Key-based access to data structures via the foo[bar] syntax.

Elixir provides two syntaxes for accessing values. user[:name] is used by dynamic structures, like maps and keywords, while user.name is used by structs. The main difference is that user[:name] won’t raise if the key :name is missing but user.name will raise if there is no :name key.

Key-based lookups

Out of the box, Access works with Keyword and Map:

iex> keywords = [a: 1, b: 2]
iex> keywords[:a]
1

iex> map = %{a: 1, b: 2}
iex> map[:a]
1

iex> star_ratings = %{1.0 => "★", 1.5 => "★☆", 2.0 => "★★"}
iex> star_ratings[1.5]
"★☆"

Access can be combined with Kernel.put_in/3 to put a value in a given key:

iex> map = %{a: 1, b: 2}
iex> put_in map[:a], 3
%{a: 3, b: 2}

This syntax is very convenient as it can be nested arbitrarily:

iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> put_in users["john"][:age], 28
%{"john" => %{age: 28}, "meg" => %{age: 23}}

Furthermore, Access transparently ignores nil values:

iex> keywords = [a: 1, b: 2]
iex> keywords[:c][:unknown]
nil

Since Access is a behaviour, it can be implemented to key-value data structures. The implementation should be added to the module that defines the struct being access. Access requires the key comparison to be implemented using the === operator.

Field-based lookups

The Access syntax (foo[bar]) cannot be used to access fields in structs. That’s by design, as Access is meant to be used for dynamic key-value structures, like maps and keywords, and not by static ones like structs.

However Elixir already provides a field-based lookup for structs. Imagine a struct named User with name and age fields. The following would raise:

user = %User{name: "john"}
user[:name]
** (UndefinedFunctionError) undefined function User.fetch/2
   (User does not implement the Access behaviour)

Structs instead use the user.name syntax:

user.name
#=> "john"

The same user.name syntax can also be used by Kernel.put_in/2 to for updating structs fields:

put_in user.name, "mary"
%User{name: "mary"}

Differently from user[:name], user.name cannot be extended by the developers, and will be always restricted to only maps and structs.

Summing up:

• user[:name] is used by dynamic structures, is extensible and does not raise on missing keys
• user.name is used by static structures, it is not extensible and it will raise on missing keys