr(&args[1])?;
+                Ok(format!(
+                    "((__a, __b) => __a.slice(0, Math.min(__a.length, __b.length)).map((__x, __i) => [__x, __b[__i]]))({})",
+                    format!("{}, {}", list1, list2)
+                ))
+            }
+            "flatten" => {
+                let list = self.emit_expr(&args[0])?;
+                Ok(format!("{}.flat()", list))
+            }
+            "contains" => {
+                let list = self.emit_expr(&args[0])?;
+                let elem = self.emit_expr(&args[1])?;
+                Ok(format!(
+                    "{}.some(__x => JSON.stringify(__x) === JSON.stringify({}))",
+                    list, elem
+                ))
+            }
+            "take" => {
+                let list = self.emit_expr(&args[0])?;
+                let n = self.emit_expr(&args[1])?;
+                Ok(format!("{}.slice(0, Math.max(0, {}))", list, n))
+            }
+            "drop" => {
+                let list = self.emit_expr(&args[0])?;
+                let n = self.emit_expr(&args[1])?;
+                Ok(format!("{}.slice(Math.max(0, {}))", list, n))
+            }
+            "forEach" => {
+                let list = self.emit_expr(&args[0])?;
+                let func = self.emit_expr(&args[1])?;
+                Ok(format!("({}.forEach({}), undefined)", list, func))
+            }
             _ => Err(JsGenError {
                 message: format!("Unknown List operation: {}", operation),
                 span: None,
@@ -2412,6 +2494,37 @@ impl JsBackend {
         }
     }
 
+    fn emit_ref_operation(
+        &mut self,
+        operation: &str,
+        args: &[Expr],
+    ) -> Result<String, JsGenError> {
+        match operation {
+            "new" => {
+                let val = self.emit_expr(&args[0])?;
+                Ok(format!("({{value: {}}})", val))
+            }
+            "get" => {
+                let r = self.emit_expr(&args[0])?;
+                Ok(format!("({}.value)", r))
+            }
+            "set" => {
+                let r = self.emit_expr(&args[0])?;
+                let val = self.emit_expr(&args[1])?;
+                Ok(format!("({}.value = {}, undefined)", r, val))
+            }
+            "update" => {
+                let r = self.emit_expr(&args[0])?;
+                let f = self.emit_expr(&args[1])?;
+                Ok(format!("({0}.value = {1}({0}.value), undefined)", r, f))
+            }
+            _ => Err(JsGenError {
+                message: format!("Unknown Ref operation: {}", operation),
+                span: None,
+            }),
+        }
+    }
+
     /// Emit Html module operations for type-safe HTML construction
     fn emit_html_operation(
         &mut self,

src/formatter.rs

@@ -3,7 +3,7 @@
 //! Formats Lux source code according to standard style guidelines.
 
 use crate::ast::{
-    BehavioralProperty, BinaryOp, Declaration, EffectDecl, ExternFnDecl, Expr, FunctionDecl,
+    BehavioralProperty, BinaryOp, Declaration, EffectDecl, ExternFnDecl, ExternLetDecl, Expr, FunctionDecl,
     HandlerDecl, ImplDecl, ImplMethod, LetDecl, Literal, LiteralKind, Pattern, Program, Statement,
     TraitDecl, TypeDecl, TypeDef, TypeExpr, UnaryOp, VariantFields, Visibility,
 };
@@ -104,6 +104,7 @@ impl Formatter {
             Declaration::Trait(t) => self.format_trait(t),
             Declaration::Impl(i) => self.format_impl(i),
             Declaration::ExternFn(e) => self.format_extern_fn(e),
+            Declaration::ExternLet(e) => self.format_extern_let(e),
         }
     }
 
@@ -152,6 +153,27 @@ impl Formatter {
         self.newline();
     }
 
+    fn format_extern_let(&mut self, ext: &ExternLetDecl) {
+        let indent = self.indent();
+        self.write(&indent);
+
+        if ext.visibility == Visibility::Public {
+            self.write("pub ");
+        }
+
+        self.write("extern let ");
+        self.write(&ext.name.name);
+        self.write(": ");
+        self.write(&self.format_type_expr(&ext.typ));
+
+        // Optional JS name
+        if let Some(js_name) = &ext.js_name {
+            self.write(&format!(" = \"{}\"", js_name));
+        }
+
+        self.newline();
+    }
+
     fn format_function(&mut self, func: &FunctionDecl) {
         let indent = self.indent();
         self.write(&indent);

src/interpreter.rs

@@ -144,6 +144,12 @@ pub enum BuiltinFn {
     MapFromList,
     MapToList,
     MapMerge,
+
+    // Ref operations
+    RefNew,
+    RefGet,
+    RefSet,
+    RefUpdate,
 }
 
 /// Runtime value
@@ -181,6 +187,8 @@ pub enum Value {
         name: String,
         arity: usize,
     },
+    /// Mutable reference cell
+    Ref(Rc<RefCell<Value>>),
 }
 
 impl Value {
@@ -203,6 +211,7 @@ impl Value {
             Value::Versioned { .. } => "Versioned",
             Value::Json(_) => "Json",
             Value::ExternFn { .. } => "ExternFn",
+            Value::Ref(_) => "Ref",
         }
     }
 
@@ -258,6 +267,7 @@ impl Value {
                 t1 == t2 && v1 == v2 && Value::values_equal(val1, val2)
             }
             (Value::Json(j1), Value::Json(j2)) => j1 == j2,
+            (Value::Ref(r1), Value::Ref(r2)) => Rc::ptr_eq(r1, r2),
             // Functions and handlers cannot be compared for equality
             _ => false,
         }
@@ -414,6 +424,7 @@ impl fmt::Display for Value {
             }
             Value::Json(json) => write!(f, "{}", json),
             Value::ExternFn { name, .. } => write!(f, "<extern fn {}>", name),
+            Value::Ref(cell) => write!(f, "<ref: {}>", cell.borrow()),
         }
     }
 }
@@ -1202,6 +1213,15 @@ impl Interpreter {
             ("merge".to_string(), Value::Builtin(BuiltinFn::MapMerge)),
         ]));
         env.define("Map", map_module);
+
+        // Ref module
+        let ref_module = Value::Record(HashMap::from([
+            ("new".to_string(), Value::Builtin(BuiltinFn::RefNew)),
+            ("get".to_string(), Value::Builtin(BuiltinFn::RefGet)),
+            ("set".to_string(), Value::Builtin(BuiltinFn::RefSet)),
+            ("update".to_string(), Value::Builtin(BuiltinFn::RefUpdate)),
+        ]));
+        env.define("Ref", ref_module);
     }
 
     /// Execute a program
@@ -1431,6 +1451,14 @@ impl Interpreter {
                 Ok(Value::Unit)
             }
 
+            Declaration::ExternLet(ext) => {
+                // Register a placeholder that errors at runtime (extern lets only work in JS)
+                let name = ext.name.name.clone();
+                self.global_env
+                    .define(&name, Value::ExternFn { name: name.clone(), arity: 0 });
+                Ok(Value::Unit)
+            }
+
             Declaration::Effect(_) | Declaration::Trait(_) | Declaration::Impl(_) => {
                 // These are compile-time only
                 Ok(Value::Unit)
@@ -3432,6 +3460,56 @@ impl Interpreter {
                 }
                 Ok(EvalResult::Value(Value::Map(map1)))
             }
+
+            BuiltinFn::RefNew => {
+                if args.len() != 1 {
+                    return Err(err("Ref.new requires 1 argument"));
+                }
+                Ok(EvalResult::Value(Value::Ref(Rc::new(RefCell::new(args.into_iter().next().unwrap())))))
+            }
+
+            BuiltinFn::RefGet => {
+                if args.len() != 1 {
+                    return Err(err("Ref.get requires 1 argument"));
+                }
+                match &args[0] {
+                    Value::Ref(cell) => Ok(EvalResult::Value(cell.borrow().clone())),
+                    v => Err(err(&format!("Ref.get expects Ref, got {}", v.type_name()))),
+                }
+            }
+
+            BuiltinFn::RefSet => {
+                if args.len() != 2 {
+                    return Err(err("Ref.set requires 2 arguments: ref, value"));
+                }
+                match &args[0] {
+                    Value::Ref(cell) => {
+                        *cell.borrow_mut() = args[1].clone();
+                        Ok(EvalResult::Value(Value::Unit))
+                    }
+                    v => Err(err(&format!("Ref.set expects Ref as first argument, got {}", v.type_name()))),
+                }
+            }
+
+            BuiltinFn::RefUpdate => {
+                if args.len() != 2 {
+                    return Err(err("Ref.update requires 2 arguments: ref, fn"));
+                }
+                match &args[0] {
+                    Value::Ref(cell) => {
+                        let old = cell.borrow().clone();
+                        let result = self.eval_call(args[1].clone(), vec![old], span)?;
+                        match result {
+                            EvalResult::Value(new_val) => {
+                                *cell.borrow_mut() = new_val;
+                            }
+                            _ => return Err(err("Ref.update callback must return a value")),
+                        }
+                        Ok(EvalResult::Value(Value::Unit))
+                    }
+                    v => Err(err(&format!("Ref.update expects Ref as first argument, got {}", v.type_name()))),
+                }
+            }
         }
     }
 

src/linter.rs

@@ -406,6 +406,9 @@ impl Linter {
                 Declaration::ExternFn(e) => {
                     self.defined_functions.insert(e.name.name.clone());
                 }
+                Declaration::ExternLet(e) => {
+                    self.define_var(&e.name.name);
+                }
                 Declaration::Let(l) => {
                     self.define_var(&l.name.name);
                 }

src/main.rs

@@ -2311,6 +2311,25 @@ fn extract_module_doc(source: &str, path: &str) -> Result<ModuleDoc, String> {
                     properties: vec![],
                 });
             }
+            ast::Declaration::ExternLet(ext) => {
+                let js_note = ext.js_name.as_ref()
+                    .map(|n| format!(" = \"{}\"", n))
+                    .unwrap_or_default();
+                let signature = format!(
+                    "extern let {}: {}{}",
+                    ext.name.name,
+                    format_type(&ext.typ),
+                    js_note
+                );
+                let doc = extract_doc_comment(source, ext.span.start);
+                functions.push(FunctionDoc {
+                    name: ext.name.name.clone(),
+                    signature,
+                    description: doc,
+                    is_public: matches!(ext.visibility, ast::Visibility::Public),
+                    properties: vec![],
+                });
+            }
             ast::Declaration::Effect(e) => {
                 let doc = extract_doc_comment(source, e.span.start);
                 let ops: Vec<String> = e.operations.iter()
@@ -5836,6 +5855,58 @@ c")"#;
         assert_eq!(eval(source).unwrap(), "Some(30)");
     }
 
+    // Ref cell tests
+    #[test]
+    fn test_ref_new_and_get() {
+        let source = r#"
+            let r = Ref.new(42)
+            let result = Ref.get(r)
+        "#;
+        assert_eq!(eval(source).unwrap(), "42");
+    }
+
+    #[test]
+    fn test_ref_set() {
+        let source = r#"
+            let r = Ref.new(0)
+            let _ = Ref.set(r, 10)
+            let result = Ref.get(r)
+        "#;
+        assert_eq!(eval(source).unwrap(), "10");
+    }
+
+    #[test]
+    fn test_ref_update() {
+        let source = r#"
+            let r = Ref.new(5)
+            let _ = Ref.update(r, fn(n) => n + 1)
+            let result = Ref.get(r)
+        "#;
+        assert_eq!(eval(source).unwrap(), "6");
+    }
+
+    #[test]
+    fn test_ref_multiple_updates() {
+        let source = r#"
+            let counter = Ref.new(0)
+            let _ = Ref.set(counter, 1)
+            let _ = Ref.update(counter, fn(n) => n * 10)
+            let _ = Ref.set(counter, Ref.get(counter) + 5)
+            let result = Ref.get(counter)
+        "#;
+        assert_eq!(eval(source).unwrap(), "15");
+    }
+
+    #[test]
+    fn test_ref_with_string() {
+        let source = r#"
+            let r = Ref.new("hello")
+            let _ = Ref.set(r, "world")
+            let result = Ref.get(r)
+        "#;
+        assert_eq!(eval(source).unwrap(), "\"world\"");
+    }
+
     #[test]
     fn test_file_copy() {
         use std::io::Write;

src/modules.rs

@@ -53,6 +53,7 @@ impl Module {
                     Declaration::Type(t) => t.visibility == Visibility::Public,
                     Declaration::Trait(t) => t.visibility == Visibility::Public,
                     Declaration::ExternFn(e) => e.visibility == Visibility::Public,
+                    Declaration::ExternLet(e) => e.visibility == Visibility::Public,
                     // Effects, handlers, and impls are always public for now
                     Declaration::Effect(_) | Declaration::Handler(_) | Declaration::Impl(_) => true,
                 }
@@ -298,6 +299,9 @@ impl ModuleLoader {
                 Declaration::ExternFn(e) if e.visibility == Visibility::Public => {
                     exports.insert(e.name.name.clone());
                 }
+                Declaration::ExternLet(e) if e.visibility == Visibility::Public => {
+                    exports.insert(e.name.name.clone());
+                }
                 _ => {}
             }
         }

src/parser.rs

@@ -238,7 +238,7 @@ impl Parser {
 
         match self.peek_kind() {
             TokenKind::Fn => Ok(Declaration::Function(self.parse_function_decl(visibility, doc)?)),
-            TokenKind::Extern => Ok(Declaration::ExternFn(self.parse_extern_fn_decl(visibility, doc)?)),
+            TokenKind::Extern => self.parse_extern_decl(visibility, doc),
             TokenKind::Effect => Ok(Declaration::Effect(self.parse_effect_decl(doc)?)),
             TokenKind::Handler => Ok(Declaration::Handler(self.parse_handler_decl()?)),
             TokenKind::Type => Ok(Declaration::Type(self.parse_type_decl(visibility, doc)?)),
@@ -324,6 +324,58 @@ impl Parser {
         })
     }
 
+    /// Parse extern declaration: dispatch to extern fn or extern let
+    fn parse_extern_decl(&mut self, visibility: Visibility, doc: Option<String>) -> Result<Declaration, ParseError> {
+        // Peek past 'extern' to see if it's 'fn' or 'let'
+        if self.pos + 1 < self.tokens.len() {
+            match &self.tokens[self.pos + 1].kind {
+                TokenKind::Fn => Ok(Declaration::ExternFn(self.parse_extern_fn_decl(visibility, doc)?)),
+                TokenKind::Let => Ok(Declaration::ExternLet(self.parse_extern_let_decl(visibility, doc)?)),
+                _ => Err(self.error("Expected 'fn' or 'let' after 'extern'")),
+            }
+        } else {
+            Err(self.error("Expected 'fn' or 'let' after 'extern'"))
+        }
+    }
+
+    /// Parse extern let declaration: extern let name: Type = "jsName"
+    fn parse_extern_let_decl(&mut self, visibility: Visibility, doc: Option<String>) -> Result<ExternLetDecl, ParseError> {
+        let start = self.current_span();
+        self.expect(TokenKind::Extern)?;
+        self.expect(TokenKind::Let)?;
+
+        let name = self.parse_ident()?;
+
+        // Type annotation
+        self.expect(TokenKind::Colon)?;
+        let typ = self.parse_type()?;
+
+        // Optional JS name override: = "jsName"
+        let js_name = if self.check(TokenKind::Eq) {
+            self.advance();
+            match self.peek_kind() {
+                TokenKind::String(s) => {
+                    let name = s.clone();
+                    self.advance();
+                    Some(name)
+                }
+                _ => return Err(self.error("Expected string literal for JS name in extern let")),
+            }
+        } else {
+            None
+        };
+
+        let span = start.merge(self.previous_span());
+        Ok(ExternLetDecl {
+            visibility,
+            doc,
+            name,
+            typ,
+            js_name,
+            span,
+        })
+    }
+
     /// Parse extern function declaration: extern fn name<T>(params): ReturnType = "jsName"
     fn parse_extern_fn_decl(&mut self, visibility: Visibility, doc: Option<String>) -> Result<ExternFnDecl, ParseError> {
         let start = self.current_span();

src/symbol_table.rs

@@ -269,6 +269,24 @@ impl SymbolTable {
                 let id = self.add_symbol(scope_idx, symbol);
                 self.add_reference(id, ext.name.span, true, true);
             }
+            Declaration::ExternLet(ext) => {
+                let is_public = matches!(ext.visibility, Visibility::Public);
+                let sig = format!(
+                    "extern let {}: {}",
+                    ext.name.name,
+                    self.type_expr_to_string(&ext.typ)
+                );
+                let mut symbol = self.new_symbol(
+                    ext.name.name.clone(),
+                    SymbolKind::Variable,
+                    ext.span,
+                    Some(sig),
+                    is_public,
+                );
+                symbol.documentation = ext.doc.clone();
+                let id = self.add_symbol(scope_idx, symbol);
+                self.add_reference(id, ext.name.span, true, true);
+            }
         }
     }
 

src/typechecker.rs

@@ -1238,6 +1238,11 @@ impl TypeChecker {
                 let fn_type = Type::function(param_types, return_type);
                 self.env.bind(&ext.name.name, TypeScheme::mono(fn_type));
             }
+            Declaration::ExternLet(ext) => {
+                // Register extern let with its declared type
+                let typ = self.resolve_type(&ext.typ);
+                self.env.bind(&ext.name.name, TypeScheme::mono(typ));
+            }
         }
     }
 
@@ -3021,6 +3026,9 @@ impl TypeChecker {
                         "Map" if resolved_args.len() == 2 => {
                             return Type::Map(Box::new(resolved_args[0].clone()), Box::new(resolved_args[1].clone()));
                         }
+                        "Ref" if resolved_args.len() == 1 => {
+                            return Type::Ref(Box::new(resolved_args[0].clone()));
+                        }
                         _ => {}
                     }
                 }

src/types.rs

@@ -49,6 +49,8 @@ pub enum Type {
     Option(Box<Type>),
     /// Map type (sugar for App(Map, [K, V]))
     Map(Box<Type>, Box<Type>),
+    /// Ref type — mutable reference cell holding a value of type T
+    Ref(Box<Type>),
     /// Versioned type (e.g., User @v2)
     Versioned {
         base: Box<Type>,
@@ -120,7 +122,7 @@ impl Type {
             }
             Type::Tuple(elements) => elements.iter().any(|e| e.contains_var(var)),
             Type::Record(fields) => fields.iter().any(|(_, t)| t.contains_var(var)),
-            Type::List(inner) | Type::Option(inner) => inner.contains_var(var),
+            Type::List(inner) | Type::Option(inner) | Type::Ref(inner) => inner.contains_var(var),
             Type::Map(k, v) => k.contains_var(var) || v.contains_var(var),
             Type::Versioned { base, .. } => base.contains_var(var),
             _ => false,
@@ -161,6 +163,7 @@ impl Type {
             ),
             Type::List(inner) => Type::List(Box::new(inner.apply(subst))),
             Type::Option(inner) => Type::Option(Box::new(inner.apply(subst))),
+            Type::Ref(inner) => Type::Ref(Box::new(inner.apply(subst))),
             Type::Map(k, v) => Type::Map(Box::new(k.apply(subst)), Box::new(v.apply(subst))),
             Type::Versioned { base, version } => Type::Versioned {
                 base: Box::new(base.apply(subst)),
@@ -211,7 +214,7 @@ impl Type {
                 }
                 vars
             }
-            Type::List(inner) | Type::Option(inner) => inner.free_vars(),
+            Type::List(inner) | Type::Option(inner) | Type::Ref(inner) => inner.free_vars(),
             Type::Map(k, v) => {
                 let mut vars = k.free_vars();
                 vars.extend(v.free_vars());
@@ -288,6 +291,7 @@ impl fmt::Display for Type {
             }
             Type::List(inner) => write!(f, "List<{}>", inner),
             Type::Option(inner) => write!(f, "Option<{}>", inner),
+            Type::Ref(inner) => write!(f, "Ref<{}>", inner),
             Type::Map(k, v) => write!(f, "Map<{}, {}>", k, v),
             Type::Versioned { base, version } => {
                 write!(f, "{} {}", base, version)
@@ -1946,6 +1950,32 @@ impl TypeEnv {
         ]);
         env.bind("Map", TypeScheme::mono(map_module_type));
 
+        // Ref module
+        let ref_inner = || Type::var();
+        let ref_type = || Type::Ref(Box::new(Type::var()));
+        let ref_module_type = Type::Record(vec![
+            (
+                "new".to_string(),
+                Type::function(vec![ref_inner()], ref_type()),
+            ),
+            (
+                "get".to_string(),
+                Type::function(vec![ref_type()], ref_inner()),
+            ),
+            (
+                "set".to_string(),
+                Type::function(vec![ref_type(), ref_inner()], Type::Unit),
+            ),
+            (
+                "update".to_string(),
+                Type::function(
+                    vec![ref_type(), Type::function(vec![ref_inner()], ref_inner())],
+                    Type::Unit,
+                ),
+            ),
+        ]);
+        env.bind("Ref", TypeScheme::mono(ref_module_type));
+
         // Result module
         let result_type = Type::App {
             constructor: Box::new(Type::Named("Result".to_string())),
@@ -2185,6 +2215,9 @@ impl TypeEnv {
             Type::Map(k, v) => {
                 Type::Map(Box::new(self.expand_type_alias(k)), Box::new(self.expand_type_alias(v)))
             }
+            Type::Ref(inner) => {
+                Type::Ref(Box::new(self.expand_type_alias(inner)))
+            }
             Type::Versioned { base, version } => {
                 Type::Versioned {
                     base: Box::new(self.expand_type_alias(base)),
@@ -2345,6 +2378,9 @@ pub fn unify(t1: &Type, t2: &Type) -> Result<Substitution, String> {
         // Option
         (Type::Option(a), Type::Option(b)) => unify(a, b),
 
+        // Ref
+        (Type::Ref(a), Type::Ref(b)) => unify(a, b),
+
         // Map
         (Type::Map(k1, v1), Type::Map(k2, v2)) => {
             let s1 = unify(k1, k2)?;