---

name: Go Core description: Go language fundamentals, concurrency, error handling, and project patterns. metadata: labels: [golang, core, language] triggers: files: ['go.mod', '**/*.go'] keywords: [func, package, import, goroutine, chan]

Go Core Standards

Goroutines & Channels

Copy// Spawn goroutine
go func() {
    result := process(data)
    resultChan <- result
}()

// Unbuffered channel (synchronous)
ch := make(chan int)

// Buffered channel
ch := make(chan int, 100)

// Send and receive
ch <- value      // Send
value := <-ch    // Receive

// Select for multiplexing
select {
case msg := <-msgChan:
    handle(msg)
case <-time.After(5 * time.Second):
    return errors.New("timeout")
case <-ctx.Done():
    return ctx.Err()
}

Patterns:

• Worker pools with buffered channels
• Fan-out/fan-in for parallel processing
• Done channel for cancellation

Error Handling

Copy// Return errors, don't panic
func fetchUser(id int) (*User, error) {
    user, err := db.FindUser(id)
    if err != nil {
        return nil, fmt.Errorf("fetch user %d: %w", id, err)
    }
    return user, nil
}

// Error wrapping (Go 1.13+)
if errors.Is(err, sql.ErrNoRows) {
    return nil, ErrNotFound
}

// Type assertion for custom errors
var apiErr *APIError
if errors.As(err, &apiErr) {
    log.Printf("API error: %d", apiErr.Code)
}

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

Rules:

• Always check errors immediately
• Wrap with context: fmt.Errorf("operation: %w", err)
• Use errors.Is and errors.As for comparison
• Never: panic for recoverable errors

Interfaces

Copy// Small interfaces
type Reader interface {
    Read(p []byte) (n int, err error)
}

type Writer interface {
    Write(p []byte) (n int, err error)
}

// Composition
type ReadWriter interface {
    Reader
    Writer
}

// Accept interfaces, return structs
func ProcessData(r Reader) (*Result, error) {
    data, err := io.ReadAll(r)
    // ...
    return &Result{}, nil
}

Best Practices:

• Define interfaces where used, not where implemented
• Keep interfaces small (1-3 methods)
• Use interface{} sparingly; prefer generics (Go 1.18+)

Context

Copy// Pass context as first parameter
func fetchData(ctx context.Context, id int) (*Data, error) {
    // Respect cancellation
    select {
    case <-ctx.Done():
        return nil, ctx.Err()
    default:
    }

    // Use context with HTTP requests
    req, _ := http.NewRequestWithContext(ctx, "GET", url, nil)
    return client.Do(req)
}

// Add timeout
ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
defer cancel()

// Add values (use sparingly)
ctx = context.WithValue(ctx, requestIDKey, reqID)

Rules:

• Never store context in structs
• Always call cancel function (use defer)
• Use for cancellation, deadlines, request-scoped values only

Project Structure

Copyproject/
├── cmd/
│   └── myapp/
│       └── main.go       # Entry point
├── internal/             # Private packages
│   ├── handler/
│   ├── service/
│   └── repository/
├── pkg/                  # Public packages
├── api/                  # API definitions (proto, OpenAPI)
├── configs/
├── go.mod
└── go.sum

Conventions:

• internal/ prevents external imports
• cmd/ for multiple entry points
• Flat structure for small projects

Testing

Copy// Table-driven tests
func TestAdd(t *testing.T) {
    tests := []struct {
        name     string
        a, b     int
        expected int
    }{
        {"positive", 1, 2, 3},
        {"negative", -1, -1, -2},
        {"zero", 0, 0, 0},
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            result := Add(tt.a, tt.b)
            if result != tt.expected {
                t.Errorf("Add(%d, %d) = %d; want %d", tt.a, tt.b, result, tt.expected)
            }
        })
    }
}

// Parallel tests
func TestParallel(t *testing.T) {
    t.Parallel()
    // ...
}

// Benchmarks
func BenchmarkProcess(b *testing.B) {
    for i := 0; i < b.N; i++ {
        Process(data)
    }
}

Performance

1. Preallocate slices: make([]T, 0, expectedLen)
2. Avoid allocations: Reuse buffers, use sync.Pool
3. Profile first: go tool pprof before optimizing
4. Escape analysis: go build -gcflags='-m' to check heap escapes
5. String building: Use strings.Builder not + concatenation