Structs

import IEx.Helpers

Defining Structs

defmodule User do
  defstruct name: "Jill", age: 25

  def new do
    %User{name: "Bruce", age: 21}
  end
end

{:module, User, <<70, 79, 82, 49, 0, 0, 8, ...>>, {:new, 0}}

defmodule Point do
  defstruct [:x, :y]
end

{:module, Point, <<70, 79, 82, 49, 0, 0, 8, ...>>, %Point{x: nil, y: nil}}

Point.__struct__()

%Point{x: nil, y: nil}

User.new()

%User{name: "Bruce", age: 21}

u = %User{age: 26}

%User{name: "Jill", age: 26}

The struct key and function

# exports from erlang
exports(User)

__struct__/0     __struct__/1     new/0            

# __ => dunder struct => double underscore struct
User.__struct__(%{age: 30})

%User{name: "Jill", age: 30}

# __ => dunder struct => double underscore struct
User.__struct__(age: 30)

%User{name: "Jill", age: 30}

Map.keys(u)

[:name, :__struct__, :age]

u.__struct__

User

Protocols

i(%{})

Term
  %{}
Data type
  Map
Reference modules
  Map
Implemented protocols
  Collectable, Enumerable, IEx.Info, Inspect

i(%Point{})

Term
  %Point{x: nil, y: nil}
Data type
  Point
Description
  This is a struct. Structs are maps with a __struct__ key.
Reference modules
  Point, Map
Implemented protocols
  IEx.Info, Inspect

is_map(%Point{})

true

is_struct(%Point{})

true

Enum.count(%{one: 1})

1

Enum.count(Point.__struct__())

map = %{one: 1, two: 2}
map[:two]

2

map.one

1

{map.one, map[:two]}

{1, 2}

user = User.new()
user.name

"Bruce"

user[:age]

Common structs in Elixir

IEx.Helpers.i(Date.utc_today())

Term
  ~D[2024-01-03]
Data type
  Date
Description
  This is a struct representing a date. It is commonly
  represented using the `~D` sigil syntax, that is
  defined in the `Kernel.sigil_D/2` macro.
Raw representation
  %Date{year: 2024, month: 1, day: 3, calendar: Calendar.ISO}
Reference modules
  Date, Calendar, Map
Implemented protocols
  IEx.Info, Inspect, String.Chars

Date.utc_today() |> Map.keys()

[:calendar, :month, :__struct__, :day, :year]

DateTime.utc_now() |> Map.keys()

[:microsecond, :second, :calendar, :month, :__struct__, :day, :year, :minute, :hour, :time_zone,
 :zone_abbr, :utc_offset, :std_offset]