diff --git a/Cargo.lock b/Cargo.lock
index 39e863c..755861a 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -88,7 +88,7 @@ dependencies = [
  "heck",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.77",
 ]
 
 [[package]]
@@ -109,6 +109,27 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f3f6d59c71e7dc3af60f0af9db32364d96a16e9310f3f5db2b55ed642162dd35"
 
+[[package]]
+name = "gc"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f73a03797d58caede765f4b19522d1a63d737088bbef81de06d2dd117d7bc8c3"
+dependencies = [
+ "gc_derive",
+]
+
+[[package]]
+name = "gc_derive"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fb6f1e0d69658b2806f60864bad1a80a9634fc0bc17f7ed55828e3b9c45bf61e"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+ "synstructure",
+]
+
 [[package]]
 name = "heck"
 version = "0.5.0"
@@ -128,6 +149,7 @@ dependencies = [
  "assert_matches",
  "clap",
  "common_macros",
+ "gc",
  "thiserror",
 ]
 
@@ -155,6 +177,17 @@ version = "0.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7da8b5736845d9f2fcb837ea5d9e2628564b3b043a70948a3f0b778838c5fb4f"
 
+[[package]]
+name = "syn"
+version = "1.0.109"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72b64191b275b66ffe2469e8af2c1cfe3bafa67b529ead792a6d0160888b4237"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
 [[package]]
 name = "syn"
 version = "2.0.77"
@@ -166,6 +199,18 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "synstructure"
+version = "0.12.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f36bdaa60a83aca3921b5259d5400cbf5e90fc51931376a9bd4a0eb79aa7210f"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+ "unicode-xid",
+]
+
 [[package]]
 name = "thiserror"
 version = "1.0.63"
@@ -183,7 +228,7 @@ checksum = "a4558b58466b9ad7ca0f102865eccc95938dca1a74a856f2b57b6629050da261"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.77",
 ]
 
 [[package]]
@@ -192,6 +237,12 @@ version = "1.0.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e91b56cd4cadaeb79bbf1a5645f6b4f8dc5bde8834ad5894a8db35fda9efa1fe"
 
+[[package]]
+name = "unicode-xid"
+version = "0.2.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ebc1c04c71510c7f702b52b7c350734c9ff1295c464a03335b00bb84fc54f853"
+
 [[package]]
 name = "utf8parse"
 version = "0.2.2"
diff --git a/Cargo.toml b/Cargo.toml
index 36c1228..631438c 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -8,3 +8,4 @@ assert_matches = "1.5.0"
 clap = { version = "4.5.8", features = ["derive"] }
 common_macros = "0.1.1"
 thiserror = "1.0.63"
+gc = { version = "0.5", features = ["derive"] }
diff --git a/src/builtins.rs b/src/builtins.rs
index c88596d..6d32d09 100644
--- a/src/builtins.rs
+++ b/src/builtins.rs
@@ -1,13 +1,34 @@
 //! Builtin functions.
-use crate::obj::{NilInst, ObjP};
+use std::collections::HashMap;
+
+use crate::obj::*;
 use crate::vm::Vm;
 
-pub(crate) fn println(_vm: &mut Vm, args: Vec<ObjP>) -> ObjP {
-    println!("{}", args[0]);
-    NilInst::create()
+pub(crate) fn println(vm: &mut Vm, args: Vec<ObjP>) -> ObjP {
+    println!("{}", args[0].borrow());
+    vm.create_nil()
 }
 
-pub(crate) fn print(_vm: &mut Vm, args: Vec<ObjP>) -> ObjP {
-    print!("{}", args[0]);
-    NilInst::create()
+pub(crate) fn print(vm: &mut Vm, args: Vec<ObjP>) -> ObjP {
+    print!("{}", args[0].borrow());
+    vm.create_nil()
+}
+
+pub fn init_builtins(builtins: &mut HashMap<String, ObjP>) {
+    macro_rules! builtins {
+        ($($builtin:ident / $argc:expr),* $(,)?) => {
+            $({
+                let builtin_function = builtins.create_builtin_function(stringify!($builtin), $builtin, $argc);
+                builtins.insert(
+                    stringify!($builtin).to_string(),
+                    builtin_function
+                );
+            })*
+        }
+    }
+
+    builtins! {
+        print/1,
+        println/1,
+    }
 }
diff --git a/src/compiler.rs b/src/compiler.rs
index acbe82c..1717374 100644
--- a/src/compiler.rs
+++ b/src/compiler.rs
@@ -1,6 +1,7 @@
 use std::collections::{HashMap, HashSet};
 use std::fmt::{self, Display};
-use std::sync::{Arc, LazyLock};
+use std::rc::Rc;
+use std::sync::LazyLock;
 
 use assert_matches::assert_matches;
 use common_macros::hash_map;
@@ -405,25 +406,25 @@ impl Display for CompileError {
 ////////////////////////////////////////////////////////////////////////////////
 
 #[derive(Debug)]
-pub struct Compiler {
+pub struct Compiler<'b> {
     chunks: Vec<Chunk>,
     scopes: Vec<Scope>,
     constants: Vec<ObjP>,
     globals: Vec<String>,
+    builtins: &'b HashMap<String, ObjP>,
 }
 
-impl Default for Compiler {
-    fn default() -> Self {
+impl<'b> Compiler<'b> {
+    pub fn new(builtins: &'b HashMap<String, ObjP>) -> Self {
         Compiler {
             chunks: Default::default(),
             scopes: Default::default(),
             constants: Default::default(),
-            globals: vec!["print".to_string(), "println".to_string()],
+            globals: builtins.keys().map(ToString::to_string).collect(),
+            builtins,
         }
     }
-}
 
-impl Compiler {
     fn chunk(&self) -> &Chunk {
         self.chunks.last().expect("no chunk")
     }
@@ -478,7 +479,7 @@ impl Compiler {
         // simple interning - try to find a constant that is exactly equal to this one and just
         // return its value instead
         for (index, interned) in self.constants.iter().enumerate() {
-            if constant.equals(interned.as_ref()) {
+            if constant.borrow().equals(&*interned.borrow()) {
                 return Ok(index as ConstantId);
             }
         }
@@ -491,6 +492,8 @@ impl Compiler {
             }
             .into());
         }
+
+        // convert this to a pointer, upcast, and then re-GC
         self.constants.push(constant);
         Ok(index as ConstantId)
     }
@@ -629,7 +632,7 @@ impl Compiler {
     }
 }
 
-impl StmtVisitor for Compiler {
+impl StmtVisitor for Compiler<'_> {
     fn visit_expr_stmt(&mut self, stmt: &ExprStmt) -> Result<()> {
         self.compile_expr(&stmt.expr)?;
         self.emit(stmt_line_number(stmt), Op::Pop);
@@ -661,9 +664,13 @@ impl StmtVisitor for Compiler {
         // TODO - maybe this would be smarter to set up in the AST. I'm 99% sure that the last
         // object created, if it were a function object, will be what we're assigning it to, but I
         // want to be 100% sure instead of 99%.
-        let obj = Arc::get_mut(self.constants.last_mut().unwrap()).unwrap();
-        if let Some(fun) = obj.as_any_mut().downcast_mut::<UserFunctionInst>() {
-            fun.set_name(Arc::new(name.to_string()));
+        let obj = self.constants.last().unwrap().as_ref();
+        if let Some(fun) = obj
+            .borrow_mut()
+            .as_any_mut()
+            .downcast_mut::<UserFunctionInst>()
+        {
+            fun.set_name(Rc::new(name.to_string()));
         }
 
         Ok(())
@@ -671,7 +678,7 @@ impl StmtVisitor for Compiler {
 
     fn visit_set_stmt(&mut self, stmt: &SetStmt) -> Result<()> {
         self.compile_expr(&stmt.expr)?;
-        let name = self.insert_constant(StrInst::create(&stmt.name.text))?;
+        let name = self.insert_constant(self.create_str(&stmt.name.text))?;
         self.compile_expr(&stmt.rhs)?;
         self.emit(stmt_line_number(stmt), Op::SetAttr(name));
         Ok(())
@@ -690,7 +697,7 @@ impl StmtVisitor for Compiler {
         if let Some(expr) = &stmt.expr {
             self.compile_expr(expr)?;
         } else {
-            let nil = self.insert_constant(NilInst::create())?;
+            let nil = self.insert_constant(self.create_nil())?;
             self.emit(stmt_line_number(stmt), Op::PushConstant(nil));
         }
         Ok(())
@@ -699,7 +706,7 @@ impl StmtVisitor for Compiler {
         // condition
         self.compile_expr(&stmt.condition)?;
         // call obj.__bool__()
-        let bool_attr = self.insert_constant(StrInst::create("__bool__"))?;
+        let bool_attr = self.insert_constant(self.create_str("__bool__"))?;
         self.emit(expr_line_number(&*stmt.condition), Op::GetAttr(bool_attr));
         self.emit(expr_line_number(&*stmt.condition), Op::Call(0));
         let condition_patch_index = self.chunk().code.len();
@@ -746,7 +753,7 @@ impl StmtVisitor for Compiler {
     }
 }
 
-impl ExprVisitor for Compiler {
+impl ExprVisitor for Compiler<'_> {
     fn visit_binary_expr(&mut self, expr: &BinaryExpr) -> Result<()> {
         static OP_NAMES: LazyLock<HashMap<TokenKind, &'static str>> = LazyLock::new(|| {
             hash_map! {
@@ -771,7 +778,7 @@ impl ExprVisitor for Compiler {
         let mut exit_patch_index = 0;
 
         if let TokenKind::And | TokenKind::Or = expr.op.kind {
-            let constant_id = self.insert_constant(StrInst::create("__bool__"))?;
+            let constant_id = self.insert_constant(self.create_str("__bool__"))?;
             self.emit(expr_line_number(&*expr.lhs), Op::GetAttr(constant_id));
             self.emit(expr_line_number(&*expr.lhs), Op::Call(0));
             exit_patch_index = self.chunk().code.len();
@@ -785,12 +792,12 @@ impl ExprVisitor for Compiler {
         let name = OP_NAMES
             .get(&expr.op.kind)
             .expect("invalid binary operator");
-        let constant_id = self.insert_constant(StrInst::create(name))?;
+        let constant_id = self.insert_constant(self.create_str(name))?;
         self.emit(expr_line_number(expr), Op::GetAttr(constant_id));
 
         // convert RHS to a bool if we're doing AND or OR
         if let TokenKind::And | TokenKind::Or = expr.op.kind {
-            let constant_id = self.insert_constant(StrInst::create("__bool__"))?;
+            let constant_id = self.insert_constant(self.create_str("__bool__"))?;
             self.emit(expr_line_number(&*expr.rhs), Op::GetAttr(constant_id));
             self.emit(expr_line_number(&*expr.rhs), Op::Call(0));
         }
@@ -828,7 +835,7 @@ impl ExprVisitor for Compiler {
         });
         self.compile_expr(&expr.expr)?;
         let name = OP_NAMES.get(&expr.op.kind).expect("invalid unary operator");
-        let constant_id = self.insert_constant(StrInst::create(name))?;
+        let constant_id = self.insert_constant(self.create_str(name))?;
         self.emit(expr_line_number(expr), Op::GetAttr(constant_id));
         self.emit(expr_line_number(expr), Op::Call(0));
         Ok(())
@@ -852,7 +859,7 @@ impl ExprVisitor for Compiler {
 
     fn visit_get_expr(&mut self, expr: &GetExpr) -> Result<()> {
         self.compile_expr(&expr.expr)?;
-        let constant_id = self.insert_constant(StrInst::create(&expr.name.text))?;
+        let constant_id = self.insert_constant(self.create_str(&expr.name.text))?;
         self.emit(expr_line_number(expr), Op::GetAttr(constant_id));
         Ok(())
     }
@@ -878,25 +885,25 @@ impl ExprVisitor for Compiler {
             }
             TokenKind::Number => {
                 let obj = if expr.token.text.contains('.') {
-                    FloatInst::create(expr.token.text.parse().unwrap()) as ObjP
+                    self.create_float(expr.token.text.parse().unwrap())
                 } else {
-                    IntInst::create(expr.token.text.parse().unwrap()) as ObjP
+                    self.create_int(expr.token.text.parse().unwrap())
                 };
                 let constant_id = self.insert_constant(obj)?;
                 self.emit(expr_line_number(expr), Op::PushConstant(constant_id));
             }
             TokenKind::String => {
                 let constant_id =
-                    self.insert_constant(StrInst::create(unescape(&expr.token.text)))?;
+                    self.insert_constant(self.create_str(unescape(&expr.token.text)))?;
                 self.emit(expr_line_number(expr), Op::PushConstant(constant_id));
             }
             TokenKind::True | TokenKind::False => {
                 let constant_id =
-                    self.insert_constant(BoolInst::create(expr.token.kind == TokenKind::True))?;
+                    self.insert_constant(self.create_bool(expr.token.kind == TokenKind::True))?;
                 self.emit(expr_line_number(expr), Op::PushConstant(constant_id));
             }
             TokenKind::Nil => {
-                let constant_id = self.insert_constant(NilInst::create())?;
+                let constant_id = self.insert_constant(self.create_nil())?;
                 self.emit(expr_line_number(expr), Op::PushConstant(constant_id));
             }
             _ => unreachable!(),
@@ -954,7 +961,7 @@ impl ExprVisitor for Compiler {
         }
 
         // always end with a "return nil"
-        let nil = self.insert_constant(NilInst::create())?;
+        let nil = self.insert_constant(self.create_nil())?;
         self.emit(end_line, Op::PushConstant(nil));
         self.emit(end_line, Op::Return);
 
@@ -962,7 +969,7 @@ impl ExprVisitor for Compiler {
 
         // create the function
         let chunk = self.chunks.pop().unwrap();
-        let fun = UserFunctionInst::create(chunk, expr.params.len() as Argc);
+        let fun = self.create_user_function(chunk, expr.params.len() as Argc);
 
         // register the function as a constant
         let fun_constant = self.insert_constant(fun)?;
@@ -982,3 +989,9 @@ impl ExprVisitor for Compiler {
         Ok(())
     }
 }
+
+impl ObjFactory for Compiler<'_> {
+    fn builtins(&self) -> &HashMap<String, ObjP> {
+        &self.builtins
+    }
+}
diff --git a/src/disassemble.rs b/src/disassemble.rs
index 55fd212..864d10d 100644
--- a/src/disassemble.rs
+++ b/src/disassemble.rs
@@ -33,7 +33,7 @@ fn disassemble_chunk(chunk: &Chunk, constants: &Vec<ObjP>, globals: &Vec<String>
             }
             Op::PushConstant(constant_id) => {
                 op_str = "PUSH_CONSTANT";
-                arg = format!("{}", &constants[*constant_id as usize]);
+                arg = format!("{}", &constants[*constant_id as usize].borrow());
                 info = format!("(constant ID {constant_id})");
             }
             Op::GetLocal(local_id) => {
@@ -60,12 +60,12 @@ fn disassemble_chunk(chunk: &Chunk, constants: &Vec<ObjP>, globals: &Vec<String>
             }
             Op::GetAttr(constant_id) => {
                 op_str = "GET_ATTR";
-                arg = format!("{}", &constants[*constant_id as usize]);
+                arg = format!("{}", &constants[*constant_id as usize].borrow());
                 info = format!("(constant ID {constant_id})");
             }
             Op::SetAttr(constant_id) => {
                 op_str = "SET_ATTR";
-                arg = format!("{}", &constants[*constant_id as usize]);
+                arg = format!("{}", &constants[*constant_id as usize].borrow());
                 info = format!("(constant ID {constant_id})");
             }
             Op::Jump(jump_offset) => {
@@ -145,7 +145,11 @@ pub fn disassemble(chunk: &Chunk, constants: &Vec<ObjP>, globals: &Vec<String>)
     disassemble_chunk(chunk, constants, globals);
 
     for constant in constants {
-        if let Some(fun) = constant.as_any().downcast_ref::<UserFunctionInst>() {
+        if let Some(fun) = constant
+            .borrow()
+            .as_any()
+            .downcast_ref::<UserFunctionInst>()
+        {
             println!();
             println!(
                 "== {} starting on line {}",
diff --git a/src/main.rs b/src/main.rs
index 819c9c8..73e05a8 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,7 @@
 // trait_upcasting - https://github.com/rust-lang/rust/issues/65991
 // stabilization in progress
 #![feature(trait_upcasting)]
+#![feature(coerce_unsized)]
 
 mod ast;
 mod builtins;
@@ -11,6 +12,7 @@ mod parser;
 mod token;
 mod vm;
 
+use std::collections::HashMap;
 use std::fmt;
 use std::fs::File;
 use std::io::prelude::*;
@@ -52,10 +54,12 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     }
 
     // initialize type system
-    obj::init_types();
+    let mut builtins = HashMap::new();
+    obj::init_types(&mut builtins);
+    crate::builtins::init_builtins(&mut builtins);
 
     // compile
-    let (chunk, constants, globals) = compiler::Compiler::default().compile(&ast)?;
+    let (chunk, constants, globals) = compiler::Compiler::new(&builtins).compile(&ast)?;
 
     if args.disassemble {
         disassemble::disassemble(&chunk, &constants, &globals);
@@ -63,7 +67,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     }
 
     // run
-    let mut vm = vm::Vm::new(chunk.into(), constants, globals);
+    let mut vm = vm::Vm::new(chunk.into(), constants, globals, builtins);
     vm.run();
 
     Ok(())
diff --git a/src/obj.rs b/src/obj.rs
index f562e5f..b797f8e 100644
--- a/src/obj.rs
+++ b/src/obj.rs
@@ -5,22 +5,23 @@ use std::any::Any;
 use std::collections::HashMap;
 use std::fmt::{self, Debug, Display};
 use std::ptr;
-use std::sync::{Arc, LazyLock, Mutex, RwLock};
+use std::rc::Rc;
 
-use common_macros::hash_map;
+use gc::{Finalize, Gc, GcCell, Trace};
 
 use crate::vm::{Argc, Chunk, Frame, Vm};
 
-pub type Ptr<T> = Arc<T>;
-pub type ObjP = Ptr<dyn Obj + 'static>;
-pub type Attrs = RwLock<HashMap<String, ObjP>>;
+pub type Ptr<T> = Gc<GcCell<T>>;
+pub type ObjP = Ptr<dyn Obj>;
+pub type Attrs = HashMap<String, ObjP>;
 
 /// Downcast an object pointer to a concrete type, and do something with that object.
 pub fn with_obj_downcast<T, Out>(ptr: ObjP, closure: impl FnOnce(&T) -> Out) -> Out
 where
     T: Obj + 'static,
 {
-    if let Some(obj) = ptr.as_any().downcast_ref::<T>() {
+    let borrowed = ptr.borrow();
+    if let Some(obj) = borrowed.as_any().downcast_ref::<T>() {
         closure(obj)
     } else {
         panic!(
@@ -35,115 +36,136 @@ pub fn obj_is_inst<T>(ptr: &ObjP) -> bool
 where
     T: Obj + 'static,
 {
-    ptr.as_any().downcast_ref::<T>().is_some()
+    let borrowed = ptr.borrow();
+    borrowed.as_any().downcast_ref::<T>().is_some()
 }
 
-/// Builtin types macro
-macro_rules! builtin_types {
-    (
-        $(
-            $type_name:ident => { $( $vtable_name:ident => $vtable_value:expr ),* $(,)? }
-        ),+ $(,)?
-    ) => {
-        pub static TYPES: LazyLock<RwLock<HashMap<String, Ptr<TypeInst>>>> = LazyLock::new(|| {
-            RwLock::new(hash_map! {
-                $(
-                    stringify!($type_name).to_string() => make_ptr(TypeInst::new(stringify!($type_name)))
-                ),+
-            })
-        });
-
-        static TYPE_SYSTEM_INIT: LazyLock<Mutex<bool>> = LazyLock::new(|| Mutex::new(false));
-
-        /// Initialize types.
-        ///
-        /// This should only be called once.
-        fn init_base_types() {
-            // instantiate
-            $(
-                if stringify!($type_name) != "Type" {
-                    let mut types_ptr = TYPES.try_write().unwrap();
-                    let mut ty = types_ptr.get_mut(stringify!($type_name)).unwrap();
-                    Arc::get_mut(&mut ty).unwrap().instantiate();
-                }
-            )+
-
-            // __name__
-            $(
-                {
-                    let name = StrInst::create(stringify!($type_name));
-                    let ty = Ptr::clone(&TYPES.try_read().unwrap()[stringify!($type_name)]);
-                    ty.set_attr("__name__", name);
-                }
-            )+
-
-            // vtable
-            $(
-                {
-                    let ptr = Ptr::clone(&TYPES.try_write().unwrap()[stringify!($type_name)]);
-                    $(
-                        ptr.vtable.insert($vtable_name.into(), $vtable_value);
-                    )*
-                }
-            )+
-        }
-    };
+pub fn upcast_obj<T: Obj>(ptr: Ptr<T>) -> ObjP {
+    unsafe {
+        let ptr = Ptr::into_raw(ptr) as *const GcCell<dyn Obj>;
+        Ptr::from_raw(ptr)
+    }
 }
 
-pub(crate) fn init_types() {
-    // Taking the lock here will lock the entire function from being run twice
-    // simulataneously and prevent race conditions.
-    //
-    // Race conditions really can only happen during testing so this is just a precaution.
-    let mut lock_guard = TYPE_SYSTEM_INIT.lock().unwrap();
-    if *lock_guard {
-        if cfg!(test) {
-            return;
-        } else {
-            panic!("do not initialize type system twice");
-        }
+pub fn init_types(builtins: &mut HashMap<String, ObjP>) {
+    #![allow(non_snake_case)]
+    macro_rules! types {
+        (base_type: $base_type:ident, $($name:ident),* $(,)?) => {
+            $(
+                let $name = make_ptr(TypeInst::new(stringify!($name)));
+            )*
+
+            $(
+                $name.borrow_mut().instantiate(upcast_obj(Gc::clone(&$base_type)));
+            )*
+
+            $(
+                builtins.insert(stringify!($name).to_string(), upcast_obj($name));
+            )*
+        };
+    }
+    types! {
+        // base type
+        base_type: Type,
+        // type definitions
+        Type,
+        Obj,
+        Str,
+        Int,
+        Float,
+        Bool,
+        Nil,
+        BuiltinFunction,
+        UserFunction,
+        Method,
+    }
+}
+
+pub trait ObjFactory {
+    fn builtins(&self) -> &HashMap<String, ObjP>;
+
+    fn create_obj(&self) -> ObjP {
+        upcast_obj(ObjInst::create(ObjP::clone(
+            self.builtins().get("Obj").unwrap(),
+        )))
     }
 
-    // Init type_type here
-    {
-        let types_ptr = TYPES.try_read().unwrap();
-        let type_ptr = types_ptr.get("Type").unwrap();
-        type_ptr.set_attr(
-            "__type__",
-            Ptr::clone(&TYPES.try_read().unwrap()["Type"]) as ObjP,
-        );
-        drop(types_ptr);
-        //let mut types_ptr = TYPES.try_write().unwrap();
-        //let mut type_ptr = types_ptr.get_mut("Type").unwrap();
-        //Arc::get_mut(&mut type_ptr).unwrap().base.is_instantiated = true;
+    fn create_type(&self, name: impl ToString) -> ObjP {
+        upcast_obj(TypeInst::create(
+            ObjP::clone(self.builtins().get("Type").unwrap()),
+            name,
+        ))
     }
 
-    // Init the rest of the types
-    init_base_types();
+    fn create_str(&self, str_value: impl ToString) -> ObjP {
+        upcast_obj(StrInst::create(
+            ObjP::clone(self.builtins().get("Str").unwrap()),
+            str_value,
+        ))
+    }
 
-    *lock_guard = true;
+    fn create_int(&self, int_value: i64) -> ObjP {
+        upcast_obj(IntInst::create(
+            ObjP::clone(self.builtins().get("Int").unwrap()),
+            int_value,
+        ))
+    }
+
+    fn create_float(&self, float_value: f64) -> ObjP {
+        upcast_obj(FloatInst::create(
+            ObjP::clone(self.builtins().get("Float").unwrap()),
+            float_value,
+        ))
+    }
+
+    fn create_bool(&self, bool_value: bool) -> ObjP {
+        upcast_obj(BoolInst::create(
+            ObjP::clone(self.builtins().get("Bool").unwrap()),
+            bool_value,
+        ))
+    }
+
+    fn create_nil(&self) -> ObjP {
+        upcast_obj(NilInst::create(ObjP::clone(
+            self.builtins().get("Nil").unwrap(),
+        )))
+    }
+
+    fn create_builtin_function(
+        &self,
+        name: impl ToString,
+        function: BuiltinFunctionPtr,
+        arity: Argc,
+    ) -> ObjP {
+        upcast_obj(BuiltinFunctionInst::create(
+            ObjP::clone(self.builtins().get("BuiltinFunction").unwrap()),
+            name,
+            function,
+            arity,
+        ))
+    }
+
+    fn create_user_function(&self, chunk: Chunk, arity: Argc) -> ObjP {
+        upcast_obj(UserFunctionInst::create(
+            ObjP::clone(self.builtins().get("UserFunction").unwrap()),
+            chunk,
+            arity,
+        ))
+    }
+
+    fn create_method(&self, self_binding: ObjP, function: ObjP) -> ObjP {
+        upcast_obj(MethodInst::create(
+            ObjP::clone(self.builtins().get("Method").unwrap()),
+            self_binding,
+            function,
+        ))
+    }
 }
 
-fn placeholder(_: &mut Vm, _: Vec<ObjP>) -> ObjP {
-    NilInst::create()
-}
-
-fn to_string(_: &mut Vm, args: Vec<ObjP>) -> ObjP {
-    let str_value = format!("{}", args[0]);
-    StrInst::create(str_value)
-}
-
-builtin_types! {
-    Type => {},
-    Obj => {},
-    Str => {},
-    Int => {},
-    Float => {},
-    Bool => {},
-    Nil => {},
-    BuiltinFunction => {},
-    UserFunction => {},
-    Method => {},
+impl ObjFactory for HashMap<String, ObjP> {
+    fn builtins(&self) -> &Self {
+        self
+    }
 }
 
 /// Convenience function for creating pointers, in case the `Arc<RwLock<T>>` pointer type has to
@@ -152,36 +174,35 @@ builtin_types! {
 /// I would implement this as a `From<T>` but it doesn't seem to work for a foreign type, and I'm
 /// not sure why.
 pub fn make_ptr<T: Obj>(obj: T) -> Ptr<T> {
-    Arc::new(obj)
+    Ptr::new(GcCell::new(obj))
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // Obj
 ////////////////////////////////////////////////////////////////////////////////
 
-pub trait Obj: Debug + Display + Any + Send + Sync {
-    fn instantiate(&mut self);
+pub trait Obj: Debug + Display + Any + Trace {
+    fn instantiate(&mut self, ty: ObjP);
 
     fn is_instantiated(&self) -> bool;
     fn attrs(&self) -> &Attrs;
+    fn attrs_mut(&mut self) -> &mut Attrs;
 
-    fn set_attr(&self, name: &str, value: ObjP) {
-        let mut borrowed = self.attrs().try_write().unwrap();
-        borrowed.insert(name.to_string(), value);
+    fn set_attr(&mut self, name: &str, value: ObjP) {
+        self.attrs_mut().insert(name.to_string(), value);
     }
 
     fn get_attr(&self, name: &str) -> Option<ObjP> {
-        let borrowed = self.attrs().try_read().unwrap();
-        borrowed.get(name).map(Arc::clone)
+        self.attrs().get(name).map(Ptr::clone)
     }
 
     fn type_inst(&self) -> ObjP {
         self.get_attr("__type__").unwrap()
     }
 
-    fn type_name(&self) -> Arc<String> {
+    fn type_name(&self) -> Rc<String> {
         with_obj_downcast(self.type_inst(), |type_inst: &TypeInst| {
-            Arc::clone(&type_inst.name)
+            Rc::clone(&type_inst.name)
         })
     }
 
@@ -210,16 +231,20 @@ pub trait Obj: Debug + Display + Any + Send + Sync {
 // BaseObjInst
 ////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug, Default)]
+#[derive(Debug, Default, Trace)]
 struct BaseObjInst {
-    attrs: RwLock<HashMap<String, ObjP>>,
+    attrs: Attrs,
     is_instantiated: bool,
 }
 
+impl Finalize for BaseObjInst {
+    fn finalize(&self) {}
+}
+
 impl Clone for BaseObjInst {
     fn clone(&self) -> Self {
         Self {
-            attrs: RwLock::new(self.attrs.try_read().unwrap().clone()),
+            attrs: self.attrs.clone(),
             is_instantiated: self.is_instantiated,
         }
     }
@@ -232,13 +257,8 @@ impl Display for BaseObjInst {
 }
 
 impl Obj for BaseObjInst {
-    fn instantiate(&mut self) {
-        if self.get_attr("__type__").is_none() {
-            self.set_attr(
-                "__type__",
-                Ptr::clone(&TYPES.try_read().unwrap()["Obj"]) as ObjP,
-            );
-        }
+    fn instantiate(&mut self, ty: ObjP) {
+        self.set_attr("__type__", ty);
 
         // TODO BaseObjInst::instantiate - instantiate VTable
         // Okay, we are running into a little snag here:
@@ -275,15 +295,18 @@ impl Obj for BaseObjInst {
         &self.attrs
     }
 
+    fn attrs_mut(&mut self) -> &mut Attrs {
+        &mut self.attrs
+    }
+
     fn equals(&self, other: &dyn Obj) -> bool {
         if let Some(other) = other.as_any().downcast_ref::<BaseObjInst>() {
             // compare all attrs
-            let borrowed = self.attrs.try_read().unwrap();
-            borrowed.iter().all(|(k1, v1)| {
-                let borrowed = other.attrs.try_read().unwrap();
-                borrowed
+            self.attrs.iter().all(|(k1, v1)| {
+                other
+                    .attrs
                     .get(k1)
-                    .map(|v2| v2.equals(v1.as_ref()))
+                    .map(|v2| v2.borrow().equals(&*v1.borrow()))
                     .unwrap_or(false)
             }) && self.is_instantiated == other.is_instantiated
         } else {
@@ -310,6 +333,10 @@ macro_rules! impl_base_obj {
             self.$base_name.attrs()
         }
 
+        fn attrs_mut(&mut self) -> &mut Attrs {
+            self.$base_name.attrs_mut()
+        }
+
         fn as_any(&self) -> &dyn Any {
             self
         }
@@ -327,7 +354,7 @@ macro_rules! impl_base_obj {
 // ObjInst
 ////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug)]
+#[derive(Debug, Trace)]
 pub struct ObjInst {
     base: BaseObjInst,
 }
@@ -339,13 +366,17 @@ impl ObjInst {
         }
     }
 
-    pub fn create() -> Ptr<Self> {
+    pub fn create(ty: ObjP) -> Ptr<Self> {
         let mut new = Self::new();
-        new.instantiate();
+        new.instantiate(ty);
         make_ptr(new)
     }
 }
 
+impl Finalize for ObjInst {
+    fn finalize(&self) {}
+}
+
 impl Display for ObjInst {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "<ObjInst at {:x}>", (self as *const _ as usize))
@@ -353,8 +384,8 @@ impl Display for ObjInst {
 }
 
 impl Obj for ObjInst {
-    fn instantiate(&mut self) {
-        self.base.instantiate();
+    fn instantiate(&mut self, ty: ObjP) {
+        self.base.instantiate(ty);
     }
 
     fn equals(&self, other: &dyn Obj) -> bool {
@@ -372,28 +403,34 @@ impl Obj for ObjInst {
 // TypeInst
 ////////////////////////////////////////////////////////////////////////////////
 
+#[derive(Trace)]
 pub struct TypeInst {
-    name: Arc<String>,
+    #[unsafe_ignore_trace]
+    name: Rc<String>,
     base: BaseObjInst,
     vtable: HashMap<String, ObjP>,
 }
 
+impl Finalize for TypeInst {
+    fn finalize(&self) {}
+}
+
 impl TypeInst {
     pub fn new(name: impl ToString) -> Self {
         Self {
-            name: Arc::new(name.to_string()),
+            name: Rc::new(name.to_string()),
             base: Default::default(),
             vtable: Default::default(),
         }
     }
 
-    pub fn create(name: impl ToString) -> Ptr<Self> {
+    pub fn create(ty: ObjP, name: impl ToString) -> Ptr<Self> {
         let mut new = Self::new(name);
-        new.instantiate();
+        new.instantiate(ty);
         make_ptr(new)
     }
 
-    pub fn name(&self) -> &Arc<String> {
+    pub fn name(&self) -> &Rc<String> {
         &self.name
     }
 }
@@ -421,12 +458,8 @@ impl Display for TypeInst {
 }
 
 impl Obj for TypeInst {
-    fn instantiate(&mut self) {
-        self.set_attr(
-            "__type__",
-            Ptr::clone(&TYPES.try_read().unwrap()["Type"]) as ObjP,
-        );
-        self.base.instantiate();
+    fn instantiate(&mut self, ty: ObjP) {
+        self.base.instantiate(ty);
     }
 
     fn equals(&self, other: &dyn Obj) -> bool {
@@ -450,44 +483,45 @@ impl Obj for TypeInst {
 // StrInst
 ////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug)]
+#[derive(Debug, Trace)]
 pub struct StrInst {
-    str_value: Arc<String>,
+    #[unsafe_ignore_trace]
+    str_value: Rc<String>,
     base: BaseObjInst,
 }
 
+impl StrInst {
+    pub fn new(str_value: impl ToString) -> Self {
+        Self {
+            str_value: Rc::new(str_value.to_string()),
+            base: Default::default(),
+        }
+    }
+
+    pub fn create(ty: ObjP, str_value: impl ToString) -> Ptr<Self> {
+        let mut new = Self::new(str_value);
+        new.instantiate(ty);
+        make_ptr(new)
+    }
+
+    pub fn str_value(&self) -> &Rc<String> {
+        &self.str_value
+    }
+}
+
+impl Finalize for StrInst {
+    fn finalize(&self) {}
+}
+
 impl Display for StrInst {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "{}", self.str_value)
     }
 }
 
-impl StrInst {
-    pub fn new(str_value: impl ToString) -> Self {
-        Self {
-            str_value: Arc::new(str_value.to_string()),
-            base: Default::default(),
-        }
-    }
-
-    pub fn create(str_value: impl ToString) -> Ptr<Self> {
-        let mut new = Self::new(str_value);
-        new.instantiate();
-        make_ptr(new)
-    }
-
-    pub fn str_value(&self) -> &Arc<String> {
-        &self.str_value
-    }
-}
-
 impl Obj for StrInst {
-    fn instantiate(&mut self) {
-        self.set_attr(
-            "__type__",
-            Ptr::clone(&TYPES.try_read().unwrap()["Str"]) as ObjP,
-        );
-        self.base.instantiate();
+    fn instantiate(&mut self, ty: ObjP) {
+        self.base.instantiate(ty);
     }
 
     fn is_truthy(&self) -> bool {
@@ -509,7 +543,7 @@ impl Obj for StrInst {
 // IntInst
 ////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug)]
+#[derive(Debug, Trace)]
 pub struct IntInst {
     int_value: i64,
     base: BaseObjInst,
@@ -523,9 +557,9 @@ impl IntInst {
         }
     }
 
-    pub fn create(int_value: i64) -> Ptr<Self> {
+    pub fn create(ty: ObjP, int_value: i64) -> Ptr<Self> {
         let mut new = Self::new(int_value);
-        new.instantiate();
+        new.instantiate(ty);
         make_ptr(new)
     }
 
@@ -534,6 +568,10 @@ impl IntInst {
     }
 }
 
+impl Finalize for IntInst {
+    fn finalize(&self) {}
+}
+
 impl Display for IntInst {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "{}", self.int_value)
@@ -541,12 +579,8 @@ impl Display for IntInst {
 }
 
 impl Obj for IntInst {
-    fn instantiate(&mut self) {
-        self.set_attr(
-            "__type__",
-            Ptr::clone(&TYPES.try_read().unwrap()["Int"]) as ObjP,
-        );
-        self.base.instantiate();
+    fn instantiate(&mut self, ty: ObjP) {
+        self.base.instantiate(ty);
     }
 
     fn is_truthy(&self) -> bool {
@@ -570,7 +604,7 @@ impl Obj for IntInst {
 // FloatInst
 ////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug)]
+#[derive(Debug, Trace)]
 pub struct FloatInst {
     float_value: f64,
     base: BaseObjInst,
@@ -584,9 +618,9 @@ impl FloatInst {
         }
     }
 
-    pub fn create(float_value: f64) -> Ptr<Self> {
+    pub fn create(ty: ObjP, float_value: f64) -> Ptr<Self> {
         let mut new = Self::new(float_value);
-        new.instantiate();
+        new.instantiate(ty);
         make_ptr(new)
     }
 
@@ -595,6 +629,10 @@ impl FloatInst {
     }
 }
 
+impl Finalize for FloatInst {
+    fn finalize(&self) {}
+}
+
 impl Display for FloatInst {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "{}", self.float_value)
@@ -602,12 +640,8 @@ impl Display for FloatInst {
 }
 
 impl Obj for FloatInst {
-    fn instantiate(&mut self) {
-        self.set_attr(
-            "__type__",
-            Ptr::clone(&TYPES.try_read().unwrap()["Float"]) as ObjP,
-        );
-        self.base.instantiate();
+    fn instantiate(&mut self, ty: ObjP) {
+        self.base.instantiate(ty);
     }
 
     fn is_truthy(&self) -> bool {
@@ -631,7 +665,7 @@ impl Obj for FloatInst {
 // BoolInst
 ////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug)]
+#[derive(Debug, Trace)]
 pub struct BoolInst {
     bool_value: bool,
     base: BaseObjInst,
@@ -645,10 +679,10 @@ impl BoolInst {
         }
     }
 
-    pub fn create(bool_value: bool) -> Ptr<Self> {
+    pub fn create(ty: ObjP, bool_value: bool) -> Ptr<Self> {
         // TODO BoolInst::create : interning
         let mut new = Self::new(bool_value);
-        new.instantiate();
+        new.instantiate(ty);
         make_ptr(new)
     }
 
@@ -657,6 +691,10 @@ impl BoolInst {
     }
 }
 
+impl Finalize for BoolInst {
+    fn finalize(&self) {}
+}
+
 impl Display for BoolInst {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "{}", self.bool_value)
@@ -664,12 +702,8 @@ impl Display for BoolInst {
 }
 
 impl Obj for BoolInst {
-    fn instantiate(&mut self) {
-        self.set_attr(
-            "__type__",
-            Ptr::clone(&TYPES.try_read().unwrap()["Bool"]) as ObjP,
-        );
-        self.base.instantiate();
+    fn instantiate(&mut self, ty: ObjP) {
+        self.base.instantiate(ty);
     }
 
     fn is_truthy(&self) -> bool {
@@ -691,7 +725,7 @@ impl Obj for BoolInst {
 // NilInst
 ////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug, Default)]
+#[derive(Debug, Default, Trace)]
 pub struct NilInst {
     base: BaseObjInst,
 }
@@ -701,14 +735,18 @@ impl NilInst {
         Default::default()
     }
 
-    pub fn create() -> Ptr<Self> {
+    pub fn create(ty: ObjP) -> Ptr<Self> {
         // TODO NilInst::create : interning
         let mut new = Self::new();
-        new.instantiate();
+        new.instantiate(ty);
         make_ptr(new)
     }
 }
 
+impl Finalize for NilInst {
+    fn finalize(&self) {}
+}
+
 impl Display for NilInst {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "nil")
@@ -716,12 +754,8 @@ impl Display for NilInst {
 }
 
 impl Obj for NilInst {
-    fn instantiate(&mut self) {
-        self.set_attr(
-            "__type__",
-            Ptr::clone(&TYPES.try_read().unwrap()["Nil"]) as ObjP,
-        );
-        self.base.instantiate();
+    fn instantiate(&mut self, ty: ObjP) {
+        self.base.instantiate(ty);
     }
 
     fn is_truthy(&self) -> bool {
@@ -741,27 +775,33 @@ impl Obj for NilInst {
 
 pub type BuiltinFunctionPtr = fn(vm: &mut Vm, args: Vec<ObjP>) -> ObjP;
 
-#[derive(Debug)]
+#[derive(Debug, Trace)]
 pub struct BuiltinFunctionInst {
     base: BaseObjInst,
     name: String,
+    #[unsafe_ignore_trace]
     function: BuiltinFunctionPtr,
     arity: Argc,
 }
 
 impl BuiltinFunctionInst {
-    pub fn new(name: String, function: BuiltinFunctionPtr, arity: Argc) -> Self {
+    pub fn new(name: impl ToString, function: BuiltinFunctionPtr, arity: Argc) -> Self {
         Self {
             base: Default::default(),
-            name,
+            name: name.to_string(),
             function,
             arity,
         }
     }
 
-    pub fn create(name: String, function: BuiltinFunctionPtr, arity: Argc) -> Ptr<Self> {
+    pub fn create(
+        ty: ObjP,
+        name: impl ToString,
+        function: BuiltinFunctionPtr,
+        arity: Argc,
+    ) -> Ptr<Self> {
         let mut new = Self::new(name, function, arity);
-        new.instantiate();
+        new.instantiate(ty);
         make_ptr(new)
     }
 
@@ -770,6 +810,10 @@ impl BuiltinFunctionInst {
     }
 }
 
+impl Finalize for BuiltinFunctionInst {
+    fn finalize(&self) {}
+}
+
 impl Display for BuiltinFunctionInst {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(
@@ -783,12 +827,8 @@ impl Display for BuiltinFunctionInst {
 }
 
 impl Obj for BuiltinFunctionInst {
-    fn instantiate(&mut self) {
-        self.set_attr(
-            "__type__",
-            Ptr::clone(&TYPES.try_read().unwrap()["UserFunction"]) as ObjP,
-        );
-        self.base.instantiate();
+    fn instantiate(&mut self, ty: ObjP) {
+        self.base.instantiate(ty);
     }
 
     fn arity(&self) -> Option<Argc> {
@@ -825,11 +865,13 @@ impl Obj for BuiltinFunctionInst {
 // UserFunctionInst
 ////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Trace)]
 pub struct UserFunctionInst {
     base: BaseObjInst,
-    name: Arc<String>,
-    chunk: Arc<Chunk>,
+    #[unsafe_ignore_trace]
+    name: Rc<String>,
+    #[unsafe_ignore_trace]
+    chunk: Rc<Chunk>,
     arity: Argc,
     captures: Vec<ObjP>,
 }
@@ -838,16 +880,16 @@ impl UserFunctionInst {
     pub fn new(chunk: Chunk, arity: Argc) -> Self {
         Self {
             base: Default::default(),
-            name: Arc::new("(anonymous)".to_string()),
-            chunk: Arc::new(chunk),
+            name: Rc::new("(anonymous)".to_string()),
+            chunk: Rc::new(chunk),
             arity,
             captures: Default::default(),
         }
     }
 
-    pub fn create(chunk: Chunk, arity: Argc) -> Ptr<Self> {
+    pub fn create(ty: ObjP, chunk: Chunk, arity: Argc) -> Ptr<Self> {
         let mut new = Self::new(chunk, arity);
-        new.instantiate();
+        new.instantiate(ty);
         make_ptr(new)
     }
 
@@ -855,7 +897,7 @@ impl UserFunctionInst {
         &self.name
     }
 
-    pub fn set_name(&mut self, name: Arc<String>) {
+    pub fn set_name(&mut self, name: Rc<String>) {
         self.name = name;
     }
 
@@ -868,6 +910,10 @@ impl UserFunctionInst {
     }
 }
 
+impl Finalize for UserFunctionInst {
+    fn finalize(&self) {}
+}
+
 impl Display for UserFunctionInst {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(
@@ -881,12 +927,8 @@ impl Display for UserFunctionInst {
 }
 
 impl Obj for UserFunctionInst {
-    fn instantiate(&mut self) {
-        self.set_attr(
-            "__type__",
-            Ptr::clone(&TYPES.try_read().unwrap()["UserFunction"]) as ObjP,
-        );
-        self.base.instantiate();
+    fn instantiate(&mut self, ty: ObjP) {
+        self.base.instantiate(ty);
     }
 
     fn arity(&self) -> Option<Argc> {
@@ -896,8 +938,8 @@ impl Obj for UserFunctionInst {
     fn call(&self, vm: &mut Vm, argc: Argc) {
         assert_eq!(argc, self.arity, "argc must match arity");
         let new_frame = Frame {
-            name: Arc::clone(&self.name),
-            chunk: Arc::clone(&self.chunk),
+            name: Rc::clone(&self.name),
+            chunk: Rc::clone(&self.chunk),
             ip: 0,
             stack_base: vm.stack().len() - (argc as usize),
         };
@@ -923,7 +965,7 @@ impl Obj for UserFunctionInst {
 // MethodInst
 ////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug)]
+#[derive(Debug, Trace)]
 pub struct MethodInst {
     base: BaseObjInst,
     self_binding: ObjP,
@@ -939,9 +981,9 @@ impl MethodInst {
         }
     }
 
-    pub fn create(self_binding: ObjP, function: ObjP) -> Ptr<Self> {
+    pub fn create(ty: ObjP, self_binding: ObjP, function: ObjP) -> Ptr<Self> {
         let mut new = Self::new(self_binding, function);
-        new.instantiate();
+        new.instantiate(ty);
         make_ptr(new)
     }
 
@@ -950,27 +992,27 @@ impl MethodInst {
     }
 }
 
+impl Finalize for MethodInst {
+    fn finalize(&self) {}
+}
+
 impl Display for MethodInst {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        write!(fmt, "{}", self.function)
+        write!(fmt, "{}", self.function.borrow())
     }
 }
 
 impl Obj for MethodInst {
-    fn instantiate(&mut self) {
-        self.set_attr(
-            "__type__",
-            Ptr::clone(&TYPES.try_read().unwrap()["Method"]) as ObjP,
-        );
-        self.base.instantiate();
+    fn instantiate(&mut self, ty: ObjP) {
+        self.base.instantiate(ty);
     }
 
     fn arity(&self) -> Option<Argc> {
-        self.function.arity()
+        self.function.borrow().arity()
     }
 
     fn call(&self, vm: &mut Vm, argc: Argc) {
-        self.function.call(vm, argc)
+        self.function.borrow().call(vm, argc)
     }
 
     fn equals(&self, other: &dyn Obj) -> bool {
@@ -991,58 +1033,60 @@ impl Obj for MethodInst {
 
 #[test]
 fn test_new_objects() {
-    init_types();
+    let mut builtins = HashMap::new();
+    init_types(&mut builtins);
 
-    let type_value = TypeInst::create("Type");
-    assert_eq!(&*type_value.type_name(), "Type");
+    let type_value = builtins.create_type("Type");
+    assert_eq!(&*type_value.borrow().type_name(), "Type");
 
-    let str_value = StrInst::create("asdfasdfasdfasdfasdf");
-    assert_eq!(&*str_value.type_name(), "Str");
+    let str_value = builtins.create_str("asdfasdfasdfasdfasdf");
+    assert_eq!(&*str_value.borrow().type_name(), "Str");
 
-    let int_value = IntInst::create(1234);
-    assert_eq!(&*int_value.type_name(), "Int");
+    let int_value = builtins.create_int(1234);
+    assert_eq!(&*int_value.borrow().type_name(), "Int");
 
-    let float_value = FloatInst::create(1234.5678);
-    assert_eq!(&*float_value.type_name(), "Float");
+    let float_value = builtins.create_float(1234.5678);
+    assert_eq!(&*float_value.borrow().type_name(), "Float");
 
-    let nil_value = NilInst::create();
-    assert_eq!(&*nil_value.type_name(), "Nil");
+    let nil_value = builtins.create_nil();
+    assert_eq!(&*nil_value.borrow().type_name(), "Nil");
 }
 
 #[test]
 fn test_obj_equals() {
-    init_types();
+    let mut builtins = HashMap::new();
+    init_types(&mut builtins);
 
-    let int1 = IntInst::create(1234);
-    let int2 = IntInst::create(1234);
+    let int1 = builtins.create_int(1234);
+    let int2 = builtins.create_int(1234);
 
-    assert!(int1.equals(int2.as_ref()));
-    assert!(int2.equals(int1.as_ref()));
+    assert!(int1.borrow().equals(&*int2.borrow()));
+    assert!(int2.borrow().equals(&*int1.borrow()));
 
-    let float1 = FloatInst::create(1234.0);
-    assert!(int1.equals(float1.as_ref()));
-    assert!(float1.equals(int2.as_ref()));
+    let float1 = builtins.create_float(1234.0);
+    assert!(int1.borrow().equals(&*float1.borrow()));
+    assert!(float1.borrow().equals(&*int2.borrow()));
 
     // self-equality
-    let str1 = StrInst::create("1234");
-    assert!(str1.equals(str1.as_ref()));
+    let str1 = builtins.create_str("1234");
+    assert!(str1.borrow().equals(&*str1.borrow()));
 
-    let str2 = StrInst::create("1234");
-    assert!(str1.equals(str2.as_ref()));
-    assert!(str2.equals(str1.as_ref()));
+    let str2 = builtins.create_str("1234");
+    assert!(str1.borrow().equals(&*str2.borrow()));
+    assert!(str2.borrow().equals(&*str1.borrow()));
 
-    assert!(!str1.equals(float1.as_ref()));
-    assert!(!str1.equals(int1.as_ref()));
+    assert!(!str1.borrow().equals(&*float1.borrow()));
+    assert!(!str1.borrow().equals(&*int1.borrow()));
 
-    let obj1 = ObjInst::create();
-    let obj2 = ObjInst::create();
-    assert!(obj1.equals(obj2.as_ref()));
+    let obj1 = builtins.create_obj();
+    let obj2 = builtins.create_obj();
+    assert!(obj1.borrow().equals(&*obj2.borrow()));
 
     // these objects aren't equal anymore
-    obj1.set_attr("my_attr", Ptr::clone(&str2) as ObjP);
-    assert!(!obj1.equals(obj2.as_ref()));
+    obj1.borrow_mut().set_attr("my_attr", ObjP::clone(&str2));
+    assert!(!obj1.borrow().equals(&*obj2.borrow()));
 
     // but now they are!
-    obj2.set_attr("my_attr", Ptr::clone(&str2) as ObjP);
-    assert!(obj2.equals(obj1.as_ref()));
+    obj2.borrow_mut().set_attr("my_attr", ObjP::clone(&str2));
+    assert!(obj2.borrow().equals(&*obj1.borrow()));
 }
diff --git a/src/vm.rs b/src/vm.rs
index e51dff1..49d8a6e 100644
--- a/src/vm.rs
+++ b/src/vm.rs
@@ -1,6 +1,6 @@
-use std::sync::Arc;
+use std::collections::HashMap;
+use std::rc::Rc;
 
-use crate::builtins;
 use crate::obj::*;
 
 #[derive(Debug, Clone, Copy, PartialEq)]
@@ -62,14 +62,14 @@ pub struct Chunk {
 
 #[derive(Debug)]
 pub struct Frame {
-    pub(crate) name: Arc<String>,
-    pub(crate) chunk: Arc<Chunk>,
+    pub(crate) name: Rc<String>,
+    pub(crate) chunk: Rc<Chunk>,
     pub(crate) ip: usize,
     pub(crate) stack_base: usize,
 }
 
 impl Frame {
-    pub fn new(name: Arc<String>, chunk: Arc<Chunk>, stack_base: usize) -> Self {
+    pub fn new(name: Rc<String>, chunk: Rc<Chunk>, stack_base: usize) -> Self {
         Self {
             name,
             chunk,
@@ -85,17 +85,20 @@ pub struct Vm {
     globals: Vec<ObjP>,
     stack: Vec<ObjP>,
     frames: Vec<Frame>,
+    builtins: HashMap<String, ObjP>,
 }
 
 impl Vm {
     /// Create a new virtual machine with the given chunk, constants, and global names.
-    pub fn new(chunk: Arc<Chunk>, constants: Vec<ObjP>, global_names: Vec<String>) -> Self {
+    pub fn new(
+        chunk: Rc<Chunk>,
+        constants: Vec<ObjP>,
+        global_names: Vec<String>,
+        builtins: HashMap<String, ObjP>,
+    ) -> Self {
         // set up globals
-        let nil = NilInst::create();
-        let mut globals: Vec<_> = global_names
-            .iter()
-            .map(|_| Ptr::clone(&nil) as ObjP)
-            .collect();
+        let nil = builtins.create_nil();
+        let mut globals: Vec<_> = global_names.iter().map(|_| ObjP::clone(&nil)).collect();
 
         let mut register_global = |name: &str, value: ObjP| {
             let index = global_names
@@ -105,14 +108,9 @@ impl Vm {
             globals[index] = value;
         };
 
-        register_global(
-            "print",
-            BuiltinFunctionInst::create("print".to_string(), builtins::print, 1),
-        );
-        register_global(
-            "println",
-            BuiltinFunctionInst::create("println".to_string(), builtins::println, 1),
-        );
+        for (name, builtin) in builtins.iter() {
+            register_global(&name, ObjP::clone(&builtin));
+        }
 
         // stack and frames
         let stack = Vec::new();
@@ -124,6 +122,7 @@ impl Vm {
             globals,
             stack,
             frames,
+            builtins,
         }
     }
 
@@ -231,9 +230,9 @@ impl Vm {
                     // need both declarations to borrow cell value
                     let name_obj = Ptr::clone(&self.constants[constant_id as usize]);
                     let name =
-                        with_obj_downcast(name_obj, |name: &StrInst| Arc::clone(&name.str_value()));
+                        with_obj_downcast(name_obj, |name: &StrInst| Rc::clone(&name.str_value()));
                     let owner = self.pop();
-                    let value = owner.get_attr(&name);
+                    let value = owner.borrow().get_attr(&name);
                     if let Some(value) = value {
                         self.push(value);
                     } else {
@@ -243,18 +242,18 @@ impl Vm {
                         todo!(
                             "throw an error because we couldn't read attr '{}' on '{}'",
                             name,
-                            owner,
+                            owner.borrow(),
                         );
                     }
                 }
                 Op::SetAttr(constant_id) => {
                     let name_obj = Ptr::clone(&self.constants[constant_id as usize]);
                     let name =
-                        with_obj_downcast(name_obj, |name: &StrInst| Arc::clone(&name.str_value()));
+                        with_obj_downcast(name_obj, |name: &StrInst| Rc::clone(&name.str_value()));
                     let value = self.pop();
                     let target = self.pop();
 
-                    target.set_attr(&name, value);
+                    target.borrow_mut().set_attr(&name, value);
                 }
                 Op::Jump(offset) => {
                     let base = (self.ip() - 1) as JumpOpArg;
@@ -264,14 +263,14 @@ impl Vm {
                 Op::JumpFalse(offset) => {
                     let base = (self.ip() - 1) as JumpOpArg;
                     let value = self.peek();
-                    if !value.is_truthy() {
+                    if !value.borrow().is_truthy() {
                         self.set_ip((base + offset) as usize);
                     }
                 }
                 Op::JumpTrue(offset) => {
                     let base = (self.ip() - 1) as JumpOpArg;
                     let value = self.peek();
-                    if value.is_truthy() {
+                    if value.borrow().is_truthy() {
                         self.set_ip((base + offset) as usize);
                     }
                 }
@@ -280,19 +279,22 @@ impl Vm {
                     let index = self.stack.len() - argc - 1;
                     let fun_ptr = Ptr::clone(&self.stack[index]);
 
-                    let arity = if let Some(arity) = fun_ptr.arity() {
+                    let arity = if let Some(arity) = fun_ptr.borrow().arity() {
                         arity as usize
                     } else {
                         // TODO Vm::run, Op::Call - throw an exception when the value isn't
                         //                          callable
                         // BLOCKED-ON: exceptions
-                        todo!("throw an error because we couldn't call {}", fun_ptr);
+                        todo!(
+                            "throw an error because we couldn't call {}",
+                            fun_ptr.borrow()
+                        );
                     };
 
                     // Methods with bound "self" parameter
                     // argc may be mutated
                     let mut argc = argc;
-                    if let Some(method) = fun_ptr.as_any().downcast_ref::<MethodInst>() {
+                    if let Some(method) = fun_ptr.borrow().as_any().downcast_ref::<MethodInst>() {
                         // shift all of the arguments over by one
                         // (duplicate the last item on the stack and then shift everyone else over)
                         self.stack
@@ -309,10 +311,10 @@ impl Vm {
                         // BLOCKED-ON: exceptions
                         todo!(
                             "throw an error because we passed the wrong number of arguments to {}",
-                            fun_ptr
+                            fun_ptr.borrow()
                         );
                     }
-                    fun_ptr.call(self, argc as Argc);
+                    fun_ptr.borrow().call(self, argc as Argc);
                 }
                 Op::Return => {
                     let return_value = self.pop();
@@ -337,7 +339,7 @@ impl Vm {
                     let stack_base = self.frames[frame_index].stack_base;
                     let value = Ptr::clone(&self.stack[stack_base + (slot as usize)]);
                     fun.push_capture(value);
-                    self.push(make_ptr(fun));
+                    self.push(upcast_obj(make_ptr(fun)));
                 }
                 Op::Halt => {
                     break;
@@ -346,3 +348,9 @@ impl Vm {
         }
     }
 }
+
+impl ObjFactory for Vm {
+    fn builtins(&self) -> &HashMap<String, ObjP> {
+        &self.builtins
+    }
+}