Revamp object system, start using gc crate

Wow, what a ride. I think everything should be working now. In short:

* Objects use the `gc` crate, which as a `Gc` garbage-collected pointer
  type. I may choose to implement my own in contiguous memory in the
  future. We will see.
* The type system is no longer global. This is a bit of a burden,
  because now, whenever you want to create a new object, you need to
  pass its type object into the `Obj::instantiate` method, as well as
  its `::create` static method.
* This burden is somewhat alleviated by the `ObjFactory` trait, which
  helps create new objects as long as you have access to a `builtins`
  hashmap. So something that would normally look like this:

    fn init_builtins(builtins: &mut HashMap<String, ObjP>) {
        let print_builtin = upcast_obj(BuiltinFunctionInst::create(
            ObjP::clone(&builtins.get("BuiltinFunction").unwrap()),
            "print",
            print,
            1
        );
        builtins.insert("print".to_string(), print_builtin)
        // other builtins inserted here...
    }

  now looks like this:

    fn init_builtins(builtins: &mut HashMap<String, ObjP>) {
        let print_builtin = builtins.create_builtin_function("print", print, 1);
        builtins.insert("print".to_string(), print_builtin);
    }

(turns out, if all you need is a HashMap<String, ObjP>, you can
implement ObjFactory for HashMap<String, ObjP> itself(!))

Overall, I'm happier with this design, and I think this is what is going
to get merged. It's a little weird to be querying type names that are
used in the language itself to get those type objects, but whatever
works, I guess.

Next up is vtables.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

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"

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"] }

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 {
+pub(crate) fn println(vm: &mut Vm, args: Vec<ObjP>) -> ObjP {
-    println!("{}", args[0]);
+    println!("{}", args[0].borrow());
-    NilInst::create()
+    vm.create_nil()
 }
 
-pub(crate) fn print(_vm: &mut Vm, args: Vec<ObjP>) -> ObjP {
+pub(crate) fn print(vm: &mut Vm, args: Vec<ObjP>) -> ObjP {
-    print!("{}", args[0]);
+    print!("{}", args[0].borrow());
-    NilInst::create()
+    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,
+    }
 }

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 {
+impl<'b> Compiler<'b> {
-    fn default() -> Self {
+    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();
+        let obj = self.constants.last().unwrap().as_ref();
-        if let Some(fun) = obj.as_any_mut().downcast_mut::<UserFunctionInst>() {
+        if let Some(fun) = obj
-            fun.set_name(Arc::new(name.to_string()));
+            .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
+    }
+}

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 {}",

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(())

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) name: Rc<String>,
-    pub(crate) chunk: Arc<Chunk>,
+    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 nil = builtins.create_nil();
-        let mut globals: Vec<_> = global_names
+        let mut globals: Vec<_> = global_names.iter().map(|_| ObjP::clone(&nil)).collect();
-            .iter()
-            .map(|_| Ptr::clone(&nil) as ObjP)
-            .collect();
 
         let mut register_global = |name: &str, value: ObjP| {
             let index = global_names
@@ -105,14 +108,9 @@ impl Vm {
             globals[index] = value;
         };
 
-        register_global(
+        for (name, builtin) in builtins.iter() {
-            "print",
+            register_global(&name, ObjP::clone(&builtin));
-            BuiltinFunctionInst::create("print".to_string(), builtins::print, 1),
+        }
-        );
-        register_global(
-            "println",
-            BuiltinFunctionInst::create("println".to_string(), builtins::println, 1),
-        );
 
         // 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
+    }
+}