Structs in

About Structs

Structs are special maps with a defined set of keys.

• Structs provide compile-time checks and default values.
• A struct is named after the module it is defined in.
• To define a struct use the defstruct construct.
  • The construct usually immediately follows after the module definition.
• defstruct accepts either a list of atoms (for nil default values) or keyword lists (for specified default values).
  • The fields without defaults must precede the fields with default values.

defmodule Plane do
  defstruct [:engine, wings: 2]
end

plane = %Plane{}
# => %Plane{engine: nil, wings: 2}

Accessing fields and updating

• Most functions that work with maps will also work with structs.

  • The Access Behaviour is an exception and is not implemented for structs.

• It is recommended to use the static access operator . to access struct fields.

• Get/fetch field values:

  plane = %Plane{}
  plane.engine
  # => nil
  Map.fetch(plane, :wings)
  # => {:ok, 2}
  

• Update field values

  plane = %Plane{}
  %{plane | wings: 4}
  # => %Plane{engine: nil, wings: 4}
  

Enforcing field value initialization

• The @enforce_keys module attribute creates a run-time check that specified fields must be initialized to a non-nil value when the struct is created.
• @enforce_keys is followed by a list of the field keys (which are atoms).
• If an enforced key is not initialized, an error is raised.

defmodule User do
  @enforce_keys [:username]
  defstruct [:username]
end

%User{}
# => (ArgumentError) the following keys must also be given when building struct User: [:username]