Monk is your everyday :ok | :error monad.

The monk macro allows you to pipe functions that accept a mere value, and returns wrapped values like {ok:, val} or {:error, val}.

If a function returns {:error, reason}, the subsequent functions in the pipe will not be called and the result of the whole expression will be the {:error, _} tuple

Of course, you need to use Monk to be able to call the macro.

Two syntaxes are supported :

result = monk data |> map |> reduce

monk do
  some |> nice |> piping
end

If you except an error from a function, you can catch the error and continue through the pipe with the ok/1 function. The next function will receive the {:error, reason} tuple as it’s first argument. Here, report_error receives any value that is returned by do_things

monk bad_data |> ok(do_things) |> report_error

This could be read as the following :

{:ok, d} = bad_data()
{:ok, {:error, e}} = ok(do_things(d))
report_error({:error, e})