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Golang / GoLang Production Patterns and Web Standards Interview Questions

How do you wrap errors in Go 1.13+ and use errors.Is and errors.As for inspection?

Go 1.13 introduced the %w verb in fmt.Errorf and the errors.Is / errors.As functions to create and inspect error chains. Wrapping preserves the original error while adding context — callers can still check for specific error types or sentinel values anywhere in the chain.

import "errors"

// Sentinel errors — comparable with errors.Is
var (
    ErrNotFound   = errors.New("not found")
    ErrPermission = errors.New("permission denied")
)

// Wrapping with %w — preserves the error chain
func fetchUser(id int) (*User, error) {
    u, err := db.QueryUser(id)
    if err != nil {
        return nil, fmt.Errorf("fetchUser(id=%d): %w", id, err)
    }
    return u, nil
}

// errors.Is — checks the entire chain for a target value
err := fetchUser(42)
if errors.Is(err, ErrNotFound) {
    http.Error(w, "user not found", http.StatusNotFound)
    return
}

// Custom structured error type
type ValidationError struct {
    Field   string
    Message string
}
func (e *ValidationError) Error() string {
    return fmt.Sprintf("validation error on %s: %s", e.Field, e.Message)
}

func validateAge(age int) error {
    if age < 0 {
        return fmt.Errorf("validateAge: %w",
            &ValidationError{Field: "age", Message: "must be non-negative"})
    }
    return nil
}

// errors.As — extracts a specific error type from the chain
var ve *ValidationError
if errors.As(err, &ve) {
    http.Error(w,
        fmt.Sprintf("invalid field %s: %s", ve.Field, ve.Message),
        http.StatusBadRequest)
}

// errors.Unwrap — one level of unwrapping
inner := errors.Unwrap(err)   // returns the wrapped error

// Go 1.20+: errors.Join — wrap multiple errors in one
combined := errors.Join(err1, err2)
errors.Is(combined, err1) // true

Key difference: %v formats the error as a plain string — the wrapped error is lost for inspection purposes. %w both formats it AND preserves the wrapped error so errors.Is and errors.As can traverse the chain.

What does 'fmt.Errorf("context: %w", err)' do that 'fmt.Errorf("context: %v", err)' does not?
What does errors.As(err, ⌖) do?

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