View Source Map (Elixir v1.11.4)

Maps are the "go to" key-value data structure in Elixir.

Maps can be created with the %{} syntax, and key-value pairs can be expressed as key => value:

iex> %{}
%{}
iex> %{"one" => :two, 3 => "four"}
%{3 => "four", "one" => :two}

Key-value pairs in a map do not follow any order (that's why the printed map in the example above has a different order than the map that was created).

Maps do not impose any restriction on the key type: anything can be a key in a map. As a key-value structure, maps do not allow duplicated keys. Keys are compared using the exact-equality operator (===/2). If colliding keys are defined in a map literal, the last one prevails.

When the key in a key-value pair is an atom, the key: value shorthand syntax can be used (as in many other special forms), provided key-value pairs are put at the end:

iex> %{"hello" => "world", a: 1, b: 2}
%{:a => 1, :b => 2, "hello" => "world"}

Keys in maps can be accessed through some of the functions in this module (such as Map.get/3 or Map.fetch/2) or through the map[] syntax provided by the Access module:

iex> map = %{a: 1, b: 2}
iex> Map.fetch(map, :a)
{:ok, 1}
iex> map[:b]
2
iex> map["non_existing_key"]
nil

To access atom keys, one may also use the map.key notation. Note that map.key will raise a KeyError if the map doesn't contain the key :key, compared to map[:key], that would return nil.

map = %{foo: "bar", baz: "bong"}
map.foo
#=> "bar"
map.non_existing_key
#=> ** (KeyError) key :non_existing_key not found in: %{baz: "bong", foo: "bar"}

Note: do not add parens when accessing fields, such as in data.key(). If parenthesis are used, Elixir will consider it to be a function call on data, which would be expected to be an atom.

The two syntaxes for accessing keys reveal the dual nature of maps. The map[key] syntax is used for dynamically created maps that may have any key, of any type. map.key is used with maps that hold a predetermined set of atoms keys, which are expected to always be present. Structs, defined via defstruct/1, are one example of such "static maps", where the keys can also be checked during compile time.

Maps can be pattern matched on. When a map is on the left-hand side of a pattern match, it will match if the map on the right-hand side contains the keys on the left-hand side and their values match the ones on the left-hand side. This means that an empty map matches every map.

iex> %{} = %{foo: "bar"}
%{foo: "bar"}
iex> %{a: a} = %{:a => 1, "b" => 2, [:c, :e, :e] => 3}
iex> a
1

But this will raise a MatchError exception:

%{:c => 3} = %{:a => 1, 2 => :b}

Variables can be used as map keys both when writing map literals as well as when matching:

iex> n = 1
1
iex> %{n => :one}
%{1 => :one}
iex> %{^n => :one} = %{1 => :one, 2 => :two, 3 => :three}
%{1 => :one, 2 => :two, 3 => :three}

Maps also support a specific update syntax to update the value stored under existing atom keys:

iex> map = %{one: 1, two: 2}
iex> %{map | one: "one"}
%{one: "one", two: 2}

When a key that does not exist in the map is updated a KeyError exception will be raised:

%{map | three: 3}

The functions in this module that need to find a specific key work in logarithmic time. This means that the time it takes to find keys grows as the map grows, but it's not directly proportional to the map size. In comparison to finding an element in a list, it performs better because lists have a linear time complexity. Some functions, such as keys/1 and values/1, run in linear time because they need to get to every element in the map.

Maps also implement the Enumerable protocol, so many functions to work with maps are found in the Enum module. Additionally, the following functions for maps are found in Kernel: