bamort/.github/instructions/go.instructions.md

---
description: 'Instructions for writing Go code following idiomatic Go practices and community standards'
applyTo: '**/*.go,**/go.mod,**/go.sum'
---

# Go Development Instructions

READ THIS FILE CAREFULLY AND COMPLETE BEFORE STARTING DEVELOPMENT!

Follow idiomatic Go practices and community standards when writing Go code. These instructions are based on [Effective Go](https://go.dev/doc/effective_go), [Go Code Review Comments](https://go.dev/wiki/CodeReviewComments), and [Google's Go Style Guide](https://google.github.io/styleguide/go/).

## General Instructions

- Write simple, clear, and idiomatic Go code
- Favor clarity and simplicity over cleverness
- Follow the principle of least surprise
- Keep the happy path left-aligned (minimize indentation)
- Return early to reduce nesting
- Make the zero value useful
- Document exported types, functions, methods, and packages
- Use Go modules for dependency management
- You write tests ONLY in _test.go files. 
- You will NEVER create files for testing with a main() function!

## Naming Conventions

### Packages

- Use lowercase, single-word package names
- Avoid underscores, hyphens, or mixedCaps
- Choose names that describe what the package provides, not what it contains
- Avoid generic names like `util`, `common`, or `base`
- Package names should be singular, not plural

### Variables and Functions

- Use mixedCaps or MixedCaps (camelCase) rather than underscores
- Keep names short but descriptive
- Use single-letter variables only for very short scopes (like loop indices)
- Exported names start with a capital letter
- Unexported names start with a lowercase letter
- Avoid stuttering (e.g., avoid `http.HTTPServer`, prefer `http.Server`)

### Interfaces

- Name interfaces with -er suffix when possible (e.g., `Reader`, `Writer`, `Formatter`)
- Single-method interfaces should be named after the method (e.g., `Read` → `Reader`)
- Keep interfaces small and focused

### Constants

- Use MixedCaps for exported constants
- Use mixedCaps for unexported constants
- Group related constants using `const` blocks
- Consider using typed constants for better type safety

## Code Style and Formatting

### Formatting

- Always use `gofmt` to format code
- Use `goimports` to manage imports automatically
- Keep line length reasonable (no hard limit, but consider readability)
- Add blank lines to separate logical groups of code

### Comments

- Write comments in complete sentences
- Start sentences with the name of the thing being described
- Package comments should start with "Package [name]"
- Use line comments (`//`) for most comments
- Use block comments (`/* */`) sparingly, mainly for package documentation
- Document why, not what, unless the what is complex

### Error Handling

- Check errors immediately after the function call
- Don't ignore errors using `_` unless you have a good reason (document why)
- Wrap errors with context using `fmt.Errorf` with `%w` verb
- Create custom error types when you need to check for specific errors
- Place error returns as the last return value
- Name error variables `err`
- Keep error messages lowercase and don't end with punctuation

## Architecture and Project Structure

### Package Organization

- Follow standard Go project layout conventions
- Keep `main` packages in `cmd/` directory
- Put reusable packages in `pkg/` or `internal/`
- Use `internal/` for packages that shouldn't be imported by external projects
- Group related functionality into packages
- Avoid circular dependencies

### Dependency Management

- Use Go modules (`go.mod` and `go.sum`)
- Keep dependencies minimal
- Regularly update dependencies for security patches
- Use `go mod tidy` to clean up unused dependencies
- Vendor dependencies only when necessary

## Type Safety and Language Features

### Type Definitions

- Define types to add meaning and type safety
- Use struct tags for JSON, XML, database mappings
- Prefer explicit type conversions
- Use type assertions carefully and check the second return value

### Pointers vs Values

- Use pointers for large structs or when you need to modify the receiver
- Use values for small structs and when immutability is desired
- Be consistent within a type's method set
- Consider the zero value when choosing pointer vs value receivers

### Interfaces and Composition

- Accept interfaces, return concrete types
- Keep interfaces small (1-3 methods is ideal)
- Use embedding for composition
- Define interfaces close to where they're used, not where they're implemented
- Don't export interfaces unless necessary

## Concurrency

### Goroutines

- Don't create goroutines in libraries; let the caller control concurrency
- Always know how a goroutine will exit
- Use `sync.WaitGroup` or channels to wait for goroutines
- Avoid goroutine leaks by ensuring cleanup

### Channels

- Use channels to communicate between goroutines
- Don't communicate by sharing memory; share memory by communicating
- Close channels from the sender side, not the receiver
- Use buffered channels when you know the capacity
- Use `select` for non-blocking operations

### Synchronization

- Use `sync.Mutex` for protecting shared state
- Keep critical sections small
- Use `sync.RWMutex` when you have many readers
- Prefer channels over mutexes when possible
- Use `sync.Once` for one-time initialization

## Error Handling Patterns

### Creating Errors

- Use `errors.New` for simple static errors
- Use `fmt.Errorf` for dynamic errors
- Create custom error types for domain-specific errors
- Export error variables for sentinel errors
- Use `errors.Is` and `errors.As` for error checking

### Error Propagation

- Add context when propagating errors up the stack
- Don't log and return errors (choose one)
- Handle errors at the appropriate level
- Consider using structured errors for better debugging

## API Design

### HTTP Handlers

- Use `http.HandlerFunc` for simple handlers
- Implement `http.Handler` for handlers that need state
- Use middleware for cross-cutting concerns
- Set appropriate status codes and headers
- Handle errors gracefully and return appropriate error responses

### JSON APIs

- Use struct tags to control JSON marshaling
- Validate input data
- Use pointers for optional fields
- Consider using `json.RawMessage` for delayed parsing
- Handle JSON errors appropriately

## Performance Optimization

### Memory Management

- Minimize allocations in hot paths
- Reuse objects when possible (consider `sync.Pool`)
- Use value receivers for small structs
- Preallocate slices when size is known
- Avoid unnecessary string conversions

### Profiling

- Use built-in profiling tools (`pprof`)
- Benchmark critical code paths
- Profile before optimizing
- Focus on algorithmic improvements first
- Consider using `testing.B` for benchmarks

## Testing

### Test Organization

- Keep tests in the same package (white-box testing)
- Use `_test` package suffix for black-box testing
- Name test files with `_test.go` suffix
- Place test files next to the code they test

### Writing Tests

- Use table-driven tests for multiple test cases
- Name tests descriptively using `Test_functionName_scenario`
- Use subtests with `t.Run` for better organization
- Test both success and error cases
- Include benchmarks when performance matters: `BenchmarkFunctionName`
- Use `testify` or similar libraries sparingly

### Test Helpers

- Mark helper functions with `t.Helper()`
- Create test fixtures for complex setup
- Use `testing.TB` interface for functions used in tests and benchmarks
- Clean up resources using `t.Cleanup()`

## Security Best Practices

### Input Validation

- Validate all external input
- Use strong typing to prevent invalid states
- Sanitize data before using in SQL queries
- Be careful with file paths from user input
- Validate and escape data for different contexts (HTML, SQL, shell)

### Cryptography

- Use standard library crypto packages
- Don't implement your own cryptography
- Use crypto/rand for random number generation
- Store passwords using bcrypt or similar
- Use TLS for network communication

## Documentation

### Code Documentation

- Document all exported symbols
- Start documentation with the symbol name
- Use examples in documentation when helpful
- Keep documentation close to code
- Update documentation when code changes

### README and Documentation Files

- Include clear setup instructions
- Document dependencies and requirements
- Provide usage examples
- Document configuration options
- Include troubleshooting section

## Tools and Development Workflow

### Essential Tools

- `go vet`: Find suspicious constructs
- `go test`: Run tests
- `go fmt`: Format code
- `golint` or `golangci-lint`: Additional linting
- `go mod`: Manage dependencies
- `go generate`: Code generation

### Development Practices

- Always use Test-Driven Development (TDD)
- Write tests before any implementation
- No implementation code until tests exist and are reviewed
- Run tests before committing
- Use pre-commit hooks for formatting and linting
- Keep commits focused and atomic
- Write meaningful commit messages
- Review diffs before committing

## Common Pitfalls to Avoid

- Not checking errors
- Ignoring race conditions
- Creating goroutine leaks
- Not using defer for cleanup
- Modifying maps concurrently
- Not understanding nil interfaces vs nil pointers
- Forgetting to close resources (files, connections)
- Using global variables unnecessarily
- Over-using empty interfaces (`interface{}`)
- Not considering the zero value of types