The idiom is {:ok, value}, {:error, reason}
So the pattern match should focus on the contents of reason.
For example:
Process.monitor/1 will result in a general message of the format:
{:DOWN, ref, :process, object, reason}
where reason can take on values like :normal, :noproc or :noconnection, i.e. values that are highly distinct and imply their context. So it’s a good idea to follow the same practice with {:error, reason} tuples.
