golang-expert

name: golang-expert description: | Expert Go/Golang development guidance enforcing best practices, functional programming principles, KISS, DRY, and idiomatic patterns. Use when: (1) Writing new Go code, (2) Reviewing Go code for quality issues, (3) Refactoring Go code, (4) Designing Go architecture with interfaces and DI, (5) Writing or improving Go tests, (6) Debugging concurrency or performance issues, (7) User asks about Go best practices or patterns.

Golang Expert

Expert guidance for writing clean, idiomatic, maintainable Go code.

Table of Contents

1. Core Principles
2. Quick Reference
3. Detailed Guides

Core Principles

The Go Philosophy (KISS, DRY, YAGNI)

1. Simplicity over cleverness - Readable beats clever
2. Explicit over implicit - No magic, clear data flow
3. Composition over inheritance - Small interfaces, embed structs
4. Errors are values - Handle them, don't ignore them

KISS - Keep It Simple

// BAD - over-engineered
type ProcessorFactory interface {
    CreateProcessor(config Config) Processor
}

// GOOD - direct and simple
func Process(data []byte) (Result, error) {
    // Direct implementation
}

DRY - Don't Repeat Yourself

// BAD - duplicated logic
func ParseUserDate(s string) time.Time { /*...*/ }
func ParseOrderDate(s string) time.Time { /*...*/ } // Same code!

// GOOD - single source of truth
func ParseDate(s string) (time.Time, error) {
    return time.Parse(time.RFC3339, s)
}

YAGNI - You Aren't Gonna Need It

Don't build for hypothetical future requirements. Only implement what's needed right now.

// BAD - over-engineered for "future flexibility"
type DataProcessor interface {
    Process(data []byte) ([]byte, error)
    ProcessBatch(data [][]byte) ([][]byte, error)
    ProcessAsync(data []byte, callback func([]byte, error))
    ProcessWithOptions(data []byte, opts ProcessOptions) ([]byte, error)
}

type ProcessOptions struct {
    Format       string
    Compression  bool
    Encryption   bool
    Retry        int
    Timeout      time.Duration
    Logger       Logger
    Metrics      MetricsCollector
    // 20 more fields "just in case"
}

// GOOD - solve today's problem
func Process(data []byte) ([]byte, error) {
    // Direct implementation of what's actually needed
}

YAGNI Anti-patterns to Avoid:

1. Premature abstraction - Don't create interfaces until you have 2+ implementations
2. Speculative generality - Don't add parameters "someone might need"
3. Gold plating - Don't add features beyond requirements
4. Framework thinking - You're building an app, not a framework

// BAD - abstraction with only one implementation
type UserRepository interface {
    GetUser(id int) (*User, error)
}
type userRepositoryImpl struct { db *sql.DB }

// GOOD - just use the concrete type until you need abstraction
type UserStore struct { db *sql.DB }
func (s *UserStore) GetUser(id int) (*User, error) { /*...*/ }

The Rule of Three: Don't abstract until you see the pattern three times.

// First time: just write the code
// Second time: note the duplication, but wait
// Third time: NOW refactor to remove duplication

Delete code freely. Unused code is a liability, not an asset. Version control remembers everything.

Functional Principles

1. No global mutable state - Use dependency injection
2. Immutability - Return new values, don't mutate inputs
3. Pure functions - Same input = same output, no side effects
4. Constants over variables - Use const when possible

// BAD - global state
var logger *Logger
func SetLogger(l *Logger) { logger = l }

// GOOD - dependency injection
type Service struct {
    logger Logger
}
func NewService(logger Logger) *Service {
    return &Service{logger: logger}
}

Quick Reference

Interface Design

// Small, focused interfaces (Interface Segregation)
type Reader interface { Read(p []byte) (n int, err error) }
type Writer interface { Write(p []byte) (n int, err error) }

// Compose when needed
type ReadWriter interface {
    Reader
    Writer
}

// Accept interfaces, return structs
func Process(r Reader) *Result { /*...*/ }

Error Handling

// Wrap errors with context
if err != nil {
    return fmt.Errorf("process user %d: %w", id, err)
}

// Sentinel errors for expected conditions
var ErrNotFound = errors.New("not found")

// Check with errors.Is/As
if errors.Is(err, ErrNotFound) { /*...*/ }

Table-Driven Tests

func TestParse(t *testing.T) {
    tests := []struct {
        name    string
        input   string
        want    Result
        wantErr bool
    }{
        {"valid input", "abc", Result{Value: "abc"}, false},
        {"empty input", "", Result{}, true},
    }
    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            got, err := Parse(tt.input)
            if (err != nil) != tt.wantErr {
                t.Errorf("Parse() error = %v, wantErr %v", err, tt.wantErr)
                return
            }
            if got != tt.want {
                t.Errorf("Parse() = %v, want %v", got, tt.want)
            }
        })
    }
}

Concurrency

// Always use context for cancellation
func Process(ctx context.Context) error {
    select {
    case <-ctx.Done():
        return ctx.Err()
    case result := <-work():
        return handle(result)
    }
}

// Use errgroup for parallel work
g, ctx := errgroup.WithContext(ctx)
for _, item := range items {
    item := item // capture loop variable
    g.Go(func() error { return process(ctx, item) })
}
return g.Wait()

Detailed Guides

Load these references as needed:

Code Review Workflow

When reviewing Go code:

1. Read code-review-checklist.md
2. Check for KISS/DRY violations
3. Verify error handling is complete
4. Assess interface design
5. Review test coverage and quality
6. Flag concurrency issues
7. Identify performance concerns

Refactoring Workflow

When refactoring:

1. Ensure tests exist before changes
2. Apply KISS - Remove unnecessary abstractions
3. Apply DRY - Extract duplicated code
4. Improve interfaces - Make them smaller
5. Add DI - Remove global state
6. Run tests after each change