Here you call string_num and use num_list as a parameter. Since such a variable does not exist in the scope of the cipher function, the compiler looks for a function but of course it does not exist as well.

Can you post the rest of your module? I’m pretty sure that the problem originates elsewhere, as the following compiles (with a bunch of warnings):

defmodule Example do
  defp string_to_num(string_grahemes, alphabets, cipher_nums, num_list \\ []) do
        string_to_num(tl(string_grahemes), alphabets, tl(cipher_nums), [(Keyword.get(alphabets, hd(string_grahemes) |> String.to_atom)) + hd(cipher_nums) | num_list])
  end

  defp string_to_num([], alphabets, cipher_nums, num_list) do
        num_list
  end
end

Not an exact answer, but you should probably swap the functions (put the last one before the first) because now string_to_num with an empty list as the first argument will be processed by the first function, where I think you would like it to use the second function.

As @zwippie suggests, you need to put the string_to_num/4 with the [] first param first, since if the first param is [], the first clause will match, with [] being assigned to string_grahemes. You should then be getting a compiler warning to that effect.

You should also be getting a compiler warning:

warning: defp string_to_num/4 has multiple clauses and also declares default values. In such cases, the default values should be defined in a header. Instead of:

    def foo(:first_clause, b \\ :default) do ... end
    def foo(:second_clause, b) do ... end

one should write:

    def foo(a, b \\ :default)
    def foo(:first_clause, b) do ... end
    def foo(:second_clause, b) do ... end