1) receive event string 2) split header and payload 3) transform each in a wellformed list of fields 4) return their concatenation or an error

gistfile1.ex

def format event
  {header, payload} = SomeModule.somefn(event)

  # then return the concatenation of the resulting lists
  {:ok, format_header(header) ++ format_payload(payload)}
  # or
  {:error, format_header_reason}
  # or
  {:error, format_payload_reason} 
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

gistfile2.ex

def format event
  {header, payload} = SomeModule.somefn(event)

  case format_header(header) do
     {:ok, f_header} ->
        case format_payload(payload) do
          {:ok, f_payload} -> {:ok, f_header ++ f_payload}
          {:error, reason -> {:error, reason}
        end 
     {:error, reason -> {:error, reason}
  end
  
end