ReefVM/CLAUDE.md

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

bun <file.ts>              # Run TypeScript files directly
bun examples/native.ts     # Run example

Testing

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: Parser that converts human-readable bytecode strings to executable bytecode. Handles label resolution, constant pool management, and function definition parsing.
• 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 parser, label resolution, constants
• 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

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 }

Native Function Registration

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

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.