ネストしたデータ構造を操作するput_inとupdate_inが便利 ref: http://qiita.com/yoavlt/items/b973210605d6f771b9d1

file0.txt

iex> foo = %{person: %{name: "foo", age: 21}, place: "Shibuya"}
iex> put_in(foo.person.age, 30)
 %{person: %{age: 30, name: "foo"}, place: "Shibuya"}
iex> foo
%{person: %{age: 21, name: "foo"}, place: "Shibuya"}

file1.txt

iex> foo = %{person: %{name: "foo", age: 21}, place: "Shibuya"}
iex> update_in(foo.person.age, fn age -> age + 1 end)
%{person: %{age: 22, name: "foo"}, place: "Shibuya"}

file2.txt

defmodule User do
  defstruct name: nil, text: nil, place: nil
end

defmodule Message do
  defstruct user: nil, text: nil, place: nil
end

message = %Message{user: %User{name: "foo", age: 21}, text "Yay!", place: "Shibuya"}
update_in(message.user.age, &(&1 + 1))
// => %Message{place: "Shibuya", text: "Yay!", user: %User{age: 22, name: "foo"}}