with example

with.ex

defmodule RetweetCalculator do
  def calculate_total_retweets(user_id) do
    with {:ok, handle} <- twitter_handle_for(user_id),
         {:ok, tweets} <- all_tweets_for(handle),
      do: { :ok, sum_retweets_for(tweets) }
  end

  defp twitter_handle_for(user_id) do
    # implementation
    {:ok, handle}
  end

  defp all_tweets_for(user) do
    # implementation
    {:ok, []}
  end

  defp sum_retweets_for(tweets) do
    tweets
    |> Enum.map(fn(tweet) -> tweet.retweets end) # => [2,3,5,7]
    |> Enum.sum # => 17
  end
end