# CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

## Project Overview

ReefVM is a stack-based bytecode virtual machine for the Shrimp programming language. It implements a complete VM with closures, tail call optimization, exception handling, variadic functions, named parameters, and Ruby-style iterators with break/continue.

**Essential reading**: Before making changes, read README.md, SPEC.md, and GUIDE.md to understand the VM architecture, instruction set, and compiler patterns.

## Development Commands

### Running Files
```bash
bun <file.ts>              # Run TypeScript files directly
bun examples/native.ts     # Run example
```

### Testing
```bash
bun test                   # Run all tests
bun test <file>            # Run specific test file
bun test --watch           # Watch mode
```

### Building
No build step required - Bun runs TypeScript directly.

## Architecture

### Core Components

**VM Execution Model** (src/vm.ts):
- Stack-based execution with program counter (PC)
- Call stack for function frames
- Exception handler stack for try/catch/finally
- Lexical scope chain with parent references
- Native function registry for TypeScript interop

**Key subsystems**:
- **bytecode.ts**: Compiler that converts both string and array formats to executable bytecode. Handles label resolution, constant pool management, and function definition parsing. The `toBytecode()` function accepts either a string (human-readable) or typed array format (programmatic).
- **value.ts**: Tagged union Value type system with type coercion functions (toNumber, toString, isTrue, isEqual)
- **scope.ts**: Linked scope chain for variable resolution with lexical scoping
- **frame.ts**: Call frame tracking for function calls and break targets
- **exception.ts**: Exception handler records for try/catch/finally blocks
- **validator.ts**: Bytecode validation to catch common errors before execution
- **opcode.ts**: OpCode enum defining all VM instructions

### 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.

**Truthiness semantics**: Only `null` and `false` are falsy. Unlike JavaScript, `0`, `""`, empty arrays, and empty dicts are truthy.

**No AND/OR opcodes**: Short-circuit logical operations are implemented at the compiler level using JUMP patterns with DUP.

**Tail call optimization**: TAIL_CALL reuses the current call frame instead of pushing a new one, enabling unbounded recursion.

**Break semantics**: CALL marks frames as break targets. BREAK unwinds the call stack to the most recent break target, enabling Ruby-style iterator patterns.

**Exception handling**: THROW jumps to finally (if present) or catch. The VM does NOT auto-jump to finally on successful try completion - compilers must explicitly generate JUMPs to finally blocks.

**Parameter binding priority**: Named args bind to fixed params first. Unmatched named args go to `@named` dict parameter. Fixed params bind in order: named arg > positional arg > default > null.

**Native function calling**: CALL_NATIVE consumes the entire stack as arguments (different from CALL which pops specific argument counts).

## Testing Strategy

Tests are organized by feature area:
- **basic.test.ts**: Stack ops, arithmetic, comparisons, variables, control flow
- **functions.test.ts**: Function creation, calls, closures, defaults, variadic, named args
- **tail-call.test.ts**: Tail call optimization and unbounded recursion
- **exceptions.test.ts**: Try/catch/finally, exception unwinding, nested handlers
- **native.test.ts**: Native function interop (sync and async)
- **bytecode.test.ts**: Bytecode string parser, label resolution, constants
- **programmatic.test.ts**: Array format API, typed tuples, labels, functions
- **validator.test.ts**: Bytecode validation rules
- **examples.test.ts**: Integration tests for example programs

When adding features:
1. Add unit tests for the specific opcode/feature
2. Add integration tests showing real-world usage
3. Update SPEC.md with formal specification
4. Update GUIDE.md with compiler patterns
5. Consider adding an example to examples/

## Common Patterns

### Writing Bytecode Tests

ReefVM supports two bytecode formats: string and array.

**String format** (human-readable):
```typescript
import { toBytecode, run } from "#reef"

const bytecode = toBytecode(`
  PUSH 42
  STORE x
  LOAD x
  HALT
`)

const result = await run(bytecode)
// result is { type: 'number', value: 42 }
```

**Array format** (programmatic, type-safe):
```typescript
import { toBytecode, run } from "#reef"

const bytecode = toBytecode([
  ["PUSH", 42],
  ["STORE", "x"],
  ["LOAD", "x"],
  ["HALT"]
])

const result = await run(bytecode)
// result is { type: 'number', value: 42 }
```

Array format features:
- Typed tuples for compile-time type checking
- Labels defined as `[".label:"]` (single-element arrays with colon suffix)
- Label references as strings: `["JUMP", ".label"]` (no colon in references)
- 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
```typescript
const vm = new VM(bytecode)
vm.registerFunction('functionName', (...args: Value[]): Value => {
  // Implementation
  return toValue(result)
})
await vm.run()
```

### Label Usage (Preferred)
Use labels instead of numeric offsets for readability:
```
JUMP .skip
PUSH 42
HALT
.skip:
  PUSH 99
  HALT
```

## TypeScript Configuration

- Import alias: `#reef` maps to `./src/index.ts`
- Module system: ES modules (`"type": "module"` in package.json)
- Bun automatically handles TypeScript compilation

## Bun-Specific Notes

- Use `bun` instead of `node`, `npm`, `pnpm`, or `vite`
- No need for dotenv - Bun loads .env automatically
- Prefer Bun APIs over Node.js equivalents when available
- See .cursor/rules/use-bun-instead-of-node-vite-npm-pnpm.mdc for detailed Bun usage

## Adding a New OpCode

When adding a new instruction to ReefVM, you must update multiple files in a specific order. Follow this checklist:

### 1. Define the OpCode (src/opcode.ts)

Add the new opcode to the `OpCode` enum with comprehensive documentation:

```typescript
export enum OpCode {
  // ... existing opcodes

  MY_NEW_OP,  // operand: <type> | stack: [inputs] → [outputs]
              // Description of what it does
              // Any important behavioral notes
}
```

### 2. Implement VM Execution (src/vm.ts)

Add a case to the `execute()` method's switch statement:

```typescript
async execute(instruction: Instruction) {
  switch (instruction.op) {
    // ... existing cases

    case OpCode.MY_NEW_OP:
      // Implementation
      // - Pop values from this.stack as needed
      // - Perform the operation
      // - Push results to this.stack
      // - Throw errors for invalid operations
      // - Use await for async operations
      break
  }
}
```

Common helper methods:
- `this.binaryOp((a, b) => ...)` - For binary arithmetic/comparison
- `toNumber(value)`, `toString(value)`, `isTrue(value)`, `isEqual(a, b)` - Type coercion
- `this.scope.get(name)`, `this.scope.set(name, value)` - Variable access

### 3. Update Validator (src/validator.ts)

Add the opcode to the appropriate set:

```typescript
// If your opcode requires an operand:
const OPCODES_WITH_OPERANDS = new Set([
  // ... existing
  OpCode.MY_NEW_OP,
])

// If your opcode takes no operand:
const OPCODES_WITHOUT_OPERANDS = new Set([
  // ... existing
  OpCode.MY_NEW_OP,
])
```

If your opcode has complex operand validation, add a specific check in the validation loop around line 154.

### 4. Update Array API (src/bytecode.ts)

Add your instruction to the `InstructionTuple` type:

```typescript
type InstructionTuple =
  // ... existing types
  | ["MY_NEW_OP"]                    // No operand
  | ["MY_NEW_OP", string]            // String operand
  | ["MY_NEW_OP", number]            // Number operand
  | ["MY_NEW_OP", string, number]    // Multiple operands
```

If your opcode has special operand handling, add a case in `toBytecodeFromArray()` around line 241.

### 5. Write Tests (REQUIRED)

Create tests in the appropriate test file:

```typescript
// tests/basic.test.ts, tests/functions.test.ts, etc.

test("MY_NEW_OP description", async () => {
  const bytecode = toBytecode([
    // Setup
    ["PUSH", 42],
    ["MY_NEW_OP"],
    ["HALT"]
  ])

  const result = await run(bytecode)
  expect(result).toEqual({ type: "number", value: 42 })
})

// Test edge cases
test("MY_NEW_OP with invalid input", async () => {
  // Test error conditions
  await expect(run(bytecode)).rejects.toThrow()
})
```

**ALWAYS write tests.** Test both success cases and error conditions. Add integration tests showing real-world usage.

### 6. Document Specification (SPEC.md)

Add a formal specification entry:

```markdown
#### MY_NEW_OP

**Operand**: `<type>`
**Stack**: `[input] → [output]`

Description of what the instruction does.

**Behavior**:
- Specific behavior point 1
- Specific behavior point 2

**Errors**:
- Error condition 1
- Error condition 2
```

### 7. Update Compiler Guide (GUIDE.md)

If your opcode introduces new patterns, add examples to GUIDE.md:

```markdown
### New Pattern Name

\```
PUSH value
MY_NEW_OP
STORE result
\```

Description of the pattern and when to use it.
```

### 8. Add Examples (Optional)

If your opcode enables new functionality, add an example to `examples/`:

```typescript
// examples/my_feature.reef or examples/my_feature.ts
const example = toBytecode([
  // Demonstrate the new opcode
])
```

### Checklist Summary

When adding an opcode, update in this order:

- [ ] `src/opcode.ts` - Add enum value with docs
- [ ] `src/vm.ts` - Implement execution logic
- [ ] `src/validator.ts` - Add to operand requirement set
- [ ] `src/bytecode.ts` - Add to InstructionTuple type
- [ ] `tests/*.test.ts` - Write comprehensive tests (**REQUIRED**)
- [ ] `SPEC.md` - Document formal specification
- [ ] `GUIDE.md` - Add compiler patterns (if applicable)
- [ ] `examples/` - Add example code (if applicable)

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.

**Stack operations**: Most binary operations pop in reverse order (second operand is popped first, then first operand).

**MAKE_ARRAY operand**: Specifies count, not a stack index. `MAKE_ARRAY #3` pops 3 items.

**CALL_NATIVE stack behavior**: Unlike CALL, it consumes all stack values as arguments and clears the stack.

**Finally blocks**: The compiler must generate explicit JUMPs to finally blocks for successful try/catch completion. The VM only auto-jumps to finally on THROW.

**Variable scoping**: STORE updates existing variables in parent scopes or creates in current scope. It does NOT shadow by default.

**Identifiers**: Variable and parameter names support Unicode and emoji! Valid: `💎`, `🌟`, `変数`, `counter`. Invalid: cannot start with digits or special prefixes (`.`, `#`, `@`, `...`), cannot contain whitespace or syntax characters.