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Elixir Structs

Elixir structs are built on top of Maps, a key/value pair data structure. Structs provide helpful features such as default values and compile time errors.

Structs are usually defined using the defstruct/1 inside of a module.

defmodule Kitty do
  defstruct name: nil, age: nil, breed: nil
  #defstruct [:name, :age, :breed]
end

defstruct/1 takes a key/value list where the values are the defaults. If you happen to be defaulting to nil for every field, you can use the alternative syntax on the second line that only provides the keys.

You can access a struct in the same way as a Map, except it’s defined by its module name:

iex(3)> meow = %Kitty{name: "Floyd"}
%Kitty{age: nil, breed: nil, name: "Floyd"}
iex(4)> meow = %Kitty{meow | age: 2}
%Kitty{age: 2, breed: nil, name: "Floyd"}
iex(5)> %Kitty{meow | color: "white"}
** (CompileError) iex:5: unknown key :color for struct Kitty
    (elixir) src/elixir_map.erl:185: :elixir_map."-assert_struct_keys/5-lc$^0/1-0-"/5
    (elixir) src/elixir_map.erl:62: :elixir_map.translate_struct/4
iex(5)>

As you can see, structs only allow entry of defined keys.

You can also provide a typespec.

defmodule Kitty do
  defstruct [:name, :age, :breed]

  @type t :: %Kitty{name: String.t, age: non_neg_integer, breed: String.t}
end