There are two "kinds" of panics: those initiated by the Go runtime, and those initiated by the program author, though technically there isn't a distinction. When you access an array out of bounds, the Go runtime initiates the panic. On the other hand, for a condition that the program should hold but doesn't, the program author should initiate the panic. For example, in the exhaustive switch below, reaching the default case means that either you have somehow incorrectly allowed the construction of an invalid token value earlier in your program, or you have missed to account for a valid token value in this function.

  func (t token) symbol() byte {
          switch t {
                  case add: return '+'
                  case sub: return '-'
                  case mul: return '*'
                  case quo: return '/'
                  default:  panic("unknown token")
          }
  }

Errors are different. For example, url.Parse returns an error if its input string is an invalidly formatted URL.

  package url
  func Parse(rawURL string) (*URL, error)