gistfile1.ex

def format event
  { header, payload } = SomeModule.somefn(event)
  
  join_results(format_header(header), format_payload(payload))
end

def format_header(header)
   # do stuff
   ....
   # and return
   {:ok, ["a", "b", "c"]}
   # or
   {:error, "Header Parse Failed"}
end
 
def format_payload(payload)
   # do stuff
   .... 
   # and return
   {:ok, [1, 2, 3, 4, 5, 6, 7]}
   # or
   {:error, "Payload Parse Failed"}
end

def join_results(header_error = { :error, _ }, _),  do: header_error
def join_results(_, payload_error = { :error, _ }), do: payload_error
def join_results({ :ok, header_stuff }, { :ok, payload_stuff }) do
  { :ok, header_stuff ++ payload_stuff }
end