remove and difference functions for Enum

collection.ex

defmodule Coll do
  @doc """
  Removes all values from the subject

      iex> Coll.remove([1, 2, 3], [2, 3])
      [1]
      iex> Coll.remove([1, 2], [2, 3, 4])
      [1]

  """
  def remove(subject, values) do
    subject
    |> Enum.filter(fn left -> !Enum.member?(values, left) end)
  end

  @doc """
  Shows which elements were added and which were removed in 2 versions of a collection.

  Returns a tuple with a collection of removed values and a collection of added values

  ## Examples

  ### Compare lists

      iex> old = ["apple", "banana", "pear"]
      iex> new = ["tomato", "apple"]
      iex> {removed, added} = Coll.difference(old, new)
      iex> removed
      ["banana", "pear"]
      iex> added
      ["tomato"]

  This tells us that "banana" and "pear" have been removed, while "tomato" has been added.
  
  ### Compare keyword lists

      iex> old = [fruit: :apple, vegetable: :cucumber]
      iex> new = [fruit: :apple, vegetable: :potato]
      iex> {removed, added} = Coll.difference(old, new)
      iex> removed
      [vegetable: :cucumber]
      iex> added
      [vegetable: :potato]

  ### Maps are special

      iex> old = %{}
      iex> new = %{foo: "bar"}
      iex> {removed, added} = Coll.difference(old, new)
      iex> removed
      []
      iex> added
      [foo: "bar"]
      
  ### Deep comparision

      iex> old = [[1, 2, 3], [4, 5, 6]]
      iex> new = [[2, 3, 4], [4, 6]]
      iex> old |> Enum.zip(new) |> Enum.map(fn {old, new} -> Coll.difference(old, new) end)
      [
        {[1], [4]},
        {[5], []}
      ]

  """
  def difference(old, new) do
    {remove(old, new), remove(new, old)}
  end
end