require labels for JUMP opcodes to avoid compiler bugs

Reviewed-on: #3

CLAUDE.md

@@ -55,7 +55,7 @@ No build step required - Bun runs TypeScript directly.
 
 ### Critical Design Decisions
 
-**Relative jumps**: All JUMP instructions use PC-relative offsets (not absolute addresses), making bytecode position-independent. PUSH_TRY/PUSH_FINALLY use absolute addresses.
+**Label-based jumps**: All JUMP instructions (`JUMP`, `JUMP_IF_FALSE`, `JUMP_IF_TRUE`) require label operands (`.label`), not numeric offsets. Labels are resolved to PC-relative offsets during compilation, making bytecode position-independent. PUSH_TRY/PUSH_FINALLY use absolute addresses and can accept either labels or numeric offsets.
 
 **Truthiness semantics**: Only `null` and `false` are falsy. Unlike JavaScript, `0`, `""`, empty arrays, and empty dicts are truthy.
 
@@ -137,40 +137,100 @@ Array format features:
 - Function params as string arrays: `["MAKE_FUNCTION", ["x", "y=10"], ".body"]`
 - See `tests/programmatic.test.ts` and `examples/programmatic.ts` for examples
 
-### Native Function Registration
+### Native Function Registration and Global Values
 
 **Option 1**: Pass to `run()` or `VM` constructor (convenience)
 ```typescript
 const result = await run(bytecode, {
   add: (a: number, b: number) => a + b,
-  greet: (name: string) => `Hello, ${name}!`
+  greet: (name: string) => `Hello, ${name}!`,
+  pi: 3.14159,
+  config: { debug: true, port: 8080 }
 })
 
 // Or with VM constructor
-const vm = new VM(bytecode, { add, greet })
+const vm = new VM(bytecode, { add, greet, pi, config })
 ```
 
-**Option 2**: Register with `vm.registerFunction()` (manual)
+**Option 2**: Set values with `vm.set()` (manual)
 ```typescript
 const vm = new VM(bytecode)
-vm.registerFunction('add', (a: number, b: number) => a + b)
+
+// Set functions (auto-wrapped to native functions)
+vm.set('add', (a: number, b: number) => a + b)
+
+// Set any other values (auto-converted to ReefVM Values)
+vm.set('pi', 3.14159)
+vm.set('config', { debug: true, port: 8080 })
+
 await vm.run()
 ```
 
-**Option 3**: Register Value-based functions (for direct Value access)
+**Option 3**: Set Value-based functions with `vm.setValueFunction()` (advanced)
+
+For functions that work directly with ReefVM Value types:
+
 ```typescript
-vm.registerValueFunction('customOp', (a: Value, b: Value): Value => {
+const vm = new VM(bytecode)
+
+// Set Value-based function (no wrapping, works directly with Values)
+vm.setValueFunction('customOp', (a: Value, b: Value): Value => {
   return toValue(toNumber(a) + toNumber(b))
 })
+
+await vm.run()
 ```
 
 Auto-wrapping handles:
-- Value ↔ native type conversion (`fromValue`/`toValue`)
+- Functions: wrapped as native functions with Value ↔ native type conversion
 - Sync and async functions
 - Arrays, objects, primitives, null, RegExp
+- All values converted via `toValue()`
 
-### Label Usage (Preferred)
-Use labels instead of numeric offsets for readability:
+### Calling Functions from TypeScript
+
+Use `vm.call()` to invoke Reef or native functions from TypeScript:
+
+```typescript
+const bytecode = toBytecode(`
+  MAKE_FUNCTION (x y=10) .add
+  STORE add
+  HALT
+
+  .add:
+    LOAD x
+    LOAD y
+    ADD
+    RETURN
+`)
+
+const vm = new VM(bytecode, {
+  log: (msg: string) => console.log(msg)  // Native function
+})
+await vm.run()
+
+// Call Reef function with positional arguments
+const result1 = await vm.call('add', 5, 3)  // → 8
+
+// Call Reef function with named arguments (pass final object)
+const result2 = await vm.call('add', 5, { y: 20 })  // → 25
+
+// Call Reef function with all named arguments
+const result3 = await vm.call('add', { x: 10, y: 15 })  // → 25
+
+// Call native function
+await vm.call('log', 'Hello!')
+```
+
+**How it works**:
+- Looks up function (Reef or native) in VM scope
+- For Reef functions: converts to callable JavaScript function using `fnFromValue`
+- For native functions: calls directly
+- Automatically converts arguments to ReefVM Values
+- Converts result back to JavaScript types
+
+### Label Usage (Required for JUMP instructions)
+All JUMP instructions must use labels:
 ```
 JUMP .skip
 PUSH 42
@@ -180,6 +240,67 @@ HALT
   HALT
 ```
 
+### Function Definition Patterns
+
+When defining functions, you MUST prevent the PC from falling through into function bodies. Two patterns:
+
+**Pattern 1: JUMP over function bodies (Recommended)**
+```
+MAKE_FUNCTION (params) .body
+STORE function_name
+JUMP .end              ; Skip over function body
+.body:
+  <function code>
+  RETURN
+.end:
+  <continue with program>
+```
+
+**Pattern 2: Function bodies after HALT**
+```
+MAKE_FUNCTION (params) .body
+STORE function_name
+<use the function>
+HALT                   ; Stop before function bodies
+.body:
+  <function code>
+  RETURN
+```
+
+Pattern 1 is required for:
+- Defining multiple functions before using them
+- REPL mode
+- Any case where execution continues after defining a function
+
+Pattern 2 only works if you HALT before reaching function bodies.
+
+### REPL Mode (Incremental Execution)
+
+For building REPLs (like the Shrimp REPL), use `vm.continue()` and `vm.appendBytecode()`:
+
+```typescript
+const vm = new VM(toBytecode([]), natives)
+await vm.run()  // Initialize (empty bytecode)
+
+// User enters: x = 42
+const line1 = compileLine("x = 42")  // No HALT!
+vm.appendBytecode(line1)
+await vm.continue()  // Execute only line 1
+
+// User enters: x + 10
+const line2 = compileLine("x + 10")  // No HALT!
+vm.appendBytecode(line2)
+await vm.continue()  // Execute only line 2, result is 52
+```
+
+**Key points**:
+- `vm.run()` resets PC to 0 (re-executes everything) - use for initial setup only
+- `vm.continue()` resumes from current PC (executes only new bytecode)
+- `vm.appendBytecode(bytecode)` properly handles constant index remapping
+- Don't use HALT in REPL lines - let VM stop naturally
+- Scope and variables persist across all lines
+- Side effects only run once
+
 ## TypeScript Configuration
 
 - Import alias: `#reef` maps to `./src/index.ts`
@@ -365,7 +486,7 @@ Run `bun test` to verify all tests pass before committing.
 
 ## Common Gotchas
 
-**Jump offsets**: JUMP/JUMP_IF_FALSE/JUMP_IF_TRUE use relative offsets from the next instruction (PC + 1). PUSH_TRY/PUSH_FINALLY use absolute instruction indices.
+**Label requirements**: JUMP/JUMP_IF_FALSE/JUMP_IF_TRUE require label operands (`.label`), not numeric offsets. The bytecode compiler resolves labels to PC-relative offsets internally. PUSH_TRY/PUSH_FINALLY can use either labels or absolute instruction indices (`#N`).
 
 **Stack operations**: Most binary operations pop in reverse order (second operand is popped first, then first operand).
 

GUIDE.md

@@ -179,12 +179,25 @@ PUSH 1              ; Named count
 CALL
 ```
 
+**Null triggers defaults**: Pass `null` to use default values:
+```
+; Function: greet(name='Guest', msg='Hello')
+LOAD greet
+PUSH null           ; Use default for 'name'
+PUSH "Hi"           ; Provide 'msg'
+PUSH 2
+PUSH 0
+CALL                ; → "Hi, Guest"
+```
+
 ## Opcodes
 
 ### Stack
 - `PUSH <const>` - Push constant
 - `POP` - Remove top
 - `DUP` - Duplicate top
+- `SWAP` - Swap top two values
+- `TYPE` - Pop value, push its type as string
 
 ### Variables
 - `LOAD <name>` - Push variable value (throws if not found)
@@ -194,6 +207,10 @@ CALL
 ### Arithmetic
 - `ADD`, `SUB`, `MUL`, `DIV`, `MOD` - Binary ops (pop 2, push result)
 
+### Bitwise
+- `BIT_AND`, `BIT_OR`, `BIT_XOR` - Bitwise logical ops (pop 2, push result)
+- `BIT_SHL`, `BIT_SHR`, `BIT_USHR` - Bitwise shift ops (pop 2, push result)
+
 ### Comparison
 - `EQ`, `NEQ`, `LT`, `GT`, `LTE`, `GTE` - Pop 2, push boolean
 
@@ -241,6 +258,40 @@ CALL
 
 ## Compiler Patterns
 
+### Function Definitions
+
+When defining functions, you must prevent the PC from "falling through" into the function body during sequential execution. There are two standard patterns:
+
+**Pattern 1: JUMP over function bodies (Recommended)**
+```
+MAKE_FUNCTION (params) .body
+STORE function_name
+JUMP .end              ; Skip over function body
+.body:
+  <function code>
+  RETURN
+.end:
+  <continue with program>
+```
+
+**Pattern 2: Function bodies after HALT**
+```
+MAKE_FUNCTION (params) .body
+STORE function_name
+<use the function>
+HALT                   ; Stop execution before function bodies
+.body:
+  <function code>
+  RETURN
+```
+
+**Important**: Pattern 2 only works if you HALT before reaching function bodies. Pattern 1 is more flexible and required for:
+- Defining multiple functions before using them
+- REPL mode (incremental execution)
+- Any case where execution continues after defining a function
+
+**Why?** `MAKE_FUNCTION` creates a function value but doesn't jump to the body—it just stores the body's address. Without JUMP or HALT, the PC increments into the function body and executes it as top-level code.
+
 ### If-Else
 ```
 <condition>
@@ -326,6 +377,125 @@ POP
 .end:                    ; Result on stack
 ```
 
+### Reversing Operand Order
+Use SWAP to reverse operand order for non-commutative operations:
+
+```
+; Compute 10 / 2 when values are in reverse order
+PUSH 2
+PUSH 10
+SWAP                     ; Now: [10, 2]
+DIV                      ; 10 / 2 = 5
+```
+
+```
+; Compute "hello" - "world" (subtraction with strings coerced to numbers)
+PUSH "world"
+PUSH "hello"
+SWAP                     ; Now: ["hello", "world"]
+SUB                      ; Result based on operand order
+```
+
+**Common Use Cases**:
+- Division and subtraction when operands are in wrong order
+- String concatenation with specific order
+- Preparing arguments for functions that care about position
+
+### Bitwise Operations
+All bitwise operations work with 32-bit signed integers:
+
+```
+; Bitwise AND (masking)
+PUSH 5
+PUSH 3
+BIT_AND                  ; → 1 (0101 & 0011 = 0001)
+
+; Bitwise OR (combining flags)
+PUSH 5
+PUSH 3
+BIT_OR                   ; → 7 (0101 | 0011 = 0111)
+
+; Bitwise XOR (toggling bits)
+PUSH 5
+PUSH 3
+BIT_XOR                  ; → 6 (0101 ^ 0011 = 0110)
+
+; Left shift (multiply by power of 2)
+PUSH 5
+PUSH 2
+BIT_SHL                  ; → 20 (5 << 2 = 5 * 4)
+
+; Arithmetic right shift (divide by power of 2, preserves sign)
+PUSH 20
+PUSH 2
+BIT_SHR                  ; → 5 (20 >> 2 = 20 / 4)
+
+PUSH -20
+PUSH 2
+BIT_SHR                  ; → -5 (sign preserved)
+
+; Logical right shift (zero-fill)
+PUSH -1
+PUSH 1
+BIT_USHR                 ; → 2147483647 (unsigned shift)
+```
+
+**Common Use Cases**:
+- Flags and bit masks: `flags band MASK` to test, `flags bor FLAG` to set
+- Fast multiplication/division by powers of 2
+- Color manipulation: extract RGB components
+- Low-level bit manipulation for protocols or file formats
+
+### Runtime Type Checking (TYPE)
+Get the type of a value as a string for runtime introspection:
+
+```
+; Basic type check
+PUSH 42
+TYPE                     ; → "number"
+
+PUSH "hello"
+TYPE                     ; → "string"
+
+MAKE_ARRAY #3
+TYPE                     ; → "array"
+```
+
+**Type Guard Pattern** (check type before operation):
+```
+; Safe addition - only add if both are numbers
+LOAD x
+DUP
+TYPE
+PUSH "number"
+EQ
+JUMP_IF_FALSE .not_number
+
+LOAD y
+DUP
+TYPE
+PUSH "number"
+EQ
+JUMP_IF_FALSE .cleanup_not_number
+
+ADD                      ; Safe to add
+JUMP .end
+
+.cleanup_not_number:
+  POP                    ; Remove y
+.not_number:
+  POP                    ; Remove x
+  PUSH null
+.end:
+```
+
+**Common Use Cases**:
+- Type validation before operations
+- Polymorphic functions that handle multiple types
+- Debugging and introspection
+- Dynamic dispatch in DSLs
+- Safe coercion with fallbacks
+
 ### Try-Catch
 ```
 PUSH_TRY .catch
@@ -362,7 +532,8 @@ Functions automatically capture current scope:
 PUSH 0
 STORE counter
 MAKE_FUNCTION () .increment
-RETURN
+STORE increment_fn
+JUMP .main
 
 .increment:
   LOAD counter          ; Captured variable
@@ -371,6 +542,18 @@ RETURN
   STORE counter
   LOAD counter
   RETURN
+
+.main:
+  LOAD increment_fn
+  PUSH 0
+  PUSH 0
+  CALL                  ; Returns 1
+  POP
+  LOAD increment_fn
+  PUSH 0
+  PUSH 0
+  CALL                  ; Returns 2 (counter persists!)
+  HALT
 ```
 
 ### Tail Recursion
@@ -378,7 +561,7 @@ Use TAIL_CALL instead of CALL for last call:
 ```
 MAKE_FUNCTION (n acc) .factorial
 STORE factorial
-<...>
+JUMP .main
 
 .factorial:
   LOAD n
@@ -398,6 +581,15 @@ STORE factorial
   PUSH 2
   PUSH 0
   TAIL_CALL             ; Reuses stack frame
+
+.main:
+  LOAD factorial
+  PUSH 5
+  PUSH 1
+  PUSH 2
+  PUSH 0
+  CALL                  ; factorial(5, 1) = 120
+  HALT
 ```
 
 ### Optional Function Calls (TRY_CALL)
@@ -541,6 +733,8 @@ Only `null` and `false` are falsy. Everything else (including `0`, `""`, empty a
 
 **Arithmetic ops** (ADD, SUB, MUL, DIV, MOD) coerce both operands to numbers.
 
+**Bitwise ops** (BIT_AND, BIT_OR, BIT_XOR, BIT_SHL, BIT_SHR, BIT_USHR) coerce both operands to 32-bit signed integers.
+
 **Comparison ops** (LT, GT, LTE, GTE) coerce both operands to numbers.
 
 **Equality ops** (EQ, NEQ) use type-aware comparison with deep equality for arrays/dicts.
@@ -565,11 +759,24 @@ Variable and function parameter names support Unicode and emoji:
 
 ### Parameter Binding Priority
 For function calls, parameters bound in order:
-1. Positional argument (if provided)
-2. Named argument (if provided and matches param name)
+1. Named argument (if provided and matches param name)
+2. Positional argument (if provided)
 3. Default value (if defined)
 4. Null
 
+**Null Triggering Defaults**: Passing `null` as an argument (positional or named) triggers the default value if one exists. This allows callers to explicitly "opt-in" to defaults:
+```
+# Function with defaults: greet(name='Guest', greeting='Hello')
+LOAD greet
+PUSH null    # Triggers default: name='Guest'
+PUSH 'Hi'    # Provided: greeting='Hi'
+PUSH 2
+PUSH 0
+CALL         # Returns "Hi, Guest"
+```
+
+This works for both ReefVM functions and native TypeScript functions. If no default exists, `null` is bound as-is.
+
 ### Exception Handlers
 - PUSH_TRY uses absolute addresses for catch blocks
 - Nested try blocks form a stack
@@ -606,18 +813,18 @@ const vm = new VM(bytecode, { add, greet })
 **Method 2**: Register after construction
 ```typescript
 const vm = new VM(bytecode)
-vm.registerFunction('add', (a: number, b: number) => a + b)
+vm.set('add', (a: number, b: number) => a + b)
 await vm.run()
 ```
 
 **Method 3**: Value-based functions (for full control)
 ```typescript
-vm.registerValueFunction('customOp', (a: Value, b: Value): Value => {
+vm.setValueFunction('customOp', (a: Value, b: Value): Value => {
   return { type: 'number', value: toNumber(a) + toNumber(b) }
 })
 ```
 
-**Auto-wrapping**: `registerFunction` automatically converts between native TypeScript types and ReefVM Value types. Both sync and async functions work.
+**Auto-wrapping**: `vm.set()` automatically converts between native TypeScript types and ReefVM Value types. Both sync and async functions work.
 
 **Usage in bytecode**:
 ```
@@ -646,12 +853,12 @@ CALL                  ; → "Hi, Alice!"
 
 ```typescript
 // Basic @named - collects all named args
-vm.registerFunction('greet', (atNamed: any = {}) => {
+vm.set('greet', (atNamed: any = {}) => {
   return `Hello, ${atNamed.name || 'World'}!`
 })
 
 // Mixed positional and @named
-vm.registerFunction('configure', (name: string, atOptions: any = {}) => {
+vm.set('configure', (name: string, atOptions: any = {}) => {
   return {
     name,
     debug: atOptions.debug || false,
@@ -676,6 +883,121 @@ CALL                ; atOptions receives {debug: true, port: 8080}
 
 Named arguments that match fixed parameter names are bound to those parameters. Remaining unmatched named arguments are collected into the `atXxx` parameter as a plain JavaScript object.
 
+### Calling Functions from TypeScript
+
+You can call both Reef and native functions from TypeScript using `vm.call()`:
+
+```typescript
+const bytecode = toBytecode(`
+  MAKE_FUNCTION (name greeting="Hello") .greet
+  STORE greet
+  HALT
+
+  .greet:
+    LOAD greeting
+    PUSH " "
+    LOAD name
+    PUSH "!"
+    STR_CONCAT #4
+    RETURN
+`)
+
+const vm = new VM(bytecode, {
+  log: (msg: string) => console.log(msg)  // Native function
+})
+await vm.run()
+
+// Call Reef function with positional arguments
+const result1 = await vm.call('greet', 'Alice')
+// Returns: "Hello Alice!"
+
+// Call Reef function with named arguments (pass as final object)
+const result2 = await vm.call('greet', 'Bob', { greeting: 'Hi' })
+// Returns: "Hi Bob!"
+
+// Call Reef function with only named arguments
+const result3 = await vm.call('greet', { name: 'Carol', greeting: 'Hey' })
+// Returns: "Hey Carol!"
+
+// Call native function
+await vm.call('log', 'Hello from TypeScript!')
+```
+
+**How it works**:
+- `vm.call(functionName, ...args)` looks up the function (Reef or native) in the VM's scope
+- For Reef functions: converts to callable JavaScript function
+- For native functions: calls directly
+- Arguments are automatically converted to ReefVM Values
+- Returns the result (automatically converted back to JavaScript types)
+
+**Named arguments**: Pass a plain object as the final argument to provide named arguments. If the last argument is a non-array object, it's treated as named arguments. All preceding arguments are treated as positional.
+
+**Type conversion**: Arguments and return values are automatically converted between JavaScript types and ReefVM Values:
+- Primitives: `number`, `string`, `boolean`, `null`
+- Arrays: converted recursively
+- Objects: converted to ReefVM dicts
+- Functions: Reef functions are converted to callable JavaScript functions
+
+### REPL Mode (Incremental Compilation)
+
+ReefVM supports incremental bytecode execution for building REPLs. This allows you to execute code line-by-line while preserving scope and avoiding re-execution of side effects.
+
+**The Problem**: By default, `vm.run()` resets the program counter (PC) to 0, re-executing all previous bytecode. This makes it impossible to implement a REPL where each line executes only once.
+
+**The Solution**: Use `vm.continue()` to resume execution from where you left off:
+
+```typescript
+// Line 1: Define variable
+const line1 = toBytecode([
+  ["PUSH", 42],
+  ["STORE", "x"]
+])
+
+const vm = new VM(line1)
+await vm.run()  // Execute first line
+
+// Line 2: Use the variable
+const line2 = toBytecode([
+  ["LOAD", "x"],
+  ["PUSH", 10],
+  ["ADD"]
+])
+
+vm.appendBytecode(line2)  // Append new bytecode with proper constant remapping
+await vm.continue()       // Execute ONLY the new bytecode
+
+// Result: 52 (42 + 10)
+// The first line never re-executed!
+```
+
+**Key methods**:
+- `vm.run()`: Resets PC to 0 and runs from the beginning (normal execution)
+- `vm.continue()`: Continues from current PC (REPL mode)
+- `vm.appendBytecode(bytecode)`: Helper that properly appends bytecode with constant index remapping
+
+**Important**: Don't use `HALT` in REPL mode! The VM naturally stops when it runs out of instructions. Using `HALT` sets `vm.stopped = true`, which prevents `continue()` from resuming.
+
+**Example REPL pattern**:
+```typescript
+const vm = new VM(toBytecode([]), { /* native functions */ })
+
+while (true) {
+  const input = await getUserInput()  // Get next line from user
+  const bytecode = compileLine(input)  // Compile to bytecode (no HALT!)
+
+  vm.appendBytecode(bytecode)  // Append to VM
+  const result = await vm.continue()  // Execute only the new code
+
+  console.log(fromValue(result))  // Show result to user
+}
+```
+
+This pattern ensures:
+- Variables persist between lines
+- Side effects (like `echo` or function calls) only run once
+- Previous bytecode never re-executes
+- Scope accumulates across all lines
+
 ### Empty Stack
 - RETURN with empty stack returns null
 - HALT with empty stack returns null

IDEAS.md

@@ -0,0 +1,500 @@
+# ReefVM Architectural Improvement Ideas
+
+This document contains architectural ideas for improving ReefVM. These focus on enhancing the VM's capabilities through structural improvements rather than just adding new opcodes.
+
+## 1. Scope Resolution Optimization
+
+**Current Issue**: Variable lookups are O(n) through the scope chain on every `LOAD`. This becomes expensive in deeply nested closures.
+
+**Architectural Solution**: Implement **static scope analysis** with **lexical addressing**:
+
+```typescript
+// Instead of: LOAD x  (runtime scope chain walk)
+// Compile to: LOAD_FAST 2 1  (scope depth 2, slot 1 - O(1) lookup)
+
+class Scope {
+  locals: Map<string, Value>
+  parent?: Scope
+
+  // NEW: Add indexed slots for fast access
+  slots: Value[]  // Direct array access
+  nameToSlot: Map<string, number>  // Compile-time mapping
+}
+```
+
+**Benefits**:
+- O(1) variable access instead of O(n)
+- Critical for hot loops and deeply nested functions
+- Compiler can still fall back to named lookup for dynamic cases
+
+---
+
+## 2. Module System Architecture
+
+**Current Gap**: No way to organize code across multiple files or create reusable libraries.
+
+**Architectural Solution**: Add first-class module support:
+
+```typescript
+// New opcodes: IMPORT, EXPORT, MAKE_MODULE
+// New bytecode structure:
+type Bytecode = {
+  instructions: Instruction[]
+  constants: Constant[]
+  exports?: Map<string, number>  // Exported symbols
+  imports?: Import[]               // Import declarations
+}
+
+type Import = {
+  modulePath: string
+  symbols: string[]  // [] means import all
+  alias?: string
+}
+```
+
+**Pattern**:
+```
+MAKE_MODULE .module_body
+EXPORT add
+EXPORT subtract
+HALT
+
+.module_body:
+  MAKE_FUNCTION (x y) .add_impl
+  RETURN
+```
+
+**Benefits**:
+- Code organization and reusability
+- Circular dependency detection at load time
+- Natural namespace isolation
+- Enables standard library architecture
+
+---
+
+## 3. Source Map Integration
+
+**Current Issue**: Runtime errors show bytecode addresses, not source locations.
+
+**Architectural Solution**: Add source mapping layer:
+
+```typescript
+type Bytecode = {
+  instructions: Instruction[]
+  constants: Constant[]
+  sourceMap?: SourceMap  // NEW
+}
+
+type SourceMap = {
+  file?: string
+  mappings: SourceMapping[]  // Instruction index → source location
+}
+
+type SourceMapping = {
+  instruction: number
+  line: number
+  column: number
+  source?: string  // Original source text
+}
+```
+
+**Benefits**:
+- Meaningful error messages with line/column
+- Debugger can show original source
+- Stack traces map to source code
+- Critical for production debugging
+
+---
+
+## 4. Debugger Hook Architecture
+
+**Current Gap**: No way to pause execution, inspect state, or step through code.
+
+**Architectural Solution**: Add debug event system:
+
+```typescript
+class VM {
+  debugger?: Debugger
+
+  async execute(instruction: Instruction) {
+    // Before execution
+    await this.debugger?.onInstruction(this.pc, instruction, this)
+
+    // Execute
+    switch (instruction.op) { ... }
+
+    // After execution
+    await this.debugger?.afterInstruction(this.pc, this)
+  }
+}
+
+interface Debugger {
+  breakpoints: Set<number>
+  onInstruction(pc: number, instruction: Instruction, vm: VM): Promise<void>
+  afterInstruction(pc: number, vm: VM): Promise<void>
+  onCall(fn: Value, args: Value[]): Promise<void>
+  onReturn(value: Value): Promise<void>
+  onException(error: Value): Promise<void>
+}
+```
+
+**Benefits**:
+- Step-through debugging
+- Breakpoints at any instruction
+- State inspection at any point
+- Non-invasive (no bytecode modification)
+- Can build IDE integrations
+
+---
+
+## 5. Bytecode Optimization Pass Framework
+
+**Current Gap**: Bytecode is emitted directly, no optimization.
+
+**Architectural Solution**: Add optimization pipeline:
+
+```typescript
+type Optimizer = (bytecode: Bytecode) => Bytecode
+
+// Framework for composable optimization passes
+class BytecodeOptimizer {
+  passes: Optimizer[] = []
+
+  add(pass: Optimizer): this {
+    this.passes.push(pass)
+    return this
+  }
+
+  optimize(bytecode: Bytecode): Bytecode {
+    return this.passes.reduce((bc, pass) => pass(bc), bytecode)
+  }
+}
+
+// Example passes:
+const optimizer = new BytecodeOptimizer()
+  .add(constantFolding)      // PUSH 2; PUSH 3; ADD → PUSH 5
+  .add(deadCodeElimination)  // Remove unreachable code after HALT/RETURN
+  .add(jumpChaining)         // JUMP .a → .a: JUMP .b → JUMP .b directly
+  .add(peepholeOptimization) // DUP; POP → (nothing)
+```
+
+**Benefits**:
+- Faster execution without changing compiler
+- Can add passes without modifying VM
+- Composable and testable
+- Enables aggressive optimizations (inlining, constant folding, etc.)
+
+---
+
+## 6. Value Memory Management Architecture
+
+**Current Issue**: No tracking of memory usage, no GC hooks, unbounded growth.
+
+**Architectural Solution**: Add memory management layer:
+
+```typescript
+class MemoryManager {
+  allocatedBytes: number = 0
+  maxBytes?: number
+
+  allocateValue(value: Value): Value {
+    const size = this.sizeOf(value)
+    if (this.maxBytes && this.allocatedBytes + size > this.maxBytes) {
+      throw new Error('Out of memory')
+    }
+    this.allocatedBytes += size
+    return value
+  }
+
+  sizeOf(value: Value): number {
+    // Estimate memory footprint
+  }
+
+  // Hook for custom GC
+  gc?: () => void
+}
+
+class VM {
+  memory: MemoryManager
+
+  // All value-creating operations check memory
+  push(value: Value) {
+    this.memory.allocateValue(value)
+    this.stack.push(value)
+  }
+}
+```
+
+**Benefits**:
+- Memory limits for sandboxing
+- Memory profiling
+- Custom GC strategies
+- Prevents runaway memory usage
+
+---
+
+## 7. Instruction Profiler Architecture
+
+**Current Gap**: No way to identify performance bottlenecks in bytecode.
+
+**Architectural Solution**: Add instrumentation layer:
+
+```typescript
+class Profiler {
+  instructionCounts: Map<number, number> = new Map()
+  instructionTime: Map<number, number> = new Map()
+  hotFunctions: Map<number, FunctionProfile> = new Map()
+
+  recordInstruction(pc: number, duration: number) {
+    this.instructionCounts.set(pc, (this.instructionCounts.get(pc) || 0) + 1)
+    this.instructionTime.set(pc, (this.instructionTime.get(pc) || 0) + duration)
+  }
+
+  getHotSpots(): HotSpot[] {
+    // Identify most-executed instructions
+  }
+
+  generateReport(): ProfileReport {
+    // Human-readable performance report
+  }
+}
+
+class VM {
+  profiler?: Profiler
+
+  async execute(instruction: Instruction) {
+    const start = performance.now()
+    // ... execute ...
+    const duration = performance.now() - start
+    this.profiler?.recordInstruction(this.pc, duration)
+  }
+}
+```
+
+**Benefits**:
+- Identify hot loops and functions
+- Guide optimization efforts
+- Measure impact of changes
+- Can feed into JIT compiler (future)
+
+---
+
+## 8. Standard Library Plugin Architecture
+
+**Current Issue**: Native functions registered manually, no standard library structure.
+
+**Architectural Solution**: Module-based native libraries:
+
+```typescript
+interface NativeModule {
+  name: string
+  exports: Record<string, any>
+  init?(vm: VM): void
+}
+
+class VM {
+  modules: Map<string, NativeModule> = new Map()
+
+  registerModule(module: NativeModule) {
+    this.modules.set(module.name, module)
+    module.init?.(this)
+
+    // Auto-register exports to global scope
+    for (const [name, value] of Object.entries(module.exports)) {
+      this.set(name, value)
+    }
+  }
+
+  loadModule(name: string): NativeModule {
+    return this.modules.get(name) || throw new Error(`Module ${name} not found`)
+  }
+}
+
+// Example usage:
+const mathModule: NativeModule = {
+  name: 'math',
+  exports: {
+    sin: Math.sin,
+    cos: Math.cos,
+    sqrt: Math.sqrt,
+    PI: Math.PI
+  }
+}
+
+vm.registerModule(mathModule)
+```
+
+**Benefits**:
+- Organized standard library
+- Lazy loading of modules
+- Third-party plugin system
+- Clear namespace boundaries
+
+---
+
+## 9. Streaming Bytecode Execution
+
+**Current Limitation**: Must load entire bytecode before execution.
+
+**Architectural Solution**: Incremental bytecode loading:
+
+```typescript
+class StreamingBytecode {
+  chunks: BytecodeChunk[] = []
+
+  append(chunk: BytecodeChunk) {
+    // Remap addresses, merge constants
+    this.chunks.push(chunk)
+  }
+
+  getInstruction(pc: number): Instruction | undefined {
+    // Resolve across chunks
+  }
+}
+
+class VM {
+  async runStreaming(stream: ReadableStream<BytecodeChunk>) {
+    for await (const chunk of stream) {
+      this.bytecode.append(chunk)
+      await this.continue()  // Execute new chunk
+    }
+  }
+}
+```
+
+**Benefits**:
+- Execute before full load (faster startup)
+- Network streaming of bytecode
+- Incremental compilation
+- Better REPL experience
+
+---
+
+## 10. Type Annotation System (Optional Runtime Types)
+
+**Current Gap**: All values dynamically typed, no way to enforce types.
+
+**Architectural Solution**: Optional type metadata:
+
+```typescript
+type TypedValue = Value & {
+  typeAnnotation?: TypeAnnotation
+}
+
+type TypeAnnotation =
+  | { kind: 'number' }
+  | { kind: 'string' }
+  | { kind: 'array', elementType?: TypeAnnotation }
+  | { kind: 'dict', valueType?: TypeAnnotation }
+  | { kind: 'function', params: TypeAnnotation[], return: TypeAnnotation }
+
+// New opcodes: TYPE_CHECK, TYPE_ASSERT
+// Functions can declare parameter types:
+MAKE_FUNCTION (x:number y:string) .body
+```
+
+**Benefits**:
+- Catch type errors earlier
+- Self-documenting code
+- Enables static analysis tools
+- Optional (doesn't break existing code)
+- Can enable optimizations (known number type → skip toNumber())
+
+---
+
+## 11. VM State Serialization
+
+**Current Gap**: Can't save/restore VM execution state.
+
+**Architectural Solution**: Serializable VM state:
+
+```typescript
+class VM {
+  serialize(): SerializedState {
+    return {
+      instructions: this.instructions,
+      constants: this.constants,
+      pc: this.pc,
+      stack: this.stack.map(serializeValue),
+      callStack: this.callStack.map(serializeFrame),
+      scope: serializeScope(this.scope),
+      handlers: this.handlers
+    }
+  }
+
+  static deserialize(state: SerializedState): VM {
+    const vm = new VM(/* ... */)
+    vm.restore(state)
+    return vm
+  }
+}
+```
+
+**Benefits**:
+- Save/restore execution state
+- Distributed computing (send state to workers)
+- Crash recovery
+- Time-travel debugging
+- Checkpoint/restart
+
+---
+
+## 12. Async Iterator Support
+
+**Current Gap**: Iterators work via break, but no async iteration.
+
+**Architectural Solution**: First-class async iteration:
+
+```typescript
+// New value type:
+type Value = ... | { type: 'async_iterator', value: AsyncIterableIterator<Value> }
+
+// New opcodes: MAKE_ASYNC_ITERATOR, AWAIT_NEXT, YIELD_ASYNC
+
+// Pattern:
+for_await (item in asyncIterable) {
+  // Compiles to AWAIT_NEXT loop
+}
+```
+
+**Benefits**:
+- Stream processing
+- Async I/O without blocking
+- Natural async patterns
+- Matches JavaScript async iterators
+
+---
+
+## Priority Recommendations
+
+### Tier 1 (Highest Impact):
+1. **Source Map Integration** - Critical for usability
+2. **Module System** - Essential for scaling beyond toy programs
+3. **Scope Resolution Optimization** - Performance multiplier
+
+### Tier 2 (High Value):
+4. **Debugger Hook Architecture** - Developer experience game-changer
+5. **Standard Library Plugin Architecture** - Enables ecosystem
+6. **Bytecode Optimization Framework** - Performance without complexity
+
+### Tier 3 (Nice to Have):
+7. **Instruction Profiler** - Guides future optimization
+8. **Memory Management** - Important for production use
+9. **VM State Serialization** - Enables advanced use cases
+
+### Tier 4 (Future/Experimental):
+10. **Type Annotations** - Optional, doesn't break existing code
+11. **Streaming Bytecode** - Mostly useful for large programs
+12. **Async Iterators** - Specialized use case
+
+---
+
+## Design Principles
+
+These improvements focus on:
+- **Performance** (scope optimization, bytecode optimization)
+- **Developer Experience** (source maps, debugger, profiler)
+- **Scalability** (modules, standard library architecture)
+- **Production Readiness** (memory management, serialization)
+
+All ideas maintain ReefVM's core design philosophy of simplicity, orthogonality, and explicit behavior.

README.md

@@ -44,11 +44,14 @@ Commands: `clear`, `reset`, `exit`.
 - Variadic functions with positional rest parameters (`...rest`)
 - Named arguments (named) that collect unmatched named args into a dict (`@named`)
 - Mixed positional and named arguments with proper priority binding
+- Default parameter values with null-triggering: passing `null` explicitly uses the default value
 - Tail call optimization with unbounded recursion (10,000+ iterations without stack overflow)
 - Exception handling (PUSH_TRY, PUSH_FINALLY, POP_TRY, THROW) with nested try/finally blocks and call stack unwinding
 - Native function interop with auto-wrapping for native TypeScript types
 - Native functions stored in scope, called via LOAD + CALL
+- Native functions support `atXxx` parameters (e.g., `atOptions`) to collect unmatched named args
 - Pass functions directly to `run(bytecode, { fnName: fn })` or `new VM(bytecode, { fnName: fn })`
+- Call Reef functions from TypeScript with `vm.call(name, ...args)` with automatic type conversion
 
 ## Design Decisions
 
@@ -58,3 +61,4 @@ Commands: `clear`, `reset`, `exit`.
 - Variadic parameters: Functions can collect remaining positional arguments into an array using `...rest` syntax
 - Named parameters: Functions can collect unmatched named arguments into a dict using `@named` syntax
 - Argument binding priority: Named args bind to regular params first, with unmatched ones going to `@named`
+- Null triggers defaults: Passing `null` to a parameter with a default value explicitly uses that default (applies to both ReefVM and native functions)

SPEC.md

@@ -138,6 +138,24 @@ type ExceptionHandler = {
 **Effect**: Duplicate top of stack
 **Stack**: [value] → [value, value]
 
+#### SWAP
+**Operand**: None
+**Effect**: Swap the top two values on the stack
+**Stack**: [value1, value2] → [value2, value1]
+
+#### TYPE
+**Operand**: None
+**Effect**: Pop value from stack, push its type as a string
+**Stack**: [value] → [typeString]
+
+Returns the type of a value as a string. 
+
+**Example**:
+```
+PUSH 42
+TYPE              ; Pushes "number"
+```
+
 ### Variable Operations
 
 #### LOAD
@@ -179,7 +197,29 @@ All arithmetic operations pop two values, perform operation, push result as numb
 
 #### ADD
 **Stack**: [a, b] → [a + b]
-**Note**: Only for numbers (use separate string concat if needed)
+
+Performs different operations depending on operand types:
+- If either operand is a string, converts both to strings and concatenates
+- Else if both operands are arrays, concatenates the arrays
+- Else if both operands are dicts, merges them (b's keys overwrite a's keys on conflict)
+- Else if both operands are numbers, performs numeric addition
+- Otherwise, throws an error
+
+**Examples**:
+- `5 + 3` → `8` (numeric addition)
+- `"hello" + " world"` → `"hello world"` (string concatenation)
+- `"count: " + 42` → `"count: 42"` (string concatenation)
+- `100 + " items"` → `"100 items"` (string concatenation)
+- `[1, 2, 3] + [4]` → `[1, 2, 3, 4]` (array concatenation)
+- `[1, 2] + [3, 4]` → `[1, 2, 3, 4]` (array concatenation)
+- `{a: 1} + {b: 2}` → `{a: 1, b: 2}` (dict merge)
+- `{a: 1, b: 2} + {b: 99}` → `{a: 1, b: 99}` (dict merge, b overwrites)
+
+**Invalid operations** (throw errors):
+- `true + false` → Error
+- `null + 5` → Error
+- `[1] + 5` → Error
+- `{a: 1} + 5` → Error
 
 #### SUB
 **Stack**: [a, b] → [a - b]
@@ -193,6 +233,62 @@ All arithmetic operations pop two values, perform operation, push result as numb
 #### MOD
 **Stack**: [a, b] → [a % b]
 
+### Bitwise Operations
+
+All bitwise operations coerce operands to 32-bit signed integers, perform the operation, and push the result as a number.
+
+#### BIT_AND
+**Operand**: None
+**Stack**: [a, b] → [a & b]
+
+Performs bitwise AND operation. Both operands are coerced to 32-bit signed integers.
+
+**Example**: `5 & 3` → `1` (binary: `0101 & 0011` → `0001`)
+
+#### BIT_OR
+**Operand**: None
+**Stack**: [a, b] → [a | b]
+
+Performs bitwise OR operation. Both operands are coerced to 32-bit signed integers.
+
+**Example**: `5 | 3` → `7` (binary: `0101 | 0011` → `0111`)
+
+#### BIT_XOR
+**Operand**: None
+**Stack**: [a, b] → [a ^ b]
+
+Performs bitwise XOR (exclusive OR) operation. Both operands are coerced to 32-bit signed integers.
+
+**Example**: `5 ^ 3` → `6` (binary: `0101 ^ 0011` → `0110`)
+
+#### BIT_SHL
+**Operand**: None
+**Stack**: [a, b] → [a << b]
+
+Performs left shift operation. Left operand is coerced to 32-bit signed integer, right operand determines shift amount (masked to 0-31).
+
+**Example**: `5 << 2` → `20` (binary: `0101` shifted left 2 positions → `10100`)
+
+#### BIT_SHR
+**Operand**: None
+**Stack**: [a, b] → [a >> b]
+
+Performs sign-preserving right shift operation. Left operand is coerced to 32-bit signed integer, right operand determines shift amount (masked to 0-31). The sign bit is preserved (arithmetic shift).
+
+**Example**:
+- `20 >> 2` → `5` (binary: `10100` shifted right 2 positions → `0101`)
+- `-20 >> 2` → `-5` (sign bit preserved)
+
+#### BIT_USHR
+**Operand**: None
+**Stack**: [a, b] → [a >>> b]
+
+Performs zero-fill right shift operation. Left operand is coerced to 32-bit signed integer, right operand determines shift amount (masked to 0-31). Zeros are shifted in from the left (logical shift).
+
+**Example**:
+- `-1 >>> 1` → `2147483647` (all bits shift right, zero fills from left)
+- `-8 >>> 1` → `2147483644`
+
 ### Comparison Operations
 
 All comparison operations pop two values, compare, push boolean result.
@@ -231,39 +327,45 @@ All comparison operations pop two values, compare, push boolean result.
 ```
 <evaluate left>
 DUP
-JUMP_IF_FALSE #2    # skip POP and <evaluate right>
+JUMP_IF_FALSE .end
 POP
 <evaluate right>
-end:
+.end:
 ```
 
 **OR pattern** (short-circuits if left side is true):
 ```
 <evaluate left>
 DUP
-JUMP_IF_TRUE #2     # skip POP and <evaluate right>
+JUMP_IF_TRUE .end
 POP
 <evaluate right>
-end:
+.end:
 ```
 
 ### Control Flow
 
 #### JUMP
-**Operand**: Offset (number)
-**Effect**: Add offset to PC (relative jump)
+**Operand**: Label (string)
+**Effect**: Jump to the specified label
 **Stack**: No change
 
+**Note**: JUMP only accepts label operands (`.label`), not numeric offsets. The VM resolves labels to relative offsets internally.
+
 #### JUMP_IF_FALSE
-**Operand**: Offset (number)
-**Effect**: If top of stack is falsy, add offset to PC (relative jump)
+**Operand**: Label (string)
+**Effect**: If top of stack is falsy, jump to the specified label
 **Stack**: [condition] → []
 
+**Note**: JUMP_IF_FALSE only accepts label operands (`.label`), not numeric offsets.
+
 #### JUMP_IF_TRUE
-**Operand**: Offset (number)
-**Effect**: If top of stack is truthy, add offset to PC (relative jump)
+**Operand**: Label (string)
+**Effect**: If top of stack is truthy, jump to the specified label
 **Stack**: [condition] → []
 
+**Note**: JUMP_IF_TRUE only accepts label operands (`.label`), not numeric offsets.
+
 #### BREAK
 **Operand**: None
 **Effect**: Unwind call stack until frame with `isBreakTarget = true`, resume there
@@ -378,11 +480,19 @@ The created function captures `currentScope` as its `parentScope`.
 3. Default value (if defined)
 4. Null
 
+**Null Value Semantics**:
+- Passing `null` as an argument explicitly triggers the default value (if one exists)
+- This allows callers to "opt-in" to defaults even when providing arguments positionally
+- If no default exists, `null` is bound as-is
+- This applies to both ReefVM functions and native TypeScript functions
+- Example: `fn(null, 20)` where `fn(x=10, y)` binds `x=10` (default triggered), `y=20`
+
 **Named Args Handling**:
 - Named args that match fixed parameter names are bound to those params
 - If the function has `named: true`, remaining named args (that don't match any fixed param) are collected into the last parameter as a dict
 - This allows flexible calling: `fn(x=10, y=20, extra=30)` where `extra` goes to the named args dict
 - **Native functions support named arguments** - parameter names are extracted from the function signature at call time
+- Passing `null` via named args also triggers defaults: `fn(x=null)` triggers `x`'s default
 
 **Errors**: Throws if top of stack is not a function (or native function)
 
@@ -622,7 +732,7 @@ const vm = new VM(bytecode, {
 })
 
 // Or after construction:
-vm.registerFunction('multiply', (a: number, b: number) => a * b)
+vm.set('multiply', (a: number, b: number) => a * b)
 ```
 
 **Usage in Bytecode**:
@@ -637,9 +747,9 @@ CALL              ; Call it like any other function
 
 **Native Function Types**:
 
-1. **Auto-wrapped functions** (via `registerFunction`): Accept and return native TypeScript types (number, string, boolean, array, object, etc.). The VM automatically converts between Value types and native types.
+1. **Auto-wrapped functions** (via `vm.set()`): Accept and return native TypeScript types (number, string, boolean, array, object, etc.). The VM automatically converts between Value types and native types.
 
-2. **Value-based functions** (via `registerValueFunction`): Accept and return `Value` types directly for full control over type handling.
+2. **Value-based functions** (via `vm.setValueFunction()`): Accept and return `Value` types directly for full control over type handling.
 
 **Auto-Wrapping Behavior**:
 - Parameters: `Value` → native type (number, string, boolean, array, object, null, RegExp)
@@ -655,27 +765,27 @@ CALL              ; Call it like any other function
 **Examples**:
 ```typescript
 // Auto-wrapped native types
-vm.registerFunction('add', (a: number, b: number) => a + b)
-vm.registerFunction('greet', (name: string) => `Hello, ${name}!`)
-vm.registerFunction('range', (n: number) => Array.from({ length: n }, (_, i) => i))
+vm.set('add', (a: number, b: number) => a + b)
+vm.set('greet', (name: string) => `Hello, ${name}!`)
+vm.set('range', (n: number) => Array.from({ length: n }, (_, i) => i))
 
 // With defaults
-vm.registerFunction('greet', (name: string, greeting = 'Hello') => {
+vm.set('greet', (name: string, greeting = 'Hello') => {
   return `${greeting}, ${name}!`
 })
 
 // Variadic functions
-vm.registerFunction('sum', (...nums: number[]) => {
+vm.set('sum', (...nums: number[]) => {
   return nums.reduce((acc, n) => acc + n, 0)
 })
 
 // Value-based for custom logic
-vm.registerValueFunction('customOp', (a: Value, b: Value): Value => {
+vm.setValueFunction('customOp', (a: Value, b: Value): Value => {
   return { type: 'number', value: toNumber(a) + toNumber(b) }
 })
 
 // Async functions
-vm.registerFunction('fetchData', async (url: string) => {
+vm.set('fetchData', async (url: string) => {
   const response = await fetch(url)
   return response.json()
 })
@@ -710,14 +820,16 @@ CALL              ; → "Hi, Bob!"
 
 ## Label Syntax
 
-The bytecode format supports labels for improved readability:
+The bytecode format requires labels for control flow jumps:
 
 **Label Definition**: `.label_name:` marks an instruction position
 **Label Reference**: `.label_name` in operands (e.g., `JUMP .loop_start`)
 
-Labels are resolved to numeric offsets during parsing. The original numeric offset syntax (`#N`) is still supported for backwards compatibility.
+Labels are resolved to relative PC offsets during bytecode compilation. All JUMP instructions (`JUMP`, `JUMP_IF_FALSE`, `JUMP_IF_TRUE`) require label operands.
 
-Example with labels:
+**Note**: Exception handling instructions (`PUSH_TRY`, `PUSH_FINALLY`) and function definitions (`MAKE_FUNCTION`) can use either labels or absolute instruction indices (`#N`).
+
+Example:
 ```
 JUMP .skip
 .middle:
@@ -728,15 +840,6 @@ JUMP .skip
   HALT
 ```
 
-Equivalent with numeric offsets:
-```
-JUMP #2
-PUSH 999
-HALT
-PUSH 42
-HALT
-```
-
 ## Common Bytecode Patterns
 
 ### If-Else Statement
@@ -812,6 +915,29 @@ PUSH 1                # namedCount
 CALL
 ```
 
+### Null Triggering Default Values
+```
+# Function: greet(name='Guest', greeting='Hello')
+MAKE_FUNCTION (name='Guest' greeting='Hello') .greet_body
+STORE 'greet'
+JUMP .main
+.greet_body:
+  LOAD 'greeting'
+  PUSH ', '
+  ADD
+  LOAD 'name'
+  ADD
+  RETURN
+.main:
+  # Call with null for first param - triggers default
+  LOAD 'greet'
+  PUSH null            # name will use default 'Guest'
+  PUSH 'Hi'            # greeting='Hi' (provided)
+  PUSH 2               # positionalCount
+  PUSH 0               # namedCount
+  CALL                 # Returns "Hi, Guest"
+```
+
 ### Tail Recursive Function
 ```
 MAKE_FUNCTION (n acc) .factorial_body
@@ -912,10 +1038,10 @@ const vm = new VM(bytecode, {
 })
 
 // Or register after construction
-vm.registerFunction('multiply', (a: number, b: number) => a * b)
+vm.set('multiply', (a: number, b: number) => a * b)
 
 // Or use Value-based functions
-vm.registerValueFunction('customOp', (a: Value, b: Value): Value => {
+vm.setValueFunction('customOp', (a: Value, b: Value): Value => {
   return { type: 'number', value: toNumber(a) + toNumber(b) }
 })
 

bin/debug

@@ -104,6 +104,8 @@ function formatValue(value: Value): string {
   } else if (value.type === 'function') {
     const params = value.params.join(', ')
     return `${colors.dim}<fn(${params})>${colors.reset}`
+  } else if (value.type === 'native') {
+    return `${colors.dim}<native>${colors.reset}`
   }
   return String(value)
 }

bin/repl

@@ -38,6 +38,8 @@ function formatValue(value: Value): string {
   } else if (value.type === 'function') {
     const params = value.params.join(', ')
     return `${colors.dim}<fn(${params})>${colors.reset}`
+  } else if (value.type === 'native') {
+    return `${colors.dim}<native>${colors.reset}`
   }
   return String(value)
 }

bunfig.toml

@@ -0,0 +1,2 @@
+[test]
+preload = ["./tests/setup.ts"]

examples/add-with-arrays.ts

@@ -0,0 +1,116 @@
+/**
+ * Demonstrates the ADD opcode working with arrays
+ *
+ * ADD now handles array concatenation:
+ * - [1, 2, 3] + [4] === [1, 2, 3, 4]
+ * - If both operands are arrays, they are concatenated
+ */
+
+import { toBytecode, run } from "#reef"
+
+// Basic array concatenation
+const basicConcat = toBytecode([
+  ["PUSH", 1],
+  ["PUSH", 2],
+  ["PUSH", 3],
+  ["MAKE_ARRAY", 3],
+  ["PUSH", 4],
+  ["MAKE_ARRAY", 1],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('Basic array concatenation ([1, 2, 3] + [4]):')
+const result1 = await run(basicConcat)
+console.log(result1)
+// Output: { type: 'array', value: [1, 2, 3, 4] }
+
+// Concatenate two multi-element arrays
+const multiConcat = toBytecode([
+  ["PUSH", 1],
+  ["PUSH", 2],
+  ["MAKE_ARRAY", 2],
+  ["PUSH", 3],
+  ["PUSH", 4],
+  ["MAKE_ARRAY", 2],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nConcatenate two arrays ([1, 2] + [3, 4]):')
+const result2 = await run(multiConcat)
+console.log(result2)
+// Output: { type: 'array', value: [1, 2, 3, 4] }
+
+// Concatenate multiple arrays in sequence
+const multipleConcat = toBytecode([
+  ["PUSH", 1],
+  ["MAKE_ARRAY", 1],
+  ["PUSH", 2],
+  ["MAKE_ARRAY", 1],
+  ["ADD"],
+  ["PUSH", 3],
+  ["MAKE_ARRAY", 1],
+  ["ADD"],
+  ["PUSH", 4],
+  ["MAKE_ARRAY", 1],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nMultiple concatenations ([1] + [2] + [3] + [4]):')
+const result3 = await run(multipleConcat)
+console.log(result3)
+// Output: { type: 'array', value: [1, 2, 3, 4] }
+
+// Concatenate arrays with mixed types
+const mixedTypes = toBytecode([
+  ["PUSH", 1],
+  ["PUSH", "hello"],
+  ["MAKE_ARRAY", 2],
+  ["PUSH", true],
+  ["PUSH", null],
+  ["MAKE_ARRAY", 2],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nConcatenate arrays with mixed types ([1, "hello"] + [true, null]):')
+const result4 = await run(mixedTypes)
+console.log(result4)
+// Output: { type: 'array', value: [1, "hello", true, null] }
+
+// Concatenate empty array with non-empty
+const emptyConcat = toBytecode([
+  ["MAKE_ARRAY", 0],
+  ["PUSH", 1],
+  ["PUSH", 2],
+  ["PUSH", 3],
+  ["MAKE_ARRAY", 3],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nConcatenate empty array with [1, 2, 3] ([] + [1, 2, 3]):')
+const result5 = await run(emptyConcat)
+console.log(result5)
+// Output: { type: 'array', value: [1, 2, 3] }
+
+// Nested arrays
+const nestedConcat = toBytecode([
+  ["PUSH", 1],
+  ["PUSH", 2],
+  ["MAKE_ARRAY", 2],
+  ["MAKE_ARRAY", 1],
+  ["PUSH", 3],
+  ["PUSH", 4],
+  ["MAKE_ARRAY", 2],
+  ["MAKE_ARRAY", 1],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nConcatenate nested arrays ([[1, 2]] + [[3, 4]]):')
+const result6 = await run(nestedConcat)
+console.log(result6)
+// Output: { type: 'array', value: [[1, 2], [3, 4]] }

examples/add-with-dicts.ts

@@ -0,0 +1,158 @@
+/**
+ * Demonstrates the ADD opcode working with dicts
+ *
+ * ADD now handles dict merging:
+ * - {a: 1} + {b: 2} === {a: 1, b: 2}
+ * - If both operands are dicts, they are merged
+ * - Keys from the second dict overwrite keys from the first on conflict
+ */
+
+import { toBytecode, run } from "#reef"
+
+// Basic dict merge
+const basicMerge = toBytecode([
+  ["PUSH", "a"],
+  ["PUSH", 1],
+  ["MAKE_DICT", 1],
+  ["PUSH", "b"],
+  ["PUSH", 2],
+  ["MAKE_DICT", 1],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('Basic dict merge ({a: 1} + {b: 2}):')
+const result1 = await run(basicMerge)
+console.log(result1)
+// Output: { type: 'dict', value: Map { a: 1, b: 2 } }
+
+// Merge with overlapping keys
+const overlapMerge = toBytecode([
+  ["PUSH", "a"],
+  ["PUSH", 1],
+  ["PUSH", "b"],
+  ["PUSH", 2],
+  ["MAKE_DICT", 2],
+  ["PUSH", "b"],
+  ["PUSH", 99],
+  ["PUSH", "c"],
+  ["PUSH", 3],
+  ["MAKE_DICT", 2],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nMerge with overlapping keys ({a: 1, b: 2} + {b: 99, c: 3}):')
+const result2 = await run(overlapMerge)
+console.log(result2)
+console.log('Note: b is overwritten from 2 to 99')
+// Output: { type: 'dict', value: Map { a: 1, b: 99, c: 3 } }
+
+// Merge multiple dicts in sequence
+const multipleMerge = toBytecode([
+  ["PUSH", "a"],
+  ["PUSH", 1],
+  ["MAKE_DICT", 1],
+  ["PUSH", "b"],
+  ["PUSH", 2],
+  ["MAKE_DICT", 1],
+  ["ADD"],
+  ["PUSH", "c"],
+  ["PUSH", 3],
+  ["MAKE_DICT", 1],
+  ["ADD"],
+  ["PUSH", "d"],
+  ["PUSH", 4],
+  ["MAKE_DICT", 1],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nMultiple merges ({a: 1} + {b: 2} + {c: 3} + {d: 4}):')
+const result3 = await run(multipleMerge)
+console.log(result3)
+// Output: { type: 'dict', value: Map { a: 1, b: 2, c: 3, d: 4 } }
+
+// Merge dicts with different value types
+const mixedTypes = toBytecode([
+  ["PUSH", "num"],
+  ["PUSH", 42],
+  ["PUSH", "str"],
+  ["PUSH", "hello"],
+  ["MAKE_DICT", 2],
+  ["PUSH", "bool"],
+  ["PUSH", true],
+  ["PUSH", "null"],
+  ["PUSH", null],
+  ["MAKE_DICT", 2],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nMerge dicts with different types ({num: 42, str: "hello"} + {bool: true, null: null}):')
+const result4 = await run(mixedTypes)
+console.log(result4)
+// Output: { type: 'dict', value: Map { num: 42, str: 'hello', bool: true, null: null } }
+
+// Merge empty dict with non-empty
+const emptyMerge = toBytecode([
+  ["MAKE_DICT", 0],
+  ["PUSH", "x"],
+  ["PUSH", 100],
+  ["PUSH", "y"],
+  ["PUSH", 200],
+  ["MAKE_DICT", 2],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nMerge empty dict with {x: 100, y: 200} ({} + {x: 100, y: 200}):')
+const result5 = await run(emptyMerge)
+console.log(result5)
+// Output: { type: 'dict', value: Map { x: 100, y: 200 } }
+
+// Merge dicts with nested structures
+const nestedMerge = toBytecode([
+  ["PUSH", "data"],
+  ["PUSH", 1],
+  ["PUSH", 2],
+  ["MAKE_ARRAY", 2],
+  ["MAKE_DICT", 1],
+  ["PUSH", "config"],
+  ["PUSH", "debug"],
+  ["PUSH", true],
+  ["MAKE_DICT", 1],
+  ["MAKE_DICT", 1],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nMerge dicts with nested structures:')
+const result6 = await run(nestedMerge)
+console.log(result6)
+// Output: { type: 'dict', value: Map { data: [1, 2], config: { debug: true } } }
+
+// Building configuration objects
+const configBuild = toBytecode([
+  // Default config
+  ["PUSH", "debug"],
+  ["PUSH", false],
+  ["PUSH", "port"],
+  ["PUSH", 3000],
+  ["PUSH", "host"],
+  ["PUSH", "localhost"],
+  ["MAKE_DICT", 3],
+  // Override with user config
+  ["PUSH", "debug"],
+  ["PUSH", true],
+  ["PUSH", "port"],
+  ["PUSH", 8080],
+  ["MAKE_DICT", 2],
+  ["ADD"],
+  ["HALT"]
+])
+
+console.log('\nBuilding config (defaults + overrides):')
+const result7 = await run(configBuild)
+console.log(result7)
+// Output: { type: 'dict', value: Map { debug: true, port: 8080, host: 'localhost' } }

examples/add-with-strings.ts

@@ -0,0 +1,73 @@
+/**
+ * Demonstrates the ADD opcode working with both numbers and strings
+ *
+ * ADD now behaves like JavaScript's + operator:
+ * - If either operand is a string, it does string concatenation
+ * - Otherwise, it does numeric addition
+ */
+
+import { toBytecode, run } from "#reef"
+
+// Numeric addition
+const numericAdd = toBytecode(`
+  PUSH 10
+  PUSH 5
+  ADD
+  HALT
+`)
+
+console.log('Numeric addition (10 + 5):')
+console.log(await run(numericAdd))
+// Output: { type: 'number', value: 15 }
+
+// String concatenation
+const stringConcat = toBytecode(`
+  PUSH "hello"
+  PUSH " world"
+  ADD
+  HALT
+`)
+
+console.log('\nString concatenation ("hello" + " world"):')
+console.log(await run(stringConcat))
+// Output: { type: 'string', value: 'hello world' }
+
+// Mixed: string + number
+const mixedConcat = toBytecode(`
+  PUSH "count: "
+  PUSH 42
+  ADD
+  HALT
+`)
+
+console.log('\nMixed concatenation ("count: " + 42):')
+console.log(await run(mixedConcat))
+// Output: { type: 'string', value: 'count: 42' }
+
+// Building a message
+const buildMessage = toBytecode(`
+  PUSH "You have "
+  PUSH 3
+  ADD
+  PUSH " new messages"
+  ADD
+  HALT
+`)
+
+console.log('\nBuilding a message:')
+console.log(await run(buildMessage))
+// Output: { type: 'string', value: 'You have 3 new messages' }
+
+// Computing then concatenating
+const computeAndConcat = toBytecode(`
+  PUSH "Result: "
+  PUSH 10
+  PUSH 5
+  ADD
+  ADD
+  HALT
+`)
+
+console.log('\nComputing then concatenating ("Result: " + (10 + 5)):')
+console.log(await run(computeAndConcat))
+// Output: { type: 'string', value: 'Result: 15' }

examples/native.ts

@@ -9,7 +9,7 @@ const bytecode = toBytecode(`
 
 const vm = new VM(bytecode)
 
-vm.registerFunction('print', (...args: Value[]): Value => {
+vm.set('print', (...args: Value[]): Value => {
   console.log(...args.map(toString))
   return toNull()
 })

src/bytecode.ts

@@ -16,13 +16,15 @@ export type Constant =
   | Value
   | FunctionDef
 
-type Atom = number | string | boolean | null
+type Atom = RegExp | number | string | boolean | null
 
 type InstructionTuple =
   // Stack
   | ["PUSH", Atom]
   | ["POP"]
   | ["DUP"]
+  | ["SWAP"]
+  | ["TYPE"]
 
   // Variables
   | ["LOAD", string]
@@ -32,6 +34,9 @@ type InstructionTuple =
   // Arithmetic
   | ["ADD"] | ["SUB"] | ["MUL"] | ["DIV"] | ["MOD"]
 
+  // Bitwise
+  | ["BIT_AND"] | ["BIT_OR"] | ["BIT_XOR"] | ["BIT_SHL"] | ["BIT_SHR"] | ["BIT_USHR"]
+
   // Comparison
   | ["EQ"] | ["NEQ"] | ["LT"] | ["GT"] | ["LTE"] | ["GTE"]
 
@@ -39,9 +44,9 @@ type InstructionTuple =
   | ["NOT"]
 
   // Control flow
-  | ["JUMP", string | number]
-  | ["JUMP_IF_FALSE", string | number]
-  | ["JUMP_IF_TRUE", string | number]
+  | ["JUMP", string]
+  | ["JUMP_IF_FALSE", string]
+  | ["JUMP_IF_TRUE", string]
   | ["BREAK"]
 
   // Exception handling
@@ -51,7 +56,7 @@ type InstructionTuple =
   | ["THROW"]
 
   // Functions
-  | ["MAKE_FUNCTION", string[], string | number]
+  | ["MAKE_FUNCTION", string[], string]
   | ["CALL"]
   | ["TAIL_CALL"]
   | ["RETURN"]
@@ -83,30 +88,6 @@ type LabelDefinition = [string] // Just ".label_name:"
 
 export type ProgramItem = InstructionTuple | LabelDefinition
 
-//
-// Parse bytecode from human-readable string format.
-// Operand types are determined by prefix/literal:
-//   #42      -> immediate number (e.g., JUMP #5, MAKE_ARRAY #3)
-//   .label   -> label reference (e.g., JUMP .loop_start, MAKE_FUNCTION (x y) .body)
-//   name     -> variable/function name (e.g., LOAD x, LOAD_NATIVE add)
-//   42       -> number constant (e.g., PUSH 42)
-//   "str"    -> string constant (e.g., PUSH "hello")
-//   'str'    -> string constant (e.g., PUSH 'hello')
-//   true     -> boolean constant (e.g., PUSH true)
-//   false    -> boolean constant (e.g., PUSH false)
-//   null     -> null constant (e.g., PUSH null)
-//
-// Labels:
-//   .label_name:   -> label definition (marks current instruction position)
-//
-// Function definitions:
-//   MAKE_FUNCTION (x y) #7           -> basic function (numeric offset)
-//   MAKE_FUNCTION (x y) .body        -> basic function (label reference)
-//   MAKE_FUNCTION (x y=42) #7        -> with defaults
-//   MAKE_FUNCTION (x ...rest) #7     -> variadic
-//   MAKE_FUNCTION (x @named) #7      -> named
-//
-
 function parseFunctionParams(paramStr: string, constants: Constant[]): {
   params: string[]
   defaults: Record<string, number>
@@ -368,6 +349,29 @@ function toBytecodeFromArray(program: ProgramItem[]): Bytecode /* throws */ {
   }
 }
 
+
+////
+// Parse bytecode from human-readable string format.
+// Operand types are determined by prefix/literal:
+//   #42      -> immediate number (e.g., JUMP #5, MAKE_ARRAY #3)
+//   .label   -> label reference (e.g., JUMP .loop_start, MAKE_FUNCTION (x y) .body)
+//   name     -> variable/function name (e.g., LOAD x, LOAD_NATIVE add)
+//   42       -> number constant (e.g., PUSH 42)
+//   "str"    -> string constant (e.g., PUSH "hello")
+//   'str'    -> string constant (e.g., PUSH 'hello')
+//   true     -> boolean constant (e.g., PUSH true)
+//   false    -> boolean constant (e.g., PUSH false)
+//   null     -> null constant (e.g., PUSH null)
+//
+// Labels:
+//   .label_name:   -> label definition (marks current instruction position)
+//
+// Function definitions:
+//   MAKE_FUNCTION (x y) #7           -> basic function (numeric offset)
+//   MAKE_FUNCTION (x y) .body        -> basic function (label reference)
+//   MAKE_FUNCTION (x y=42) #7        -> with defaults
+//   MAKE_FUNCTION (x ...rest) #7     -> variadic
+//   MAKE_FUNCTION (x @named) #7      -> named
 function toBytecodeFromString(str: string): Bytecode /* throws */ {
   const lines = str.trim().split("\n")
 
@@ -386,7 +390,7 @@ function toBytecodeFromString(str: string): Bytecode /* throws */ {
     if (!trimmed) continue
 
     // Check for label definition (.label_name:)
-    if (/^\.[a-zA-Z_][a-zA-Z0-9_]*:$/.test(trimmed)) {
+    if (/^\.[a-zA-Z_][a-zA-Z0-9_.]*:$/.test(trimmed)) {
       const labelName = trimmed.slice(1, -1)
       labels.set(labelName, cleanLines.length)
       continue

src/function.ts

@@ -1,4 +1,5 @@
 import { type Value, type NativeFunction, fromValue, toValue } from "./value"
+import { VM } from "./vm"
 
 export type ParamInfo = {
   params: string[]
@@ -9,11 +10,13 @@ export type ParamInfo = {
 
 const WRAPPED_MARKER = Symbol('reef-wrapped')
 
-export function wrapNative(fn: Function): (...args: Value[]) => Promise<Value> {
-  const wrapped = async (...values: Value[]) => {
-    const nativeArgs = values.map(fromValue)
-    const result = await fn(...nativeArgs)
-    return toValue(result)
+export function wrapNative(vm: VM, fn: Function): (this: VM, ...args: Value[]) => Promise<Value> {
+  if ((fn as any).raw) return fn as (this: VM, ...args: Value[]) => Promise<Value>
+
+  const wrapped = async function (this: VM, ...values: Value[]) {
+    const nativeArgs = values.map(arg => fromValue(arg, vm))
+    const result = await fn.call(this, ...nativeArgs)
+    return toValue(result, this)
   }
 
   const wrappedObj = wrapped as any

src/index.ts

@@ -1,14 +1,17 @@
 import type { Bytecode } from "./bytecode"
-import { type Value } from "./value"
+import type { Value } from "./value"
 import { VM } from "./vm"
 
-export async function run(bytecode: Bytecode, functions?: Record<string, Function>): Promise<Value> {
-  const vm = new VM(bytecode, functions)
+export async function run(bytecode: Bytecode, globals?: Record<string, any>): Promise<Value> {
+  const vm = new VM(bytecode, globals)
   return await vm.run()
 }
 
 export { type Bytecode, toBytecode, type ProgramItem } from "./bytecode"
-export { wrapNative } from "./function"
-export { type Value, toValue, toString, toNumber, fromValue, toNull } from "./value"
-export { VM } from "./vm"
 export { bytecodeToString } from "./format"
+export { wrapNative, isWrapped, type ParamInfo, extractParamInfo, getOriginalFunction } from "./function"
+export { OpCode } from "./opcode"
+export { Scope } from "./scope"
+export type { Value, TypeScriptFunction, NativeFunction } from "./value"
+export { isValue, toValue, toString, toNumber, fromValue, toNull, fnFromValue } from "./value"
+export { VM } from "./vm"

src/opcode.ts

@@ -3,12 +3,16 @@ export enum OpCode {
   PUSH,  // operand: constant index (number) | stack: [] → [value]
   POP,   // operand: none | stack: [value] → []
   DUP,   // operand: none | stack: [value] → [value, value]
+  SWAP,  // operand: none | stack: [value1, value2] → [value2, value1]
 
   // variables
   LOAD,     // operand: variable name (identifier) | stack: [] → [value]
   STORE,    // operand: variable name (identifier) | stack: [value] → []
   TRY_LOAD, // operand: variable name (identifier) | stack: [] → [value] | load a variable (if found) or a string
 
+  // information
+  TYPE,      // operand: none | stack: [a] → []
+
   // math (coerce to number, pop 2, push result)
   ADD,   // operand: none | stack: [a, b] → [a + b]
   SUB,   // operand: none | stack: [a, b] → [a - b]
@@ -16,6 +20,14 @@ export enum OpCode {
   DIV,   // operand: none | stack: [a, b] → [a / b]
   MOD,   // operand: none | stack: [a, b] → [a % b]
 
+  // bitwise operations (coerce to 32-bit integers, pop 2, push result)
+  BIT_AND,   // operand: none | stack: [a, b] → [a & b]
+  BIT_OR,    // operand: none | stack: [a, b] → [a | b]
+  BIT_XOR,   // operand: none | stack: [a, b] → [a ^ b]
+  BIT_SHL,   // operand: none | stack: [a, b] → [a << b]
+  BIT_SHR,   // operand: none | stack: [a, b] → [a >> b] (sign-preserving)
+  BIT_USHR,  // operand: none | stack: [a, b] → [a >>> b] (zero-fill)
+
   // comparison (pop 2, push boolean)
   EQ,    // operand: none | stack: [a, b] → [a == b] (deep equality)
   NEQ,   // operand: none | stack: [a, b] → [a != b]

src/scope.ts

@@ -30,6 +30,15 @@ export class Scope {
   has(name: string): boolean {
     return this.locals.has(name) || this.parent?.has(name) || false
   }
+
+  vars(): string[] {
+    const vars = new Set(this.parent?.vars())
+
+    for (const name of this.locals.keys())
+      vars.add(name)
+
+    return [...vars].sort()
+  }
 }
 
 

src/validator.ts

@@ -50,11 +50,19 @@ const OPCODES_WITH_OPERANDS = new Set([
 const OPCODES_WITHOUT_OPERANDS = new Set([
   OpCode.POP,
   OpCode.DUP,
+  OpCode.SWAP,
+  OpCode.TYPE,
   OpCode.ADD,
   OpCode.SUB,
   OpCode.MUL,
   OpCode.DIV,
   OpCode.MOD,
+  OpCode.BIT_AND,
+  OpCode.BIT_OR,
+  OpCode.BIT_XOR,
+  OpCode.BIT_SHL,
+  OpCode.BIT_SHR,
+  OpCode.BIT_USHR,
   OpCode.EQ,
   OpCode.NEQ,
   OpCode.LT,
@@ -79,11 +87,15 @@ const OPCODES_WITHOUT_OPERANDS = new Set([
   OpCode.DOT_GET,
 ])
 
-// immediate = immediate number, eg #5
-const OPCODES_REQUIRING_IMMEDIATE_OR_LABEL = new Set([
+// JUMP* instructions require labels only (no numeric immediates)
+const OPCODES_REQUIRING_LABEL = new Set([
   OpCode.JUMP,
   OpCode.JUMP_IF_FALSE,
   OpCode.JUMP_IF_TRUE,
+])
+
+// PUSH_TRY/PUSH_FINALLY still allow immediate or label
+const OPCODES_REQUIRING_IMMEDIATE_OR_LABEL = new Set([
   OpCode.PUSH_TRY,
   OpCode.PUSH_FINALLY,
 ])
@@ -189,6 +201,16 @@ export function validateBytecode(source: string): ValidationResult {
 
     // Validate specific operand formats
     if (operand) {
+      if (OPCODES_REQUIRING_LABEL.has(opCode)) {
+        if (!operand.startsWith('.')) {
+          errors.push({
+            line: lineNum,
+            message: `${opName} requires label (.label), got: ${operand}`,
+          })
+          continue
+        }
+      }
+
       if (OPCODES_REQUIRING_IMMEDIATE_OR_LABEL.has(opCode)) {
         if (!operand.startsWith('#') && !operand.startsWith('.')) {
           errors.push({
@@ -302,11 +324,11 @@ export function validateBytecode(source: string): ValidationResult {
           }
         }
 
-        // Validate body address
-        if (!bodyAddr!.startsWith('.') && !bodyAddr!.startsWith('#')) {
+        // Validate body address (must be a label)
+        if (!bodyAddr!.startsWith('.')) {
           errors.push({
             line: lineNum,
-            message: `Invalid body address: expected .label or #offset`,
+            message: `Invalid body address: expected .label, got: ${bodyAddr}`,
           })
         }
 

src/value.ts

@@ -1,6 +1,12 @@
+import { wrapNative, getOriginalFunction } from "./function"
+import { OpCode } from "./opcode"
 import { Scope } from "./scope"
+import { VM } from "./vm"
 
 export type NativeFunction = (...args: Value[]) => Promise<Value> | Value
+export type TypeScriptFunction = (this: VM, ...args: any[]) => any
+const REEF_FUNCTION = Symbol('__reefFunction')
+const VALUE_TYPES = new Set(['null', 'boolean', 'number', 'string', 'array', 'dict', 'regex', 'native', 'function'])
 
 export type Value =
   | { type: 'null', value: null }
@@ -33,15 +39,19 @@ export type FunctionDef = {
   named: boolean
 }
 
-export function toValue(v: any): Value /* throws */ {
+export function isValue(v: any): boolean {
+  return !!(v && typeof v === 'object' && VALUE_TYPES.has(v.type) && 'value' in v)
+}
+
+export function toValue(v: any, vm?: VM): Value /* throws */ {
   if (v === null || v === undefined)
     return { type: 'null', value: null }
 
-  if (v && typeof v === 'object' && 'type' in v && 'value' in v)
+  if (isValue(v))
     return v as Value
 
   if (Array.isArray(v))
-    return { type: 'array', value: v.map(toValue) }
+    return { type: 'array', value: v.map(x => toValue(x, vm)) }
 
   if (v instanceof RegExp)
     return { type: 'regex', value: v }
@@ -54,15 +64,38 @@ export function toValue(v: any): Value /* throws */ {
     case 'string':
       return { type: 'string', value: v }
     case 'function':
-      throw "can't toValue() a js function yet"
+      if ((v as any)[REEF_FUNCTION])
+        return (v as any)[REEF_FUNCTION]
+
+      let fn = vm ? wrapNative(vm, v) : cantCallFunctionWithoutVM(v)
+      return { type: 'native', fn, value: '<function>' }
     case 'object':
       const dict: Dict = new Map()
 
-      for (const key of Object.keys(v)) dict.set(key, toValue(v[key]))
+      for (const key of Object.keys(v)) dict.set(key, toValue(v[key], vm))
 
       return { type: 'dict', value: dict }
     default:
-      throw `can't toValue this: ${v}`
+      throw new Error(`can't toValue this: ${v}`)
+  }
+}
+
+function cantCallFunctionWithoutVM(fn: Function) {
+  const name = fn.name || '<anonymous>'
+  const str = fn.toString().slice(0, 100)
+  return (...args: Value[]) => {
+    const stack = new Error().stack || ''
+    const stackLines = stack.split('\n')
+      .slice(1)
+      .filter(line => !line.includes('toValue'))
+      .map(line => '    ' + line.trim())
+      .join('\n')
+    throw new Error(
+      `can't call function that was converted without a vm\n` +
+      `  Function: ${name}\n` +
+      `  Source: ${str}${str.length > 100 ? '...' : ''}\n` +
+      `  Called from:\n${stackLines}`
+    )
   }
 }
 
@@ -154,28 +187,87 @@ export function isEqual(a: Value, b: Value): boolean {
   }
 }
 
-export function fromValue(v: Value): any {
+export function fromValue(v: Value, vm?: VM): any {
   switch (v.type) {
     case 'null':
-      return null
     case 'boolean':
-      return v.value
     case 'number':
-      return v.value
     case 'string':
       return v.value
     case 'array':
-      return v.value.map(fromValue)
+      return v.value.map(x => fromValue(x, vm))
     case 'dict':
-      return Object.fromEntries(v.value.entries().map(([k, v]) => [k, fromValue(v)]))
+      return Object.fromEntries(v.value.entries().map(([k, v]) => [k, fromValue(v, vm)]))
     case 'regex':
       return v.value
     case 'function':
+      if (!vm || !(vm instanceof VM)) {
+        const stack = new Error().stack || ''
+        const stackLines = stack.split('\n')
+          .slice(1)
+          .filter(line => !line.includes('fromValue'))
+          .map(line => '    ' + line.trim())
+          .join('\n')
+        throw new Error(
+          `VM is required for function conversion\n` +
+          `  Function params: [${v.params.join(', ')}]\n` +
+          `  Function body at instruction: ${v.body}\n` +
+          `  Called from:\n${stackLines}`
+        )
+      }
+      return fnFromValue(v, vm)
     case 'native':
-      return '<function>'
+      return getOriginalFunction(v.fn)
   }
 }
 
 export function toNull(): Value {
   return toValue(null)
 }
+
+export function fnFromValue(fn: Value, vm: VM): Function {
+  if (fn.type !== 'function')
+    throw new Error('Value is not a function')
+
+  const wrapper = async function (...args: any[]) {
+    let positional: any[] = args
+    let named: Record<string, any> = {}
+
+    if (args.length > 0 && !Array.isArray(args[args.length - 1]) && args[args.length - 1].constructor === Object) {
+      named = args[args.length - 1]
+      positional = args.slice(0, -1)
+    }
+
+    const newVM = new VM({
+      instructions: vm.instructions,
+      constants: vm.constants,
+      labels: vm.labels
+    })
+    newVM.scope = fn.parentScope
+
+    newVM.stack.push(fn)
+    newVM.stack.push(...positional.map(x => toValue(x, vm)))
+    for (const [key, val] of Object.entries(named)) {
+      newVM.stack.push(toValue(key))
+      newVM.stack.push(toValue(val, vm))
+    }
+    newVM.stack.push(toValue(positional.length))
+    newVM.stack.push(toValue(Object.keys(named).length))
+
+    const targetDepth = newVM.callStack.length
+    await newVM.execute({ op: OpCode.CALL })
+    newVM.pc++
+
+    while (newVM.callStack.length > targetDepth && newVM.pc < newVM.instructions.length) {
+      await newVM.execute(newVM.instructions[newVM.pc]!)
+      newVM.pc++
+    }
+
+    return fromValue(newVM.stack.pop() || toNull(), vm)
+  }
+
+    // support roundtrips, eg fromValue(toValue(fn))
+    ; (wrapper as any)[REEF_FUNCTION] = fn
+
+  return wrapper
+}

src/vm.ts

@@ -3,8 +3,9 @@ import type { ExceptionHandler } from "./exception"
 import { type Frame } from "./frame"
 import { OpCode } from "./opcode"
 import { Scope } from "./scope"
-import { type Value, type NativeFunction, toValue, toNumber, isTrue, isEqual, toString, fromValue } from "./value"
-import { extractParamInfo, wrapNative, isWrapped, getOriginalFunction } from "./function"
+import type { Value, NativeFunction, TypeScriptFunction } from "./value"
+import { toValue, toNumber, isTrue, isEqual, toString, fromValue, fnFromValue } from "./value"
+import { extractParamInfo, getOriginalFunction } from "./function"
 
 export class VM {
   pc = 0
@@ -18,26 +19,71 @@ export class VM {
   labels: Map<number, string> = new Map()
   nativeFunctions: Map<string, NativeFunction> = new Map()
 
-  constructor(bytecode: Bytecode, functions?: Record<string, Function>) {
+  constructor(bytecode: Bytecode, globals?: Record<string, any>) {
     this.instructions = bytecode.instructions
     this.constants = bytecode.constants
     this.labels = bytecode.labels || new Map()
     this.scope = new Scope()
 
-    if (functions)
-      for (const name of Object.keys(functions))
-        this.registerFunction(name, functions[name]!)
+    if (globals) {
+      for (const name of Object.keys(globals ?? {}))
+        this.set(name, globals[name])
+
+      this.scope = new Scope(this.scope)
+    }
   }
 
-  registerFunction(name: string, fn: Function) {
-    const wrapped = isWrapped(fn) ? fn as NativeFunction : wrapNative(fn)
-    this.scope.set(name, { type: 'native', fn: wrapped, value: '<function>' })
+  async call(name: string, ...args: any) {
+    const value = this.scope.get(name)
+
+    if (!value) throw new Error(`Can't find ${name}`)
+    if (value.type !== 'function' && value.type !== 'native')
+      throw new Error(`Can't call ${name}`)
+
+    if (value.type === 'native') {
+      return await this.callNative(value.fn, args)
+    } else {
+      const fn = fnFromValue(value, this)
+      return await fn(...args)
+    }
   }
 
-  registerValueFunction(name: string, fn: NativeFunction) {
+  has(name: string): boolean {
+    return this.scope.has(name)
+  }
+
+  vars(): string[] {
+    return this.scope.vars()
+  }
+
+  get(name: string) {
+    return this.scope.get(name)
+  }
+
+  set(name: string, value: any) {
+    this.scope.set(name, toValue(value, this))
+  }
+
+  setFunction(name: string, fn: TypeScriptFunction) {
+    this.scope.set(name, toValue(fn, this))
+  }
+
+  setValueFunction(name: string, fn: NativeFunction) {
     this.scope.set(name, { type: 'native', fn, value: '<function>' })
   }
 
+  pushScope(locals?: Record<string, any>) {
+    this.scope = new Scope(this.scope)
+
+    if (locals)
+      for (const [name, value] of Object.entries(locals))
+        this.set(name, value)
+  }
+
+  popScope() {
+    this.scope = this.scope.parent!
+  }
+
   async run(): Promise<Value> {
     this.pc = 0
     this.stopped = false
@@ -51,6 +97,57 @@ export class VM {
     return this.stack[this.stack.length - 1] || toValue(null)
   }
 
+  // Resume execution from current PC without resetting
+  // Useful for REPL mode where you append bytecode incrementally
+  async continue(): Promise<Value> {
+    this.stopped = false
+
+    while (!this.stopped && this.pc < this.instructions.length) {
+      const instruction = this.instructions[this.pc]!
+      await this.execute(instruction)
+      this.pc++
+    }
+
+    return this.stack[this.stack.length - 1] || toValue(null)
+  }
+
+  // Helper for REPL mode: append new bytecode with proper constant index remapping
+  appendBytecode(bytecode: Bytecode): void {
+    const constantOffset = this.constants.length
+    const instructionOffset = this.instructions.length
+
+    // Remap function body addresses in constants before adding them
+    for (const constant of bytecode.constants) {
+      if (constant.type === 'function_def') {
+        this.constants.push({ ...constant, body: constant.body + instructionOffset })
+      } else {
+        this.constants.push(constant)
+      }
+    }
+
+    for (const instruction of bytecode.instructions) {
+      if (instruction.operand !== undefined && typeof instruction.operand === 'number') {
+        // Opcodes that reference constants need their operand adjusted
+        if (instruction.op === OpCode.PUSH || instruction.op === OpCode.MAKE_FUNCTION) {
+          this.instructions.push({
+            op: instruction.op,
+            operand: instruction.operand + constantOffset
+          })
+        } else {
+          this.instructions.push(instruction)
+        }
+      } else {
+        this.instructions.push(instruction)
+      }
+    }
+
+    if (bytecode.labels) {
+      for (const [addr, label] of bytecode.labels.entries()) {
+        this.labels.set(addr + instructionOffset, label)
+      }
+    }
+  }
+
   async execute(instruction: Instruction) /* throws */ {
     switch (instruction.op) {
       case OpCode.PUSH:
@@ -71,8 +168,32 @@ export class VM {
         this.stack.push(this.stack[this.stack.length - 1]!)
         break
 
+      case OpCode.SWAP:
+        const first = this.stack.pop()!
+        const second = this.stack.pop()!
+        this.stack.push(first)
+        this.stack.push(second)
+        break
+
       case OpCode.ADD:
-        this.binaryOp((a, b) => toNumber(a) + toNumber(b))
+        const b = this.stack.pop()!
+        const a = this.stack.pop()!
+
+        if (a.type === 'string' || b.type === 'string') {
+          this.stack.push(toValue(toString(a) + toString(b)))
+        } else if (a.type === 'array' && b.type === 'array') {
+          this.stack.push({ type: 'array', value: [...a.value, ...b.value] })
+        } else if (a.type === 'dict' && b.type === 'dict') {
+          const merged = new Map(a.value)
+          for (const [key, value] of b.value) {
+            merged.set(key, value)
+          }
+          this.stack.push({ type: 'dict', value: merged })
+        } else if (a.type === 'number' && b.type === 'number') {
+          this.stack.push(toValue(a.value + b.value))
+        } else {
+          throw new Error(`ADD: Cannot add ${a.type} and ${b.type}`)
+        }
         break
 
       case OpCode.SUB:
@@ -120,6 +241,31 @@ export class VM {
         this.stack.push({ type: 'boolean', value: !isTrue(val) })
         break
 
+      // Bitwise operations
+      case OpCode.BIT_AND:
+        this.binaryOp((a, b) => (toNumber(a) | 0) & (toNumber(b) | 0))
+        break
+
+      case OpCode.BIT_OR:
+        this.binaryOp((a, b) => (toNumber(a) | 0) | (toNumber(b) | 0))
+        break
+
+      case OpCode.BIT_XOR:
+        this.binaryOp((a, b) => (toNumber(a) | 0) ^ (toNumber(b) | 0))
+        break
+
+      case OpCode.BIT_SHL:
+        this.binaryOp((a, b) => (toNumber(a) | 0) << (toNumber(b) | 0))
+        break
+
+      case OpCode.BIT_SHR:
+        this.binaryOp((a, b) => (toNumber(a) | 0) >> (toNumber(b) | 0))
+        break
+
+      case OpCode.BIT_USHR:
+        this.binaryOp((a, b) => (toNumber(a) | 0) >>> (toNumber(b) | 0))
+        break
+
       case OpCode.HALT:
         this.stopped = true
         break
@@ -140,13 +286,18 @@ export class VM {
         const value = this.scope.get(varName)
 
         if (value === undefined)
-          this.stack.push(toValue(varName))
+          this.stack.push(toValue(varName, this))
         else
           this.stack.push(value)
 
         break
       }
 
+      case OpCode.TYPE:
+        const value = this.stack.pop()!
+        this.stack.push(toValue(value.type))
+        break
+
       case OpCode.STORE:
         const name = instruction.operand as string
         const toStore = this.stack.pop()!
@@ -342,10 +493,10 @@ export class VM {
         const target = this.stack.pop()!
 
         if (target.type === 'array')
-          this.stack.push(toValue(target.value?.[Number(index.value)]))
+          this.stack.push(toValue(target.value?.[Number(index.value)], this))
 
         else if (target.type === 'dict')
-          this.stack.push(toValue(target.value?.get(String(index.value))))
+          this.stack.push(toValue(target.value?.get(String(index.value)), this))
 
         else
           throw new Error(`DOT_GET: ${target.type} not supported`)
@@ -451,16 +602,25 @@ export class VM {
           let nativePositionalArgIndex = 0
 
           // Bind fixed parameters using priority: named arg > positional arg > default > null
+          // Note: null values trigger defaults (null acts as "use default")
           for (let i = 0; i < nativeFixedParamCount; i++) {
             const paramName = paramInfo.params[i]!
+            let paramValue: Value | undefined
 
             // Check if named argument was provided for this param
             if (namedArgs.has(paramName)) {
-              nativeArgs.push(namedArgs.get(paramName)!)
+              paramValue = namedArgs.get(paramName)!
               namedArgs.delete(paramName) // Remove from named args so it won't go to @named
             } else if (nativePositionalArgIndex < positionalArgs.length) {
-              nativeArgs.push(positionalArgs[nativePositionalArgIndex]!)
+              paramValue = positionalArgs[nativePositionalArgIndex]!
               nativePositionalArgIndex++
+            }
+
+            // If the parameter value is null and a default exists, use the default
+            if (paramValue?.type === 'null' && paramInfo.defaults[paramName] !== undefined) {
+              nativeArgs.push(paramInfo.defaults[paramName]!)
+            } else if (paramValue) {
+              nativeArgs.push(paramValue)
             } else if (paramInfo.defaults[paramName] !== undefined) {
               nativeArgs.push(paramInfo.defaults[paramName]!)
             } else {
@@ -468,14 +628,6 @@ export class VM {
             }
           }
 
-          // Handle variadic parameter (TypeScript rest parameters)
-          // For TypeScript functions with ...rest, we spread the remaining args
-          // rather than wrapping them in an array
-          if (paramInfo.variadic) {
-            const remainingArgs = positionalArgs.slice(nativePositionalArgIndex)
-            nativeArgs.push(...remainingArgs)
-          }
-
           // Handle named parameter (collect remaining unmatched named args)
           // Parameter names matching atXxx pattern (e.g., atOptions, atNamed) collect extra named args
           if (paramInfo.named) {
@@ -484,18 +636,54 @@ export class VM {
               namedDict.set(key, value)
             }
             // Convert dict to plain JavaScript object for the native function
-            const namedObj = fromValue({ type: 'dict', value: namedDict })
-            nativeArgs.push(toValue(namedObj))
+            const namedObj = fromValue({ type: 'dict', value: namedDict }, this)
+            nativeArgs.push(toValue(namedObj, this))
+          }
+
+          // Handle variadic parameter (TypeScript rest parameters)
+          // For TypeScript functions with ...rest, we spread the remaining args
+          // rather than wrapping them in an array
+          if (paramInfo.variadic) {
+            const remainingArgs = positionalArgs.slice(nativePositionalArgIndex)
+            nativeArgs.push(...remainingArgs)
           }
 
           // Call the native function with bound args
-          const result = await fn.fn(...nativeArgs)
-          this.stack.push(result)
-          break
+          try {
+            const result = await fn.fn.call(this, ...nativeArgs)
+            this.stack.push(result)
+            break
+          } catch (error) {
+            const errorMessage = error instanceof Error ? error.message : String(error)
+            const errorValue = toValue(errorMessage)
+
+            // no exception handlers, let it crash
+            if (this.exceptionHandlers.length === 0) {
+              throw new Error(errorMessage)
+            }
+
+            // use existing THROW logic
+            const throwHandler = this.exceptionHandlers.pop()!
+
+            while (this.callStack.length > throwHandler.callStackDepth)
+              this.callStack.pop()
+
+            this.scope = throwHandler.scope
+            this.stack.push(errorValue)
+
+            // Jump to `finally` if present, otherwise jump to `catch`
+            const targetAddress = throwHandler.finallyAddress !== undefined
+              ? throwHandler.finallyAddress
+              : throwHandler.catchAddress
+
+            // subtract 1 because pc will be incremented
+            this.pc = targetAddress - 1
+            break
+          }
         }
 
         if (fn.type !== 'function')
-          throw new Error('CALL: not a function')
+          throw new Error(`CALL: ${fn.value} is not a function`)
 
         if (this.callStack.length > 0)
           this.callStack[this.callStack.length - 1]!.isBreakTarget = true
@@ -517,16 +705,29 @@ export class VM {
         let positionalArgIndex = 0
 
         // Bind fixed parameters using priority: named arg > positional arg > default > null
+        // Note: null values trigger defaults (null acts as "use default")
         for (let i = 0; i < fixedParamCount; i++) {
           const paramName = fn.params[i]!
+          let paramValue: Value | undefined
 
           // Check if named argument was provided for this param
           if (namedArgs.has(paramName)) {
-            this.scope.set(paramName, namedArgs.get(paramName)!)
+            paramValue = namedArgs.get(paramName)!
             namedArgs.delete(paramName) // Remove from named args so it won't go to named
           } else if (positionalArgIndex < positionalArgs.length) {
-            this.scope.set(paramName, positionalArgs[positionalArgIndex]!)
+            paramValue = positionalArgs[positionalArgIndex]!
             positionalArgIndex++
+          }
+
+          // If the parameter value is null and a default exists, use the default
+          if (paramValue && paramValue.type === 'null' && fn.defaults[paramName] !== undefined) {
+            const defaultIdx = fn.defaults[paramName]!
+            const defaultValue = this.constants[defaultIdx]!
+            if (defaultValue.type === 'function_def')
+              throw new Error('Default value cannot be a function definition')
+            this.scope.set(paramName, defaultValue)
+          } else if (paramValue) {
+            this.scope.set(paramName, paramValue)
           } else if (fn.defaults[paramName] !== undefined) {
             const defaultIdx = fn.defaults[paramName]!
             const defaultValue = this.constants[defaultIdx]!
@@ -587,7 +788,7 @@ export class VM {
         const tailFn = this.stack.pop()!
 
         if (tailFn.type !== 'function')
-          throw new Error('TAIL_CALL: not a function')
+          throw new Error(`TAIL_CALL: ${tailFn.value} is not a function`)
 
         this.scope = new Scope(tailFn.parentScope)
 
@@ -656,7 +857,7 @@ export class VM {
         break
 
       default:
-        throw `Unknown op: ${instruction.op}`
+        throw new Error(`Unknown op: ${instruction.op}`)
     }
   }
 
@@ -673,4 +874,26 @@ export class VM {
     const result = fn(a, b)
     this.stack.push({ type: 'boolean', value: result })
   }
+
+  async callNative(nativeFn: NativeFunction, args: any[]): Promise<Value> {
+    const originalFn = getOriginalFunction(nativeFn)
+
+    const lastArg = args[args.length - 1]
+    if (lastArg && !Array.isArray(lastArg) && typeof lastArg === 'object') {
+      const paramInfo = extractParamInfo(originalFn)
+      const positional = args.slice(0, -1)
+      const named = lastArg
+
+      args = [...positional]
+      for (let i = positional.length; i < paramInfo.params.length; i++) {
+        const paramName = paramInfo.params[i]!
+        if (named[paramName] !== undefined) {
+          args[i] = named[paramName]
+        }
+      }
+    }
+
+    const result = await originalFn.call(this, ...args)
+    return toValue(result, this)
+  }
 }

tests/bitwise.test.ts

@@ -0,0 +1,107 @@
+import { expect, describe, test } from 'bun:test'
+import { toBytecode, run } from '#reef'
+
+describe('bitwise operations', () => {
+  test('BIT_AND', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 5], ["PUSH", 3], ["BIT_AND"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(1)
+  })
+
+  test('BIT_AND with zero', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 5], ["PUSH", 0], ["BIT_AND"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(0)
+  })
+
+  test('BIT_OR', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 5], ["PUSH", 3], ["BIT_OR"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(7)
+  })
+
+  test('BIT_OR with zero', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 5], ["PUSH", 0], ["BIT_OR"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(5)
+  })
+
+  test('BIT_XOR', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 5], ["PUSH", 3], ["BIT_XOR"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(6)
+  })
+
+  test('BIT_XOR with itself returns zero', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 5], ["PUSH", 5], ["BIT_XOR"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(0)
+  })
+
+  test('BIT_SHL', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 5], ["PUSH", 2], ["BIT_SHL"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(20)
+  })
+
+  test('BIT_SHL by zero', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 5], ["PUSH", 0], ["BIT_SHL"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(5)
+  })
+
+  test('BIT_SHR', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 20], ["PUSH", 2], ["BIT_SHR"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(5)
+  })
+
+  test('BIT_SHR preserves sign for negative numbers', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", -20], ["PUSH", 2], ["BIT_SHR"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(-5)
+  })
+
+  test('BIT_USHR', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", -1], ["PUSH", 1], ["BIT_USHR"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(2147483647)
+  })
+
+  test('BIT_USHR does not preserve sign', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", -8], ["PUSH", 1], ["BIT_USHR"], ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(2147483644)
+  })
+
+  test('compound bitwise operations', async () => {
+    const bytecode = toBytecode([
+      // (5 & 3) | (8 ^ 12)
+      ["PUSH", 5], ["PUSH", 3], ["BIT_AND"],    // stack: [1]
+      ["PUSH", 8], ["PUSH", 12], ["BIT_XOR"],   // stack: [1, 4]
+      ["BIT_OR"],                                // stack: [5]
+      ["HALT"]
+    ])
+    await expect(bytecode).toBeNumber(5)
+  })
+
+  test('shift with large shift amounts', async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 1], ["PUSH", 31], ["BIT_SHL"], ["HALT"]
+    ])
+    // 1 << 31 = -2147483648 (most significant bit set)
+    await expect(bytecode).toBeNumber(-2147483648)
+  })
+})

tests/exceptions.test.ts

@@ -15,7 +15,7 @@ test("PUSH_TRY and POP_TRY - no exception thrown", async () => {
     PUSH 999
     HALT
   `
-  expect(await run(toBytecode(str))).toEqual({ type: 'number', value: 52 })
+  await expect(str).toBeNumber(52)
 })
 
 test("THROW - catch exception with error value", async () => {
@@ -29,7 +29,7 @@ test("THROW - catch exception with error value", async () => {
     .catch:
     HALT
   `
-  expect(await run(toBytecode(str))).toEqual({ type: 'string', value: 'error occurred' })
+  await expect(str).toBeString('error occurred')
 })
 
 test("THROW - uncaught exception throws JS error", async () => {
@@ -58,7 +58,7 @@ test("THROW - exception with nested try blocks", async () => {
     PUSH "outer error"
     HALT
   `
-  expect(await run(toBytecode(str))).toEqual({ type: 'string', value: 'inner error' })
+  await expect(str).toBeString('inner error')
 })
 
 test("THROW - exception skips outer handler", async () => {
@@ -75,7 +75,7 @@ test("THROW - exception skips outer handler", async () => {
     .outer_catch:
     HALT
   `
-  expect(await run(toBytecode(str))).toEqual({ type: 'string', value: 'error message' })
+  await expect(str).toBeString('error message')
 })
 
 test("THROW - exception unwinds call stack", async () => {
@@ -150,7 +150,7 @@ test("PUSH_FINALLY - finally executes after successful try", async () => {
     ADD
     HALT
   `
-  expect(await run(toBytecode(str))).toEqual({ type: 'number', value: 110 })
+  await expect(str).toBeNumber(110)
 })
 
 test("PUSH_FINALLY - finally executes after exception", async () => {
@@ -169,7 +169,7 @@ test("PUSH_FINALLY - finally executes after exception", async () => {
     PUSH "finally ran"
     HALT
   `
-  expect(await run(toBytecode(str))).toEqual({ type: 'string', value: 'finally ran' })
+  await expect(str).toBeString('finally ran')
 })
 
 test("PUSH_FINALLY - finally without catch", async () => {
@@ -189,7 +189,7 @@ test("PUSH_FINALLY - finally without catch", async () => {
     ADD
     HALT
   `
-  expect(await run(toBytecode(str))).toEqual({ type: 'number', value: 52 })
+  await expect(str).toBeNumber(52)
 })
 
 test("PUSH_FINALLY - nested try-finally blocks", async () => {
@@ -214,7 +214,7 @@ test("PUSH_FINALLY - nested try-finally blocks", async () => {
     ADD
     HALT
   `
-  expect(await run(toBytecode(str))).toEqual({ type: 'number', value: 11 })
+  await expect(str).toBeNumber(11)
 })
 
 test("PUSH_FINALLY - error when no handler", async () => {

tests/functions-parameter.test.ts

@@ -104,7 +104,7 @@ describe("functions parameter", () => {
     expect(result).toEqual({ type: 'number', value: 200 })
   })
 
-  test("can combine with manual registerFunction", async () => {
+  test("can combine with manual vm.set", async () => {
     const bytecode = toBytecode(`
       LOAD add
       PUSH 5
@@ -127,7 +127,7 @@ describe("functions parameter", () => {
     })
 
     // Register another function manually
-    vm.registerFunction('subtract', (a: number, b: number) => a - b)
+    vm.set('subtract', (a: number, b: number) => a - b)
 
     const result = await vm.run()
     expect(result).toEqual({ type: 'number', value: 6 })
@@ -224,7 +224,7 @@ describe("functions parameter", () => {
     })
 
     // Override with manual registration
-    vm.registerFunction('getValue', () => 200)
+    vm.set('getValue', () => 200)
 
     const result = await vm.run()
     expect(result).toEqual({ type: 'number', value: 200 })

tests/functions.test.ts

@@ -487,7 +487,7 @@ test("TRY_CALL - handles null values", async () => {
 
 test("TRY_CALL - function can access its parameters", async () => {
   const bytecode = toBytecode([
-    ["MAKE_FUNCTION", ["x"], ".body"],
+    ["MAKE_FUNCTION", ["x=0"], ".body"],
     ["STORE", "addFive"],
     ["PUSH", 10],
     ["STORE", "x"],
@@ -501,8 +501,8 @@ test("TRY_CALL - function can access its parameters", async () => {
   ])
 
   const result = await run(bytecode)
-  // Function is called with 0 args, so x inside function should be null
-  // Then we add 5 to null (which coerces to 0)
+  // Function is called with 0 args, so x defaults to 0
+  // Then we add 5 to 0
   expect(result).toEqual({ type: 'number', value: 5 })
 })
 
@@ -519,3 +519,206 @@ test("TRY_CALL - with string format", async () => {
   const result = await run(bytecode)
   expect(result).toEqual({ type: 'number', value: 100 })
 })
+
+test("CALL - passing null triggers default value for single parameter", async () => {
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", ["x=42"], ".body"],
+    ["STORE", "func"],
+    ["JUMP", ".end"],
+    [".body:"],
+    ["LOAD", "x"],
+    ["RETURN"],
+    [".end:"],
+    ["LOAD", "func"],
+    ["PUSH", null],
+    ["PUSH", 1],
+    ["PUSH", 0],
+    ["CALL"],
+    ["HALT"]
+  ])
+
+  const result = await run(bytecode)
+  // Passing null should trigger the default value of 42
+  expect(result).toEqual({ type: 'number', value: 42 })
+})
+
+test("CALL - passing null triggers default value for multiple parameters", async () => {
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", ["a=10", "b=20", "c=30"], ".body"],
+    ["STORE", "func"],
+    ["JUMP", ".end"],
+    [".body:"],
+    ["LOAD", "a"],
+    ["LOAD", "b"],
+    ["ADD"],
+    ["LOAD", "c"],
+    ["ADD"],
+    ["RETURN"],
+    [".end:"],
+    ["LOAD", "func"],
+    ["PUSH", 5],
+    ["PUSH", null],
+    ["PUSH", null],
+    ["PUSH", 3],
+    ["PUSH", 0],
+    ["CALL"],
+    ["HALT"]
+  ])
+
+  const result = await run(bytecode)
+  // a=5 (provided), b=20 (null triggers default), c=30 (null triggers default)
+  // Result: 5 + 20 + 30 = 55
+  expect(result).toEqual({ type: 'number', value: 55 })
+})
+
+test("CALL - null in middle parameter triggers default", async () => {
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", ["x=100", "y=200", "z=300"], ".body"],
+    ["STORE", "func"],
+    ["JUMP", ".end"],
+    [".body:"],
+    ["LOAD", "x"],
+    ["LOAD", "y"],
+    ["ADD"],
+    ["LOAD", "z"],
+    ["ADD"],
+    ["RETURN"],
+    [".end:"],
+    ["LOAD", "func"],
+    ["PUSH", 1],
+    ["PUSH", null],
+    ["PUSH", 3],
+    ["PUSH", 3],
+    ["PUSH", 0],
+    ["CALL"],
+    ["HALT"]
+  ])
+
+  const result = await run(bytecode)
+  // x=1, y=200 (null triggers default), z=3
+  // Result: 1 + 200 + 3 = 204
+  expect(result).toEqual({ type: 'number', value: 204 })
+})
+
+test("CALL - null with named arguments triggers default", async () => {
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", ["x=50", "y=75"], ".body"],
+    ["STORE", "func"],
+    ["JUMP", ".end"],
+    [".body:"],
+    ["LOAD", "x"],
+    ["LOAD", "y"],
+    ["ADD"],
+    ["RETURN"],
+    [".end:"],
+    ["LOAD", "func"],
+    ["PUSH", "x"],
+    ["PUSH", null],
+    ["PUSH", "y"],
+    ["PUSH", 25],
+    ["PUSH", 0],
+    ["PUSH", 2],
+    ["CALL"],
+    ["HALT"]
+  ])
+
+  const result = await run(bytecode)
+  // x=50 (null triggers default), y=25 (provided via named arg)
+  // Result: 50 + 25 = 75
+  expect(result).toEqual({ type: 'number', value: 75 })
+})
+
+test("CALL - null with string default value", async () => {
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", ["name='Guest'"], ".body"],
+    ["STORE", "greet"],
+    ["JUMP", ".end"],
+    [".body:"],
+    ["PUSH", "Hello, "],
+    ["LOAD", "name"],
+    ["ADD"],
+    ["RETURN"],
+    [".end:"],
+    ["LOAD", "greet"],
+    ["PUSH", null],
+    ["PUSH", 1],
+    ["PUSH", 0],
+    ["CALL"],
+    ["HALT"]
+  ])
+
+  const result = await run(bytecode)
+  // Passing null should trigger the default value 'Guest'
+  expect(result).toEqual({ type: 'string', value: 'Hello, Guest' })
+})
+
+test("CALL - null with no default still results in null", async () => {
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", ["x"], ".body"],
+    ["STORE", "func"],
+    ["JUMP", ".end"],
+    [".body:"],
+    ["LOAD", "x"],
+    ["RETURN"],
+    [".end:"],
+    ["LOAD", "func"],
+    ["PUSH", null],
+    ["PUSH", 1],
+    ["PUSH", 0],
+    ["CALL"],
+    ["HALT"]
+  ])
+
+  const result = await run(bytecode)
+  // No default value, so null should be returned
+  expect(result).toEqual({ type: 'null', value: null })
+})
+
+test("CALL - null triggers default with variadic parameters", async () => {
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", ["x=99", "...rest"], ".body"],
+    ["STORE", "func"],
+    ["JUMP", ".end"],
+    [".body:"],
+    ["LOAD", "x"],
+    ["RETURN"],
+    [".end:"],
+    ["LOAD", "func"],
+    ["PUSH", null],
+    ["PUSH", 1],
+    ["PUSH", 2],
+    ["PUSH", 3],
+    ["PUSH", 4],
+    ["PUSH", 0],
+    ["CALL"],
+    ["HALT"]
+  ])
+
+  const result = await run(bytecode)
+  // x should be 99 (null triggers default), rest gets [1, 2, 3]
+  expect(result).toEqual({ type: 'number', value: 99 })
+})
+
+test("CALL - null triggers default with @named parameter", async () => {
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", ["x=777", "@named"], ".body"],
+    ["STORE", "func"],
+    ["JUMP", ".end"],
+    [".body:"],
+    ["LOAD", "x"],
+    ["RETURN"],
+    [".end:"],
+    ["LOAD", "func"],
+    ["PUSH", null],
+    ["PUSH", "foo"],
+    ["PUSH", "bar"],
+    ["PUSH", 1],
+    ["PUSH", 1],
+    ["CALL"],
+    ["HALT"]
+  ])
+
+  const result = await run(bytecode)
+  // x should be 777 (null triggers default)
+  expect(result).toEqual({ type: 'number', value: 777 })
+})

tests/regex.test.ts

@@ -159,14 +159,14 @@ describe("RegExp", () => {
     PUSH /bar/
     NEQ
     `
-    expect(await run(toBytecode(str))).toEqual({ type: 'boolean', value: true })
+    await expect(str).toBeBoolean(true)
 
     const str2 = `
     PUSH /test/i
     PUSH /test/i
     NEQ
     `
-    expect(await run(toBytecode(str2))).toEqual({ type: 'boolean', value: false })
+    await expect(str2).toBeBoolean(false)
   })
 
   test("is truthy", async () => {
@@ -177,7 +177,7 @@ describe("RegExp", () => {
     PUSH 42
     .end:
     `
-    expect(await run(toBytecode(str))).toEqual({ type: 'number', value: 42 })
+    await expect(str).toBeNumber(42)
   })
 
   test("NOT returns false (regex is truthy)", async () => {
@@ -185,7 +185,7 @@ describe("RegExp", () => {
     PUSH /pattern/
     NOT
     `
-    expect(await run(toBytecode(str))).toEqual({ type: 'boolean', value: false })
+    await expect(str).toBeBoolean(false)
   })
 
   test("in arrays", async () => {
@@ -301,7 +301,7 @@ describe("RegExp", () => {
     PUSH /bar/i
     STR_CONCAT #3
     `
-    expect(await run(toBytecode(str))).toEqual({ type: 'string', value: '/foo/ and /bar/i' })
+    await expect(str).toBeString('/foo/ and /bar/i')
   })
 
   test("DUP with regex", async () => {
@@ -311,7 +311,7 @@ describe("RegExp", () => {
     EQ
     `
     // Same regex duplicated should be equal
-    expect(await run(toBytecode(str))).toEqual({ type: 'boolean', value: true })
+    await expect(str).toBeBoolean(true)
   })
 
   test("empty pattern", async () => {
@@ -365,7 +365,7 @@ describe("RegExp", () => {
     PUSH /xyz/
     EQ
     `
-    expect(await run(toBytecode(str1))).toEqual({ type: 'boolean', value: false })
+    await expect(str1).toBeBoolean(false)
 
     // Same pattern, different flags
     const str2 = `
@@ -373,7 +373,7 @@ describe("RegExp", () => {
     PUSH /test/i
     EQ
     `
-    expect(await run(toBytecode(str2))).toEqual({ type: 'boolean', value: false })
+    await expect(str2).toBeBoolean(false)
 
     // Different order of flags (should be equal)
     const str3 = `
@@ -381,7 +381,7 @@ describe("RegExp", () => {
     PUSH /test/gi
     EQ
     `
-    expect(await run(toBytecode(str3))).toEqual({ type: 'boolean', value: true })
+    await expect(str3).toBeBoolean(true)
   })
 
   test("with native functions", async () => {
@@ -399,7 +399,7 @@ describe("RegExp", () => {
     const vm = new VM(bytecode)
 
     // Register a native function that takes a string and regex
-    vm.registerFunction('match', (str: string, pattern: RegExp) => {
+    vm.set('match', (str: string, pattern: RegExp) => {
       return pattern.test(str)
     })
 
@@ -422,7 +422,7 @@ describe("RegExp", () => {
 
     const vm = new VM(bytecode)
 
-    vm.registerFunction('replace', (str: string, pattern: RegExp, replacement: string) => {
+    vm.set('replace', (str: string, pattern: RegExp, replacement: string) => {
       return str.replace(pattern, replacement)
     })
 
@@ -444,7 +444,7 @@ describe("RegExp", () => {
 
     const vm = new VM(bytecode)
 
-    vm.registerFunction('extractNumbers', (str: string, pattern: RegExp) => {
+    vm.set('extractNumbers', (str: string, pattern: RegExp) => {
       return str.match(pattern) || []
     })
 

tests/repl.test.ts

@@ -0,0 +1,149 @@
+import { test, expect } from "bun:test"
+import { VM, toBytecode } from "#reef"
+
+test("REPL mode - demonstrates PC reset problem", async () => {
+  // Track how many times each line executes
+  let line1Count = 0
+  let line2Count = 0
+
+  // Line 1: Set x = 5, track execution
+  const line1 = toBytecode([
+    ["LOAD", "trackLine1"],
+    ["PUSH", 0],
+    ["PUSH", 0],
+    ["CALL"],
+    ["POP"],
+    ["PUSH", 5],
+    ["STORE", "x"]
+  ])
+
+  const vm = new VM(line1, {
+    trackLine1: () => { line1Count++; return null },
+    trackLine2: () => { line2Count++; return null }
+  })
+
+  await vm.run()
+  expect(vm.scope.get("x")).toEqual({ type: "number", value: 5 })
+  expect(line1Count).toBe(1)
+  expect(line2Count).toBe(0)
+
+  // Line 2: Track execution, load x
+  const line2 = toBytecode([
+    ["LOAD", "trackLine2"],
+    ["PUSH", 0],
+    ["PUSH", 0],
+    ["CALL"],
+    ["POP"],
+    ["LOAD", "x"]
+  ])
+
+  // Append line2 bytecode to VM (what a REPL would do)
+  vm.instructions.push(...line2.instructions)
+  for (const constant of line2.constants) {
+    if (!vm.constants.includes(constant)) {
+      vm.constants.push(constant)
+    }
+  }
+
+  // Current behavior: run() resets PC to 0, re-executing everything
+  await vm.run()
+
+  // PROBLEM: Line 1 ran AGAIN (count is now 2, not 1)
+  // This is the issue for REPL - side effects run multiple times
+  expect(line1Count).toBe(2) // Ran twice! Should still be 1
+  expect(line2Count).toBe(1) // This is correct
+
+  // What we WANT for REPL:
+  // - Only execute the NEW bytecode (line 2)
+  // - line1Count should stay at 1
+  // - line2Count should be 1
+})
+
+test("REPL mode - continue() executes only new bytecode", async () => {
+  // Track how many times each line executes
+  let line1Count = 0
+  let line2Count = 0
+
+  // Line 1: Set x = 5, track execution
+  const line1 = toBytecode([
+    ["LOAD", "trackLine1"],
+    ["PUSH", 0],
+    ["PUSH", 0],
+    ["CALL"],
+    ["POP"],
+    ["PUSH", 5],
+    ["STORE", "x"]
+  ])
+
+  const vm = new VM(line1, {
+    trackLine1: () => { line1Count++; return null },
+    trackLine2: () => { line2Count++; return null }
+  })
+
+  await vm.run()
+  expect(vm.scope.get("x")).toEqual({ type: "number", value: 5 })
+  expect(line1Count).toBe(1)
+  expect(line2Count).toBe(0)
+
+  // Line 2: Track execution, load x and add 10
+  const line2 = toBytecode([
+    ["LOAD", "trackLine2"],
+    ["PUSH", 0],
+    ["PUSH", 0],
+    ["CALL"],
+    ["POP"],
+    ["LOAD", "x"],
+    ["PUSH", 10],
+    ["ADD"]
+  ])
+
+  // Append line2 bytecode to VM using the helper method
+  vm.appendBytecode(line2)
+
+  // SOLUTION: Use continue() instead of run() to resume from current PC
+  await vm.continue()
+
+  const result = vm.stack[vm.stack.length - 1]
+
+  // SUCCESS: Line 1 only ran once, line 2 ran once
+  expect(line1Count).toBe(1) // Still 1! Side effect didn't re-run
+  expect(line2Count).toBe(1) // Ran once as expected
+  expect(result).toEqual({ type: "number", value: 15 }) // 5 + 10
+})
+
+test("REPL mode - function calls work across chunks", async () => {
+  const vm = new VM(toBytecode([]))
+  await vm.run()
+
+  // Line 1: Define a function
+  const line1 = toBytecode([
+    ["MAKE_FUNCTION", ["x"], ".body"],
+    ["STORE", "add1"],
+    ["JUMP", ".end"],
+    [".body:"],
+    ["LOAD", "x"],
+    ["PUSH", 1],
+    ["ADD"],
+    ["RETURN"],
+    [".end:"]
+  ])
+
+  vm.appendBytecode(line1)
+  await vm.continue()
+
+  expect(vm.scope.get("add1")?.type).toBe("function")
+
+  // Line 2: Call the function
+  const line2 = toBytecode([
+    ["LOAD", "add1"],
+    ["PUSH", 10],
+    ["PUSH", 1],
+    ["PUSH", 0],
+    ["CALL"]
+  ])
+
+  vm.appendBytecode(line2)
+  const result = await vm.continue()
+
+  expect(result).toEqual({ type: "number", value: 11 })
+})

tests/scope.test.ts

@@ -0,0 +1,243 @@
+import { test, expect } from "bun:test"
+import { Scope, toValue } from "#reef"
+
+test("Scope - create empty scope", () => {
+  const scope = new Scope()
+  expect(scope.parent).toBeUndefined()
+  expect(scope.locals.size).toBe(0)
+})
+
+test("Scope - create child scope with parent", () => {
+  const parent = new Scope()
+  const child = new Scope(parent)
+
+  expect(child.parent).toBe(parent)
+  expect(child.locals.size).toBe(0)
+})
+
+test("Scope - set and get variable in same scope", () => {
+  const scope = new Scope()
+  const value = toValue(42)
+
+  scope.set("x", value)
+  expect(scope.get("x")).toBe(value)
+})
+
+test("Scope - get returns undefined for non-existent variable", () => {
+  const scope = new Scope()
+  expect(scope.get("x")).toBeUndefined()
+})
+
+test("Scope - set updates existing variable in same scope", () => {
+  const scope = new Scope()
+
+  scope.set("x", toValue(10))
+  expect(scope.get("x")).toEqual({ type: "number", value: 10 })
+
+  scope.set("x", toValue(20))
+  expect(scope.get("x")).toEqual({ type: "number", value: 20 })
+})
+
+test("Scope - get searches parent scope", () => {
+  const parent = new Scope()
+  const child = new Scope(parent)
+
+  parent.set("x", toValue(42))
+  expect(child.get("x")).toEqual({ type: "number", value: 42 })
+})
+
+test("Scope - get searches multiple levels up", () => {
+  const grandparent = new Scope()
+  const parent = new Scope(grandparent)
+  const child = new Scope(parent)
+
+  grandparent.set("x", toValue(100))
+  expect(child.get("x")).toEqual({ type: "number", value: 100 })
+})
+
+test("Scope - child updates parent variable when it exists (no shadowing)", () => {
+  const parent = new Scope()
+  const child = new Scope(parent)
+
+  parent.set("x", toValue(10))
+  child.set("x", toValue(20))
+
+  // set() updates parent's variable, not shadow it
+  expect(parent.get("x")).toEqual({ type: "number", value: 20 })
+  expect(child.get("x")).toEqual({ type: "number", value: 20 })
+  expect(child.locals.has("x")).toBe(false)
+})
+
+test("Scope - set updates parent scope variable when it exists", () => {
+  const parent = new Scope()
+  const child = new Scope(parent)
+
+  parent.set("x", toValue(10))
+  child.set("x", toValue(20))
+
+  // Should update parent's variable, not create new local one
+  expect(parent.get("x")).toEqual({ type: "number", value: 20 })
+  expect(child.get("x")).toEqual({ type: "number", value: 20 })
+  expect(child.locals.has("x")).toBe(false)
+})
+
+test("Scope - set creates new local variable when not found in parent", () => {
+  const parent = new Scope()
+  const child = new Scope(parent)
+
+  parent.set("x", toValue(10))
+  child.set("y", toValue(20))
+
+  expect(parent.get("x")).toEqual({ type: "number", value: 10 })
+  expect(parent.get("y")).toBeUndefined()
+  expect(child.get("x")).toEqual({ type: "number", value: 10 })
+  expect(child.get("y")).toEqual({ type: "number", value: 20 })
+  expect(child.locals.has("y")).toBe(true)
+})
+
+test("Scope - has returns true for variable in current scope", () => {
+  const scope = new Scope()
+  scope.set("x", toValue(42))
+
+  expect(scope.has("x")).toBe(true)
+  expect(scope.has("y")).toBe(false)
+})
+
+test("Scope - has returns true for variable in parent scope", () => {
+  const parent = new Scope()
+  const child = new Scope(parent)
+
+  parent.set("x", toValue(42))
+
+  expect(child.has("x")).toBe(true)
+  expect(child.has("y")).toBe(false)
+})
+
+test("Scope - has searches multiple levels up", () => {
+  const grandparent = new Scope()
+  const parent = new Scope(grandparent)
+  const child = new Scope(parent)
+
+  grandparent.set("x", toValue(100))
+
+  expect(child.has("x")).toBe(true)
+  expect(parent.has("x")).toBe(true)
+  expect(grandparent.has("x")).toBe(true)
+})
+
+test("Scope.vars() - returns empty array for empty scope", () => {
+  const scope = new Scope()
+  expect(scope.vars()).toEqual([])
+})
+
+test("Scope.vars() - returns single variable from current scope", () => {
+  const scope = new Scope()
+  scope.set("x", toValue(42))
+
+  expect(scope.vars()).toEqual(["x"])
+})
+
+test("Scope.vars() - returns multiple variables from current scope", () => {
+  const scope = new Scope()
+  scope.set("x", toValue(1))
+  scope.set("y", toValue(2))
+  scope.set("z", toValue(3))
+
+  const vars = scope.vars()
+  expect(vars.length).toBe(3)
+  expect(vars).toContain("x")
+  expect(vars).toContain("y")
+  expect(vars).toContain("z")
+})
+
+test("Scope.vars() - includes variables from parent scope", () => {
+  const parent = new Scope()
+  const child = new Scope(parent)
+
+  parent.set("x", toValue(1))
+  child.set("y", toValue(2))
+
+  const vars = child.vars()
+  expect(vars.length).toBe(2)
+  expect(vars).toContain("x")
+  expect(vars).toContain("y")
+})
+
+test("Scope.vars() - includes variables from multiple parent scopes", () => {
+  const grandparent = new Scope()
+  const parent = new Scope(grandparent)
+  const child = new Scope(parent)
+
+  grandparent.set("x", toValue(1))
+  parent.set("y", toValue(2))
+  child.set("z", toValue(3))
+
+  const vars = child.vars()
+  expect(vars.length).toBe(3)
+  expect(vars).toContain("x")
+  expect(vars).toContain("y")
+  expect(vars).toContain("z")
+})
+
+test("Scope.vars() - no duplicates when child updates parent variable", () => {
+  const parent = new Scope()
+  const child = new Scope(parent)
+
+  parent.set("x", toValue(10))
+  child.set("x", toValue(20))  // Updates parent, doesn't create local
+
+  // Only one "x" since child doesn't have its own local x
+  const vars = child.vars()
+  expect(vars).toEqual(["x"])
+})
+
+test("Scope.vars() - can't have duplicates if manually set in locals", () => {
+  const parent = new Scope()
+  const child = new Scope(parent)
+
+  parent.set("x", toValue(10))
+  child.locals.set("x", toValue(20))
+  const vars = child.vars()
+  expect(vars).toEqual(["x"])
+
+  expect(child.get("x")).toEqual({ type: "number", value: 20 })
+  expect(parent.get("x")).toEqual({ type: "number", value: 10 })
+})
+
+test("Scope.vars() - handles deep scope chains", () => {
+  let scope = new Scope()
+
+  // Create a deep chain: level0 -> level1 -> ... -> level5
+  for (let i = 0; i < 5; i++) {
+    scope.set(`var${i}`, toValue(i))
+    scope = new Scope(scope)
+  }
+
+  // Final scope should have all variables from the chain
+  const vars = scope.vars()
+  expect(vars.length).toBe(5)
+  expect(vars).toContain("var0")
+  expect(vars).toContain("var1")
+  expect(vars).toContain("var2")
+  expect(vars).toContain("var3")
+  expect(vars).toContain("var4")
+})
+
+test("Scope - can store and retrieve functions", () => {
+  const scope = new Scope()
+
+  const fnValue = {
+    type: 'function' as const,
+    value: '<function>' as const,
+    params: ['x'],
+    defaults: {},
+    body: 0,
+    variadic: false,
+    named: false,
+    parentScope: scope
+  }
+
+  scope.set("myFunc", fnValue)
+  expect(scope.get("myFunc")).toBe(fnValue)
+  expect(scope.get("myFunc")?.type).toBe("function")
+})

tests/setup.ts

@@ -0,0 +1,249 @@
+import { expect } from "bun:test"
+import { toValue, fromValue, type Value, toBytecode, run, type Bytecode } from "#reef"
+import { isEqual } from "../src/value"
+
+declare module "bun:test" {
+  interface Matchers<T> {
+    /**
+     * Run bytecode and assert that the result equals a JavaScript value after conversion via toValue()
+     * @example expect(bytecode).toEqualValue(42)
+     * @example expect("PUSH 5\nPUSH 3\nADD").toEqualValue(8)
+     * @example expect([["PUSH", 42]]).toEqualValue(42)
+     */
+    toEqualValue(expected: any): Promise<void>
+
+    /**
+     * Run bytecode and assert that the result is null
+     * @example expect(bytecode).toBeNull()
+     * @example expect("PUSH null").toBeNull()
+     */
+    toBeNull(): Promise<void>
+
+    /**
+     * Run bytecode and assert that the result is a boolean with the expected value
+     * @example expect(bytecode).toBeBoolean(true)
+     * @example expect("PUSH true").toBeBoolean(true)
+     */
+    toBeBoolean(expected: boolean): Promise<void>
+
+    /**
+     * Run bytecode and assert that the result is a number with the expected value
+     * @example expect(bytecode).toBeNumber(42)
+     * @example expect("PUSH 42").toBeNumber(42)
+     */
+    toBeNumber(expected: number): Promise<void>
+
+    /**
+     * Run bytecode and assert that the result is a string with the expected value
+     * @example expect(bytecode).toBeString("hello")
+     * @example expect("PUSH \"hello\"").toBeString("hello")
+     */
+    toBeString(expected: string): Promise<void>
+
+    /**
+     * Run bytecode and assert that the result is an array with the expected values
+     * @example expect(bytecode).toBeArray([1, 2, 3])
+     */
+    toBeArray(expected: any[]): Promise<void>
+
+    /**
+     * Run bytecode and assert that the result is a dict with the expected key-value pairs
+     * @example expect(bytecode).toBeDict({ x: 10, y: 20 })
+     */
+    toBeDict(expected: Record<string, any>): Promise<void>
+
+    /**
+     * Run bytecode and assert that the result is a function (Reef or native)
+     * @example expect(bytecode).toBeFunction()
+     */
+    toBeFunction(): Promise<void>
+
+    /**
+     * Run bytecode and assert that the result is truthy according to ReefVM semantics
+     * (only null and false are falsy)
+     * @example expect(bytecode).toBeTruthy()
+     */
+    toBeTruthy(): Promise<void>
+
+    /**
+     * Run bytecode and assert that the result is falsy according to ReefVM semantics
+     * (only null and false are falsy)
+     * @example expect(bytecode).toBeFalsy()
+     */
+    toBeFalsy(): Promise<void>
+  }
+}
+
+expect.extend({
+  async toEqualValue(this: void, received: unknown, expected: any) {
+    const bytecode = typeof received === "string" || Array.isArray(received) ? toBytecode(received as string | any[]) : received as Bytecode
+    const result = await run(bytecode)
+    const expectedValue = toValue(expected)
+    const pass = isEqual(result, expectedValue)
+
+    return {
+      pass,
+      message: () =>
+        pass
+          ? `Expected value NOT to equal ${formatValue(expectedValue)}, but it did`
+          : `Expected value to equal ${formatValue(expectedValue)}, but received ${formatValue(result)}`,
+    }
+  },
+
+  async toBeNull(this: void, received: unknown) {
+    const bytecode = typeof received === "string" || Array.isArray(received) ? toBytecode(received as string | any[]) : received as Bytecode
+    const result = await run(bytecode)
+    const pass = result.type === "null"
+
+    return {
+      pass,
+      message: () =>
+        pass
+          ? `Expected value NOT to be null, but it was`
+          : `Expected value to be null, but received ${formatValue(result)}`,
+    }
+  },
+
+  async toBeBoolean(this: void, received: unknown, expected: boolean) {
+    const bytecode = typeof received === "string" || Array.isArray(received) ? toBytecode(received as string | any[]) : received as Bytecode
+    const result = await run(bytecode)
+    const pass = result.type === "boolean" && result.value === expected
+
+    return {
+      pass,
+      message: () =>
+        pass
+          ? `Expected value NOT to be boolean ${expected}, but it was`
+          : `Expected value to be boolean ${expected}, but received ${formatValue(result)}`,
+    }
+  },
+
+  async toBeNumber(this: void, received: unknown, expected: number) {
+    const bytecode = typeof received === "string" || Array.isArray(received) ? toBytecode(received as string | any[]) : received as Bytecode
+    const result = await run(bytecode)
+    const pass = result.type === "number" && result.value === expected
+
+    return {
+      pass,
+      message: () =>
+        pass
+          ? `Expected value NOT to be number ${expected}, but it was`
+          : `Expected value to be number ${expected}, but received ${formatValue(result)}`,
+    }
+  },
+
+  async toBeString(this: void, received: unknown, expected: string) {
+    const bytecode = typeof received === "string" || Array.isArray(received) ? toBytecode(received as string | any[]) : received as Bytecode
+    const result = await run(bytecode)
+    const pass = result.type === "string" && result.value === expected
+
+    return {
+      pass,
+      message: () =>
+        pass
+          ? `Expected value NOT to be string "${expected}", but it was`
+          : `Expected value to be string "${expected}", but received ${formatValue(result)}`,
+    }
+  },
+
+  async toBeArray(this: void, received: unknown, expected: any[]) {
+    const bytecode = typeof received === "string" || Array.isArray(received) ? toBytecode(received as string | any[]) : received as Bytecode
+    const result = await run(bytecode)
+    const expectedValue = toValue(expected)
+    const pass = result.type === "array" && isEqual(result, expectedValue)
+
+    return {
+      pass,
+      message: () =>
+        pass
+          ? `Expected value NOT to be array ${formatValue(expectedValue)}, but it was`
+          : `Expected value to be array ${formatValue(expectedValue)}, but received ${formatValue(result)}`,
+    }
+  },
+
+  async toBeDict(this: void, received: unknown, expected: Record<string, any>) {
+    const bytecode = typeof received === "string" || Array.isArray(received) ? toBytecode(received as string | any[]) : received as Bytecode
+    const result = await run(bytecode)
+    const expectedValue = toValue(expected)
+    const pass = result.type === "dict" && isEqual(result, expectedValue)
+
+    return {
+      pass,
+      message: () =>
+        pass
+          ? `Expected value NOT to be dict ${formatValue(expectedValue)}, but it was`
+          : `Expected value to be dict ${formatValue(expectedValue)}, but received ${formatValue(result)}`,
+    }
+  },
+
+  async toBeFunction(this: void, received: unknown) {
+    const bytecode = typeof received === "string" || Array.isArray(received) ? toBytecode(received as string | any[]) : received as Bytecode
+    const result = await run(bytecode)
+    const pass = result.type === "function" || result.type === "native"
+
+    return {
+      pass,
+      message: () =>
+        pass
+          ? `Expected value NOT to be a function, but it was`
+          : `Expected value to be a function, but received ${formatValue(result)}`,
+    }
+  },
+
+  async toBeTruthy(this: void, received: unknown) {
+    const bytecode = typeof received === "string" || Array.isArray(received) ? toBytecode(received as string | any[]) : received as Bytecode
+    const result = await run(bytecode)
+    // ReefVM semantics: only null and false are falsy
+    const pass = !(result.type === "null" || (result.type === "boolean" && !result.value))
+
+    return {
+      pass,
+      message: () =>
+        pass
+          ? `Expected value NOT to be truthy, but it was: ${formatValue(result)}`
+          : `Expected value to be truthy, but received ${formatValue(result)}`,
+    }
+  },
+
+  async toBeFalsy(this: void, received: unknown) {
+    const bytecode = typeof received === "string" || Array.isArray(received) ? toBytecode(received as string | any[]) : received as Bytecode
+    const result = await run(bytecode)
+    // ReefVM semantics: only null and false are falsy
+    const pass = result.type === "null" || (result.type === "boolean" && !result.value)
+
+    return {
+      pass,
+      message: () =>
+        pass
+          ? `Expected value NOT to be falsy, but it was: ${formatValue(result)}`
+          : `Expected value to be falsy, but received ${formatValue(result)}`,
+    }
+  },
+})
+
+function formatValue(value: Value): string {
+  switch (value.type) {
+    case "null":
+      return "null"
+    case "boolean":
+    case "number":
+      return String(value.value)
+    case "string":
+      return `"${value.value}"`
+    case "array":
+      return `[${value.value.map(formatValue).join(", ")}]`
+    case "dict": {
+      const entries = Array.from(value.value.entries())
+        .map(([k, v]) => `${k}: ${formatValue(v)}`)
+        .join(", ")
+      return `{${entries}}`
+    }
+    case "regex":
+      return String(value.value)
+    case "function":
+    case "native":
+      return "<function>"
+    default:
+      return String(value)
+  }
+}

tests/validator.test.ts

@@ -201,17 +201,17 @@ test("formatValidationErrors produces readable output", () => {
   expect(formatted).toContain("UNKNOWN")
 })
 
-test("detects JUMP without # or .label", () => {
+test("detects JUMP without .label", () => {
   const source = `
     JUMP 5
     HALT
   `
   const result = validateBytecode(source)
   expect(result.valid).toBe(false)
-  expect(result.errors[0]!.message).toContain("JUMP requires immediate (#number) or label (.label)")
+  expect(result.errors[0]!.message).toContain("JUMP requires label (.label)")
 })
 
-test("detects JUMP_IF_TRUE without # or .label", () => {
+test("detects JUMP_IF_TRUE without .label", () => {
   const source = `
     PUSH true
     JUMP_IF_TRUE 2
@@ -219,10 +219,10 @@ test("detects JUMP_IF_TRUE without # or .label", () => {
   `
   const result = validateBytecode(source)
   expect(result.valid).toBe(false)
-  expect(result.errors[0]!.message).toContain("JUMP_IF_TRUE requires immediate (#number) or label (.label)")
+  expect(result.errors[0]!.message).toContain("JUMP_IF_TRUE requires label (.label)")
 })
 
-test("detects JUMP_IF_FALSE without # or .label", () => {
+test("detects JUMP_IF_FALSE without .label", () => {
   const source = `
     PUSH false
     JUMP_IF_FALSE 2
@@ -230,17 +230,18 @@ test("detects JUMP_IF_FALSE without # or .label", () => {
   `
   const result = validateBytecode(source)
   expect(result.valid).toBe(false)
-  expect(result.errors[0]!.message).toContain("JUMP_IF_FALSE requires immediate (#number) or label (.label)")
+  expect(result.errors[0]!.message).toContain("JUMP_IF_FALSE requires label (.label)")
 })
 
-test("allows JUMP with immediate number", () => {
+test("rejects JUMP with immediate number", () => {
   const source = `
     JUMP #2
     PUSH 999
     HALT
   `
   const result = validateBytecode(source)
-  expect(result.valid).toBe(true)
+  expect(result.valid).toBe(false)
+  expect(result.errors[0]!.message).toContain("JUMP requires label (.label)")
 })
 
 test("detects MAKE_ARRAY without #", () => {

tests/value.test.ts

@@ -0,0 +1,302 @@
+import { test, expect } from "bun:test"
+import { isValue, toValue } from "#reef"
+
+test("isValue - recognizes valid Value objects", () => {
+  expect(isValue({ type: 'number', value: 42 })).toBe(true)
+  expect(isValue({ type: 'number', value: 0 })).toBe(true)
+  expect(isValue({ type: 'string', value: 'hello' })).toBe(true)
+  expect(isValue({ type: 'string', value: '' })).toBe(true)
+  expect(isValue({ type: 'boolean', value: true })).toBe(true)
+  expect(isValue({ type: 'boolean', value: false })).toBe(true)
+  expect(isValue({ type: 'null', value: null })).toBe(true)
+  expect(isValue({ type: 'array', value: [] })).toBe(true)
+  expect(isValue({ type: 'array', value: [toValue(1), toValue(2)] })).toBe(true)
+  expect(isValue({ type: 'dict', value: new Map() })).toBe(true)
+  expect(isValue({ type: 'regex', value: /test/ })).toBe(true)
+  expect(isValue({
+    type: 'function',
+    value: '<function>',
+    params: [],
+    defaults: {},
+    body: 0,
+    variadic: false,
+    named: false,
+    parentScope: null as any
+  })).toBe(true)
+  expect(isValue({
+    type: 'native',
+    value: '<function>',
+    fn: (() => { }) as any
+  })).toBe(true)
+})
+
+test("isValue - rejects primitives", () => {
+  expect(isValue(42)).toBe(false)
+  expect(isValue(0)).toBe(false)
+  expect(isValue('hello')).toBe(false)
+  expect(isValue('')).toBe(false)
+  expect(isValue(true)).toBe(false)
+  expect(isValue(false)).toBe(false)
+  expect(isValue(null)).toBe(false)
+  expect(isValue(undefined)).toBe(false)
+})
+
+test("isValue - rejects plain objects", () => {
+  expect(isValue({})).toBe(false)
+  expect(isValue({ foo: 'bar' })).toBe(false)
+  expect(isValue({ type: 'number' })).toBe(false)
+  expect(isValue({ value: 42 })).toBe(false)
+})
+
+test("isValue - rejects arrays and functions", () => {
+  expect(isValue([])).toBe(false)
+  expect(isValue([1, 2, 3])).toBe(false)
+  expect(isValue(() => { })).toBe(false)
+  expect(isValue(function () { })).toBe(false)
+})
+
+test("isValue - rejects other object types", () => {
+  expect(isValue(new Date())).toBe(false)
+  expect(isValue(/regex/)).toBe(false)
+  expect(isValue(new Map())).toBe(false)
+  expect(isValue(new Set())).toBe(false)
+})
+
+test("isValue - used by toValue to detect already-converted values", () => {
+  const value = toValue(42)
+  expect(isValue(value)).toBe(true)
+
+  const result = toValue(value)
+  expect(result).toBe(value)
+})
+
+test("isValue - edge cases with type and value properties", () => {
+  expect(isValue({ type: 'number', value: 42, extra: 'data' })).toBe(true)
+
+  expect(isValue({ type: null, value: 42 })).toBe(false)
+  expect(isValue({ type: 'number', value: undefined })).toBe(true)
+
+  expect(isValue({ type: 'number', val: 42 })).toBe(false)
+  expect(isValue({ typ: 'number', value: 42 })).toBe(false)
+})
+
+test("isValue - rejects objects with invalid type values", () => {
+  expect(isValue({ type: 'text', value: 'Bob' })).toBe(false)
+  expect(isValue({ type: 'email', value: 'test@example.com' })).toBe(false)
+  expect(isValue({ type: 'password', value: 'secret' })).toBe(false)
+  expect(isValue({ type: 'checkbox', value: true })).toBe(false)
+  expect(isValue({ type: 'custom', value: 123 })).toBe(false)
+  expect(isValue({ type: 'unknown', value: null })).toBe(false)
+})
+
+test("toValue - correctly handles HTML input props", async () => {
+  const { VM, toBytecode, toValue } = await import("#reef")
+
+  const bytecode = toBytecode([["HALT"]])
+  const vm = new VM(bytecode)
+
+  const inputProps = { type: 'text', value: 'Bob' }
+  const converted = toValue(inputProps, vm)
+
+  expect(converted.type).toBe('dict')
+  expect(converted.value.get('type')).toEqual({ type: 'string', value: 'text' })
+  expect(converted.value.get('value')).toEqual({ type: 'string', value: 'Bob' })
+})
+
+test("toValue - converts wrapped Reef functions back to original Value", async () => {
+  const { VM, toBytecode, fnFromValue } = await import("#reef")
+
+  // Create a Reef function
+  const bytecode = toBytecode([
+    ["MAKE_FUNCTION", ["x"], ".body"],
+    ["STORE", "add1"],
+    ["JUMP", ".end"],
+    [".body:"],
+    ["LOAD", "x"],
+    ["PUSH", 1],
+    ["ADD"],
+    ["RETURN"],
+    [".end:"],
+    ["HALT"]
+  ])
+
+  const vm = new VM(bytecode)
+  await vm.run()
+
+  const reefFunction = vm.scope.get("add1")!
+  expect(reefFunction.type).toBe("function")
+
+  // Convert to JS function
+  const jsFunction = fnFromValue(reefFunction, vm)
+  expect(typeof jsFunction).toBe("function")
+
+  // Convert back to Value - should return the original Reef function
+  const backToValue = toValue(jsFunction)
+  expect(backToValue).toBe(reefFunction)  // Same reference
+  expect(backToValue.type).toBe("function")
+})
+
+test("fromValue - converts native function back to original JS function", async () => {
+  const { VM, toBytecode, toValue, fromValue } = await import("#reef")
+
+  const bytecode = toBytecode([["HALT"]])
+  const vm = new VM(bytecode)
+
+  // Create a native JS function
+  const originalFn = (x: number, y: number) => x * y
+
+  // Convert to Value (wraps it as a native function)
+  const nativeValue = toValue(originalFn, vm)
+  expect(nativeValue.type).toBe("native")
+
+  // Convert back to JS - should get the original function
+  const convertedFn = fromValue(nativeValue, vm)
+  expect(typeof convertedFn).toBe("function")
+
+  // Verify it's the same function
+  expect(convertedFn).toBe(originalFn)
+
+  // Verify it works correctly
+  expect(convertedFn(3, 4)).toBe(12)
+})
+
+test("fromValue - native function roundtrip preserves functionality", async () => {
+  const { VM, toBytecode, toValue, fromValue } = await import("#reef")
+
+  const bytecode = toBytecode([["HALT"]])
+  const vm = new VM(bytecode)
+
+  // Create a native function with state
+  let callCount = 0
+  const countingFn = (n: number) => {
+    callCount++
+    return n * callCount
+  }
+
+  // Roundtrip through Value
+  const nativeValue = toValue(countingFn, vm)
+  const roundtrippedFn = fromValue(nativeValue, vm)
+
+  // Verify it maintains state across calls
+  expect(roundtrippedFn(10)).toBe(10)  // 10 * 1
+  expect(roundtrippedFn(10)).toBe(20)  // 10 * 2
+  expect(roundtrippedFn(10)).toBe(30)  // 10 * 3
+  expect(callCount).toBe(3)
+})
+
+test("fromValue - async native function roundtrip", async () => {
+  const { VM, toBytecode, toValue, fromValue } = await import("#reef")
+
+  const bytecode = toBytecode([["HALT"]])
+  const vm = new VM(bytecode)
+
+  const asyncFn = async (x: number, y: number) => {
+    await new Promise(resolve => setTimeout(resolve, 1))
+    return x + y
+  }
+
+  const nativeValue = toValue(asyncFn, vm)
+  expect(nativeValue.type).toBe("native")
+
+  const roundtrippedFn = fromValue(nativeValue, vm)
+
+  const result = await roundtrippedFn(5, 7)
+  expect(result).toBe(12)
+})
+
+test("toValue - throws helpful error when calling function converted without VM", async () => {
+  function myFunction(x: number) {
+    return x * 2
+  }
+
+  const value = toValue(myFunction)
+  expect(value.type).toBe('native')
+
+  // Error is thrown when calling the function, not when converting
+  await expect(async () => {
+    await value.fn({ type: 'number', value: 5 })
+  }).toThrow(/can't call function that was converted without a vm/)
+
+  await expect(async () => {
+    await value.fn({ type: 'number', value: 5 })
+  }).toThrow(/Function: myFunction/)
+
+  await expect(async () => {
+    await value.fn({ type: 'number', value: 5 })
+  }).toThrow(/Source:/)
+
+  await expect(async () => {
+    await value.fn({ type: 'number', value: 5 })
+  }).toThrow(/Called from:/)
+})
+
+test("toValue - error message shows function info for arrow functions", async () => {
+  const anonymousFn = (x: number) => x * 2
+
+  const value = toValue(anonymousFn)
+  expect(value.type).toBe('native')
+
+  // Arrow functions show as <anonymous>
+  await expect(async () => {
+    await value.fn({ type: 'number', value: 5 })
+  }).toThrow(/Function: <anonymous>/)
+
+  await expect(async () => {
+    await value.fn({ type: 'number', value: 5 })
+  }).toThrow(/Source:/)
+})
+
+test("toValue - error when function is nested in object without VM", async () => {
+  const obj = {
+    name: "test",
+    handler: (x: number) => x * 2
+  }
+
+  const value = toValue(obj)
+  expect(value.type).toBe('dict')
+
+  const handlerValue = value.value.get('handler')!
+  expect(handlerValue.type).toBe('native')
+
+  await expect(async () => {
+    await (handlerValue as any).fn({ type: 'number', value: 5 })
+  }).toThrow(/can't call function that was converted without a vm/)
+
+  await expect(async () => {
+    await (handlerValue as any).fn({ type: 'number', value: 5 })
+  }).toThrow(/Function: handler/)
+})
+
+test("toValue - error when function is nested in array without VM", async () => {
+  const arr = [1, 2, (x: number) => x * 2]
+
+  const value = toValue(arr)
+  expect(value.type).toBe('array')
+
+  const fnValue = value.value[2]!
+  expect(fnValue.type).toBe('native')
+
+  await expect(async () => {
+    await (fnValue as any).fn({ type: 'number', value: 5 })
+  }).toThrow(/can't call function that was converted without a vm/)
+})
+
+test("fromValue - throws helpful error when converting function without VM", async () => {
+  const { Scope, fromValue } = await import("#reef")
+
+  const reefFunction = {
+    type: 'function' as const,
+    params: ['x', 'y'],
+    defaults: {},
+    body: 10,
+    parentScope: new Scope(),
+    variadic: false,
+    named: false,
+    value: '<function>' as const
+  }
+
+  expect(() => fromValue(reefFunction)).toThrow(/VM is required for function conversion/)
+  expect(() => fromValue(reefFunction)).toThrow(/Function params: \[x, y\]/)
+  expect(() => fromValue(reefFunction)).toThrow(/Function body at instruction: 10/)
+  expect(() => fromValue(reefFunction)).toThrow(/Called from:/)
+})

tests/vm.test.ts

@@ -0,0 +1,54 @@
+import { test, expect, describe } from "bun:test"
+import { VM } from "#vm"
+import { toBytecode } from "#bytecode"
+import { toValue } from "#value"
+
+describe("VM scope methods", () => {
+  test("pushScope creates isolated child scope", async () => {
+    const bytecode = toBytecode([["HALT"]])
+    const vm = new VM(bytecode)
+
+    vm.set("x", 42)
+    vm.pushScope()
+
+    const xValue = vm.scope.get("x")
+    expect(xValue).toEqual({ type: "number", value: 42 })
+
+    vm.set("y", 100)
+    expect(vm.scope.get("x")).toEqual({ type: "number", value: 42 })
+    expect(vm.scope.get("y")).toEqual({ type: "number", value: 100 })
+  })
+
+  test("popScope returns to parent scope and child variables are not accessible", async () => {
+    const bytecode = toBytecode([["HALT"]])
+    const vm = new VM(bytecode)
+
+    vm.set("x", 42)
+    vm.pushScope()
+    vm.set("y", 100)
+
+    expect(vm.scope.get("x")).toEqual({ type: "number", value: 42 })
+    expect(vm.scope.get("y")).toEqual({ type: "number", value: 100 })
+
+    vm.popScope()
+    expect(vm.scope.get("x")).toEqual({ type: "number", value: 42 })
+    expect(vm.scope.get("y")).toBeUndefined()
+  })
+
+  test("pushScope with locals initializes child scope with variables", async () => {
+    const bytecode = toBytecode([["HALT"]])
+    const vm = new VM(bytecode)
+
+    vm.set("x", 42)
+    vm.pushScope({ y: 100, z: "hello" })
+
+    expect(vm.scope.get("x")).toEqual({ type: "number", value: 42 })
+    expect(vm.scope.get("y")).toEqual({ type: "number", value: 100 })
+    expect(vm.scope.get("z")).toEqual({ type: "string", value: "hello" })
+
+    vm.popScope()
+    expect(vm.scope.get("x")).toEqual({ type: "number", value: 42 })
+    expect(vm.scope.get("y")).toBeUndefined()
+    expect(vm.scope.get("z")).toBeUndefined()
+  })
+})