Didn't know whether to respond here or above to parent, but ! In stdlib of Ruby does not indicate mutation but "dangerous version"... Often the same thing but I believe all methods in std lib with a bang also have a noo-bang version, which usually does not mutate receiver, but could also be, for example, File.read vs File.read!... I forget if Ruby has those but Elixir does and they indicate there that one returns an error or ok prefixed tuple but the dangerous version just raises an error if the file isn't present for example.