group_by

group_by.ex

  def group_by(collection, fun) do
    collection |> map(fn x -> { fun.(x), x } end) |>
    List.foldr(HashDict.new, fn ({k, v}, d) ->
      Dict.update(d, k, [v], fn([h | t]) ->
        List.concat([v], [h | t])
      end)
    end)
  end