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Golang / GoLang Production Patterns and Web Standards Interview Questions

What are the best practices for defining custom error types in Go?

Custom error types are used when callers need to inspect error details beyond a simple message. The key decision is whether to use a pointer receiver (most common for structs) or a value receiver, and whether to implement the optional Unwrap() method to participate in the error chain.

// Pattern 1: simple sentinel error (no extra data)
var ErrRateLimited = errors.New("rate limited")

// Pattern 2: structured error with context
type HTTPError struct {
    Code    int
    Message string
    Cause   error // optional wrapped cause
}

func (e *HTTPError) Error() string {
    if e.Cause != nil {
        return fmt.Sprintf("HTTP %d: %s: %v", e.Code, e.Message, e.Cause)
    }
    return fmt.Sprintf("HTTP %d: %s", e.Code, e.Message)
}

// Implement Unwrap() to participate in errors.Is / errors.As chain
func (e *HTTPError) Unwrap() error { return e.Cause }

// Constructor Ҁ” always return concrete type as error interface
func newHTTPError(code int, msg string, cause error) error {
    return &HTTPError{Code: code, Message: msg, Cause: cause}
}

// Usage
func callAPI(url string) error {
    resp, err := http.Get(url)
    if err != nil {
        return newHTTPError(0, "request failed", err)
    }
    if resp.StatusCode == http.StatusTooManyRequests {
        return newHTTPError(429, "rate limited", ErrRateLimited)
    }
    return nil
}

// Caller
err := callAPI("https://api.example.com/data")
var httpErr *HTTPError
if errors.As(err, &httpErr) {
    log.Printf("HTTP error %d: %s", httpErr.Code, httpErr.Message)
}
if errors.Is(err, ErrRateLimited) {
    time.Sleep(time.Second) // back off and retry
}

// ANTI-PATTERN: typed nil trap
// func getUser() error {
//     var e *HTTPError
//     return e    // NOT nil interface! {type=*HTTPError, data=nil}
// }
// Always: return nil (bare), not a typed nil pointer

Why must a custom error type implement Unwrap() to work correctly with errors.Is?
What is the 'typed nil trap' in Go error handling?

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