def show(conn, _params) do
   invoice_id = get_session(conn, :invoice_id)
   invoice_access_token = get_session(conn, :access_token)
   do_render(invoice_id, invoice_access_token)
end

def w_error({:error, e}, key), do: {:error, {key, e}}
def w_error({:ok, v}, key), do: {key, v}

defmodule StringCharsImplementations do
 defimpl String.Chars, for: Tuple do
   def to_string(term) do
     {first, last} = term
     "#{first}..#{last}"
   end
 end
end

iex(1)> "#{{1, 2}}"
"1..2"