defmodule UserExtraction do
  def extract(user) do
    case Enum.filter(["login", "email", "password"], &(not Map.has_key?(user, &1))) do
      [] ->
        {:ok, %{login: user["login"], email: user["email"], password: user["password"]}}

      missing_fields ->
        {:error, "missing fields: #{Enum.join(missing_fields, ", ")}"}
    end
  end
end