Programming Elixir ≥ 1.6

Designing with Exceptions

If File.open succeeds, it returns {:ok, file}, where file is the service that gives you access to the file. If it fails, it returns {:error, reason}. So, for code that knows a file open might not succeed and wants to handle the fact, you might write

case File.open(user_file_name) do
{:ok, file} ->
  process(file)
{:error, message} ->
  IO.puts :stderr, "Couldn't open #{user_file_name}: #{message}"
end

If instead you expect the file to open successfully every time, you could raise an exception on failure.

case File.open("config_file") do
{:ok, file} ->
  process(file)
{:error, message} ->
  raise "Failed to open config file: #{message}"
end

Or you could let ...