Go

Errors are values

• Return error; do not panic across package boundaries.
• Wrap with fmt.Errorf("doing X: %w", err) to preserve the chain.
• Inspect with errors.Is and errors.As. Never string-match messages.
• Sentinel errors (var ErrNotFound = errors.New(...)) for expected cases; typed errors when callers need structured data.

if err != nil {
    return fmt.Errorf("load user %s: %w", id, err)
}

Interfaces — small, defined by the consumer

• Define interfaces in the package that uses them, not where concrete types live.
• Single-method interfaces are normal (io.Reader, io.Closer).
• Accept interfaces, return concrete types.