#
View Source Map (Elixir v1.15.7)

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):

    iex> %{a: 1, b: 2}
    %{a: 1, b: 2}

If you want to mix the shorthand syntax with `=>`, the shorthand syntax must come 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"}

> #### Avoid parentheses

> Do not add parentheses when accessing fields, such as in `data.key()`. If parentheses are used, Elixir will expect `data` to be an atom representing a module and attempt to call the function `key/0` in it.

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 keys. You can update using the atom keys syntax:

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

Or any other key:

    iex> other_map = %{"three" => 3, "four" => 4}
    iex> %{other_map | "three" => "three"}
    %{"four" => 4, "three" => "three"}

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`:

- `map_size/1`

# Summary

## Types

key()
value()

## Functions

delete(map, key)

Deletes the entry in `map` for a specific `key`.

drop(map, keys)

Drops the given `keys` from `map`.

equal?(map1, map2)

Checks if two maps are equal.

fetch(map, key)

Fetches the value for a specific `key` in the given `map`.

fetch!(map, key)

Fetches the value for a specific `key` in the given `map`, erroring out if `map` doesn't contain `key`.

filter(map, fun)

Returns a map containing only those pairs from `map` for which `fun` returns a truthy value.

from_keys(keys, value)

Builds a map from the given `keys` and the fixed `value`.

from_struct(struct)

Converts a `struct` to map.

get(map, key, default \\ nil)

Gets the value for a specific `key` in `map`.

get_and_update(map, key, fun)

Gets the value from `key` and updates it, all in one pass.

get_and_update!(map, key, fun)

Gets the value from `key` and updates it, all in one pass. Raises if there is no `key`.

get_lazy(map, key, fun)

Gets the value for a specific `key` in `map`.

has_key?(map, key)

Returns whether the given `key` exists in the given `map`.

intersect(map1, map2)

Intersects two maps, returning a map with the common keys.

intersect(map1, map2, fun)

Intersects two maps, returning a map with the common keys and resolving conflicts through a function.

keys(map)

Returns all keys from `map`.

merge(map1, map2)

Merges two maps into one.

merge(map1, map2, fun)

Merges two maps into one, resolving conflicts through the given `fun`.

new()

Returns a new empty map.

new(enumerable)

Creates a map from an `enumerable`.

new(enumerable, transform)

Creates a map from an `enumerable` via the given transformation function.

pop(map, key, default \\ nil)

Removes the value associated with `key` in `map` and returns the value and the updated map.

pop!(map, key)

Removes the value associated with `key` in `map` and returns the value and the updated map, or it raises if `key` is not present.

pop_lazy(map, key, fun)

Lazily returns and removes the value associated with `key` in `map`.

put(map, key, value)

Puts the given `value` under `key` in `map`.

put_new(map, key, value)

Puts the given `value` under `key` unless the entry `key` already exists in `map`.

put_new_lazy(map, key, fun)

Evaluates `fun` and puts the result under `key` in `map` unless `key` is already present.

reject(map, fun)

Returns map excluding the pairs from `map` for which `fun` returns a truthy value.

replace(map, key, value)

Puts a value under `key` only if the `key` already exists in `map`.

replace!(map, key, value)

Puts a value under `key` only if the `key` already exists in `map`.

replace_lazy(map, key, fun)

Replaces the value under `key` using the given function only if `key` already exists in `map`.

split(map, keys)

Takes all entries corresponding to the given `keys` in `map` and extracts them into a separate map.

split_with(map, fun)

Splits the `map` into two maps according to the given function `fun`.

take(map, keys)

Returns a new map with all the key-value pairs in `map` where the key is in `keys`.

to_list(map)

Converts `map` to a list.

update(map, key, default, fun)

Updates the `key` in `map` with the given function.

update!(map, key, fun)

Updates `key` with the given function.

values(map)

Returns all values from `map`.

# Types

Link to this type

# key()

View Source

    @type key() :: any()

Link to this type

# value()

View Source

    @type value() :: any()

# Functions

Link to this function

# delete(map, key)

View Source

    @spec delete(map(), key()) :: map()

Deletes the entry in `map` for a specific `key`.

If the `key` does not exist, returns `map` unchanged.

Inlined by the compiler.

## Examples

    iex> Map.delete(%{a: 1, b: 2}, :a)
    %{b: 2}
    iex> Map.delete(%{b: 2}, :a)
    %{b: 2}

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# drop(map, keys)

View Source

    @spec drop(map(), [key()]) :: map()

Drops the given `keys` from `map`.

If `keys` contains keys that are not in `map`, they're simply ignored.

## Examples

    iex> Map.drop(%{a: 1, b: 2, c: 3}, [:b, :d])
    %{a: 1, c: 3}

Link to this function

# equal?(map1, map2)

View Source

    @spec equal?(map(), map()) :: boolean()

Checks if two maps are equal.

Two maps are considered to be equal if they contain the same keys and those keys contain the same values.

Note this function exists for completeness so the `Map` and `Keyword` modules provide similar APIs. In practice, developers often compare maps using `==/2` or `===/2` directly.

## Examples

    iex> Map.equal?(%{a: 1, b: 2}, %{b: 2, a: 1})
    true
    iex> Map.equal?(%{a: 1, b: 2}, %{b: 1, a: 2})
    false

Comparison between keys and values is done with `===/3`, which means integers are not equivalent to floats:

    iex> Map.equal?(%{a: 1.0}, %{a: 1})
    false

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# fetch(map, key)

View Source

    @spec fetch(map(), key()) :: {:ok, value()} | :error

Fetches the value for a specific `key` in the given `map`.

If `map` contains the given `key` then its value is returned in the shape of `{:ok, value}`. If `map` doesn't contain `key`, `:error` is returned.

Inlined by the compiler.

## Examples

    iex> Map.fetch(%{a: 1}, :a)
    {:ok, 1}
    iex> Map.fetch(%{a: 1}, :b)
    :error

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# fetch!(map, key)

View Source

    @spec fetch!(map(), key()) :: value()

Fetches the value for a specific `key` in the given `map`, erroring out if `map` doesn't contain `key`.

If `map` contains `key`, the corresponding value is returned. If `map` doesn't contain `key`, a `KeyError` exception is raised.

Inlined by the compiler.

## Examples

    iex> Map.fetch!(%{a: 1}, :a)
    1

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# filter(map, fun)

View Source (since 1.13.0)

    @spec filter(map(), ({key(), value()} -> as_boolean(term()))) :: map()

Returns a map containing only those pairs from `map` for which `fun` returns a truthy value.

`fun` receives the key and value of each of the elements in the map as a key-value pair.

See also `reject/2` which discards all elements where the function returns a truthy value.

> #### Performance considerations

> If you find yourself doing multiple calls to `Map.filter/2` and `Map.reject/2` in a pipeline, it is likely more efficient to use `Enum.map/2` and `Enum.filter/2` instead and convert to a map at the end using `Map.new/1`.

## Examples

    iex> Map.filter(%{one: 1, two: 2, three: 3}, fn {_key, val} -> rem(val, 2) == 1 end)
    %{one: 1, three: 3}

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# from_keys(keys, value)

View Source (since 1.14.0)

    @spec from_keys([key()], value()) :: map()

Builds a map from the given `keys` and the fixed `value`.

## Examples

    iex> Map.from_keys([1, 2, 3], :number)
    %{1 => :number, 2 => :number, 3 => :number}

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# from_struct(struct)

View Source

    @spec from_struct(atom() | struct()) :: map()

Converts a `struct` to map.

It accepts the struct module or a struct itself and simply removes the `__struct__` field from the given struct or from a new struct generated from the given module.

## Example

    defmodule User do
      defstruct [:name]
    end

    Map.from_struct(User)
    #=> %{name: nil}

    Map.from_struct(%User{name: "john"})
    #=> %{name: "john"}

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# get(map, key, default \\ nil)

View Source

    @spec get(map(), key(), value()) :: value()

Gets the value for a specific `key` in `map`.

If `key` is present in `map` then its value `value` is returned. Otherwise, `default` is returned.

If `default` is not provided, `nil` is used.

## Examples

    iex> Map.get(%{}, :a)
    nil
    iex> Map.get(%{a: 1}, :a)
    1
    iex> Map.get(%{a: 1}, :b)
    nil
    iex> Map.get(%{a: 1}, :b, 3)
    3
    iex> Map.get(%{a: nil}, :a, 1)
    nil

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# get_and_update(map, key, fun)

View Source

    @spec get_and_update(
      map(),
      key(),
      (value() | nil -> {current_value, new_value :: value()} | :pop)
    ) ::
      {current_value, new_map :: map()}
    when current_value: value()

Gets the value from `key` and updates it, all in one pass.

`fun` is called with the current value under `key` in `map` (or `nil` if `key` is not present in `map`) and must return a two-element tuple: the current value (the retrieved value, which can be operated on before being returned) and the new value to be stored under `key` in the resulting new map. `fun` may also return `:pop`, which means the current value shall be removed from `map` and returned (making this function behave like `Map.pop(map, key)`).

The returned value is a two-element tuple with the current value returned by `fun` and a new map with the updated value under `key`.

## Examples

    iex> Map.get_and_update(%{a: 1}, :a, fn current_value ->
    ...>   {current_value, "new value!"}
    ...> end)
    {1, %{a: "new value!"}}

    iex> Map.get_and_update(%{a: 1}, :b, fn current_value ->
    ...>   {current_value, "new value!"}
    ...> end)
    {nil, %{a: 1, b: "new value!"}}

    iex> Map.get_and_update(%{a: 1}, :a, fn _ -> :pop end)
    {1, %{}}

    iex> Map.get_and_update(%{a: 1}, :b, fn _ -> :pop end)
    {nil, %{a: 1}}

Link to this function

# get_and_update!(map, key, fun)

View Source

    @spec get_and_update!(
      map(),
      key(),
      (value() -> {current_value, new_value :: value()} | :pop)
    ) ::
      {current_value, map()}
    when current_value: value()

Gets the value from `key` and updates it, all in one pass. Raises if there is no `key`.

Behaves exactly like `get_and_update/3`, but raises a `KeyError` exception if `key` is not present in `map`.

## Examples

    iex> Map.get_and_update!(%{a: 1}, :a, fn current_value ->
    ...>   {current_value, "new value!"}
    ...> end)
    {1, %{a: "new value!"}}

    iex> Map.get_and_update!(%{a: 1}, :b, fn current_value ->
    ...>   {current_value, "new value!"}
    ...> end)
    ** (KeyError) key :b not found in: %{a: 1}

    iex> Map.get_and_update!(%{a: 1}, :a, fn _ ->
    ...>   :pop
    ...> end)
    {1, %{}}

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# get_lazy(map, key, fun)

View Source

    @spec get_lazy(map(), key(), (-> value())) :: value()

Gets the value for a specific `key` in `map`.

If `key` is present in `map` then its value `value` is returned. Otherwise, `fun` is evaluated and its result is returned.

This is useful if the default value is very expensive to calculate or generally difficult to setup and teardown again.

## Examples

    iex> map = %{a: 1}
    iex> fun = fn ->
    ...>   # some expensive operation here
    ...>   13
    ...> end
    iex> Map.get_lazy(map, :a, fun)
    1
    iex> Map.get_lazy(map, :b, fun)
    13

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# has_key?(map, key)

View Source

    @spec has_key?(map(), key()) :: boolean()

Returns whether the given `key` exists in the given `map`.

Inlined by the compiler.

## Examples

    iex> Map.has_key?(%{a: 1}, :a)
    true
    iex> Map.has_key?(%{a: 1}, :b)
    false

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# intersect(map1, map2)

View Source (since 1.15.0)

    @spec intersect(map(), map()) :: map()

Intersects two maps, returning a map with the common keys.

The values in the returned map are the values of the intersected keys in `map2`.

Inlined by the compiler.

## Examples

    iex> Map.intersect(%{a: 1, b: 2}, %{b: "b", c: "c"})
    %{b: "b"}

Link to this function

# intersect(map1, map2, fun)

View Source (since 1.15.0)

    @spec intersect(map(), map(), (key(), value(), value() -> value())) :: map()

Intersects two maps, returning a map with the common keys and resolving conflicts through a function.

The given function will be invoked when there are duplicate keys; its arguments are `key` (the duplicate key), `value1` (the value of `key` in `map1`), and `value2` (the value of `key` in `map2`). The value returned by `fun` is used as the value under `key` in the resulting map.

## Examples

    iex> Map.intersect(%{a: 1, b: 2}, %{b: 2, c: 3}, fn _k, v1, v2 ->
    ...>   v1 + v2
    ...> end)
    %{b: 4}

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# keys(map)

View Source

    @spec keys(map()) :: [key()]

Returns all keys from `map`.

Inlined by the compiler.

## Examples

    iex> Map.keys(%{a: 1, b: 2})
    [:a, :b]

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# merge(map1, map2)

View Source

    @spec merge(map(), map()) :: map()

Merges two maps into one.

All keys in `map2` will be added to `map1`, overriding any existing one (i.e., the keys in `map2` "have precedence" over the ones in `map1`).

If you have a struct and you would like to merge a set of keys into the struct, do not use this function, as it would merge all keys on the right side into the struct, even if the key is not part of the struct. Instead, use `struct/2`.

Inlined by the compiler.

## Examples

    iex> Map.merge(%{a: 1, b: 2}, %{a: 3, d: 4})
    %{a: 3, b: 2, d: 4}

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# merge(map1, map2, fun)

View Source

    @spec merge(map(), map(), (key(), value(), value() -> value())) :: map()

Merges two maps into one, resolving conflicts through the given `fun`.

All keys in `map2` will be added to `map1`. The given function will be invoked when there are duplicate keys; its arguments are `key` (the duplicate key), `value1` (the value of `key` in `map1`), and `value2` (the value of `key` in `map2`). The value returned by `fun` is used as the value under `key` in the resulting map.

## Examples

    iex> Map.merge(%{a: 1, b: 2}, %{a: 3, d: 4}, fn _k, v1, v2 ->
    ...>   v1 + v2
    ...> end)
    %{a: 4, b: 2, d: 4}

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# new()

View Source

    @spec new() :: map()

Returns a new empty map.

## Examples

    iex> Map.new()
    %{}

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# new(enumerable)

View Source

    @spec new(Enumerable.t()) :: map()

Creates a map from an `enumerable`.

Duplicated keys are removed; the latest one prevails.

## Examples

    iex> Map.new([{:b, 1}, {:a, 2}])
    %{a: 2, b: 1}
    iex> Map.new(a: 1, a: 2, a: 3)
    %{a: 3}

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# new(enumerable, transform)

View Source

    @spec new(Enumerable.t(), (term() -> {key(), value()})) :: map()

Creates a map from an `enumerable` via the given transformation function.

Duplicated keys are removed; the latest one prevails.

## Examples

    iex> Map.new([:a, :b], fn x -> {x, x} end)
    %{a: :a, b: :b}

    iex> Map.new(%{a: 2, b: 3, c: 4}, fn {key, val} -> {key, val * 2} end)
    %{a: 4, b: 6, c: 8}

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# pop(map, key, default \\ nil)

View Source

    @spec pop(map(), key(), default) :: {value(), updated_map :: map()} | {default, map()}
    when default: value()

Removes the value associated with `key` in `map` and returns the value and the updated map.

If `key` is present in `map`, it returns `{value, updated_map}` where `value` is the value of the key and `updated_map` is the result of removing `key` from `map`. If `key` is not present in `map`, `{default, map}` is returned.

## Examples

    iex> Map.pop(%{a: 1}, :a)
    {1, %{}}
    iex> Map.pop(%{a: 1}, :b)
    {nil, %{a: 1}}
    iex> Map.pop(%{a: 1}, :b, 3)
    {3, %{a: 1}}

Link to this function

# pop!(map, key)

View Source (since 1.10.0)

    @spec pop!(map(), key()) :: {value(), updated_map :: map()}

Removes the value associated with `key` in `map` and returns the value and the updated map, or it raises if `key` is not present.

Behaves the same as `pop/3` but raises if `key` is not present in `map`.

## Examples

    iex> Map.pop!(%{a: 1}, :a)
    {1, %{}}
    iex> Map.pop!(%{a: 1, b: 2}, :a)
    {1, %{b: 2}}
    iex> Map.pop!(%{a: 1}, :b)
    ** (KeyError) key :b not found in: %{a: 1}

Link to this function

# pop_lazy(map, key, fun)

View Source

    @spec pop_lazy(map(), key(), (-> value())) :: {value(), map()}

Lazily returns and removes the value associated with `key` in `map`.

If `key` is present in `map`, it returns `{value, new_map}` where `value` is the value of the key and `new_map` is the result of removing `key` from `map`. If `key` is not present in `map`, `{fun_result, map}` is returned, where `fun_result` is the result of applying `fun`.

This is useful if the default value is very expensive to calculate or generally difficult to setup and teardown again.

## Examples

    iex> map = %{a: 1}
    iex> fun = fn ->
    ...>   # some expensive operation here
    ...>   13
    ...> end
    iex> Map.pop_lazy(map, :a, fun)
    {1, %{}}
    iex> Map.pop_lazy(map, :b, fun)
    {13, %{a: 1}}

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# put(map, key, value)

View Source

    @spec put(map(), key(), value()) :: map()

Puts the given `value` under `key` in `map`.

Inlined by the compiler.

## Examples

    iex> Map.put(%{a: 1}, :b, 2)
    %{a: 1, b: 2}
    iex> Map.put(%{a: 1, b: 2}, :a, 3)
    %{a: 3, b: 2}

Link to this function

# put_new(map, key, value)

View Source

    @spec put_new(map(), key(), value()) :: map()

Puts the given `value` under `key` unless the entry `key` already exists in `map`.

## Examples

    iex> Map.put_new(%{a: 1}, :b, 2)
    %{a: 1, b: 2}
    iex> Map.put_new(%{a: 1, b: 2}, :a, 3)
    %{a: 1, b: 2}

Link to this function

# put_new_lazy(map, key, fun)

View Source

    @spec put_new_lazy(map(), key(), (-> value())) :: map()

Evaluates `fun` and puts the result under `key` in `map` unless `key` is already present.

This function is useful in case you want to compute the value to put under `key` only if `key` is not already present, as for example, when the value is expensive to calculate or generally difficult to setup and teardown again.

## Examples

    iex> map = %{a: 1}
    iex> fun = fn ->
    ...>   # some expensive operation here
    ...>   3
    ...> end
    iex> Map.put_new_lazy(map, :a, fun)
    %{a: 1}
    iex> Map.put_new_lazy(map, :b, fun)
    %{a: 1, b: 3}

Link to this function

# reject(map, fun)

View Source (since 1.13.0)

    @spec reject(map(), ({key(), value()} -> as_boolean(term()))) :: map()

Returns map excluding the pairs from `map` for which `fun` returns a truthy value.

See also `filter/2`.

## Examples

    iex> Map.reject(%{one: 1, two: 2, three: 3}, fn {_key, val} -> rem(val, 2) == 1 end)
    %{two: 2}

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# replace(map, key, value)

View Source (since 1.11.0)

    @spec replace(map(), key(), value()) :: map()

Puts a value under `key` only if the `key` already exists in `map`.

## Examples

    iex> Map.replace(%{a: 1, b: 2}, :a, 3)
    %{a: 3, b: 2}

    iex> Map.replace(%{a: 1}, :b, 2)
    %{a: 1}

Link to this function

# replace!(map, key, value)

View Source (since 1.5.0)

    @spec replace!(map(), key(), value()) :: map()

Puts a value under `key` only if the `key` already exists in `map`.

If `key` is not present in `map`, a `KeyError` exception is raised.

Inlined by the compiler.

## Examples

    iex> Map.replace!(%{a: 1, b: 2}, :a, 3)
    %{a: 3, b: 2}

    iex> Map.replace!(%{a: 1}, :b, 2)
    ** (KeyError) key :b not found in: %{a: 1}

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# replace_lazy(map, key, fun)

View Source (since 1.14.0)

    @spec replace_lazy(map(), key(), (existing_value :: value() -> new_value :: value())) ::
      map()

Replaces the value under `key` using the given function only if `key` already exists in `map`.

In comparison to `replace/3`, this can be useful when it's expensive to calculate the value.

If `key` does not exist, the original map is returned unchanged.

## Examples

    iex> Map.replace_lazy(%{a: 1, b: 2}, :a, fn v -> v * 4 end)
    %{a: 4, b: 2}

    iex> Map.replace_lazy(%{a: 1, b: 2}, :c, fn v -> v * 4 end)
    %{a: 1, b: 2}

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# split(map, keys)

View Source

    @spec split(map(), [key()]) :: {map(), map()}

Takes all entries corresponding to the given `keys` in `map` and extracts them into a separate map.

Returns a tuple with the new map and the old map with removed keys.

Keys for which there are no entries in `map` are ignored.

## Examples

    iex> Map.split(%{a: 1, b: 2, c: 3}, [:a, :c, :e])
    {%{a: 1, c: 3}, %{b: 2}}

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# split_with(map, fun)

View Source (since 1.15.0)

    @spec split_with(map(), ({key(), value()} -> as_boolean(term()))) :: {map(), map()}

Splits the `map` into two maps according to the given function `fun`.

`fun` receives each `{key, value}` pair in the `map` as its only argument. Returns a tuple with the first map containing all the elements in `map` for which applying `fun` returned a truthy value, and a second map with all the elements for which applying `fun` returned a falsy value (`false` or `nil`).

## Examples

    iex> Map.split_with(%{a: 1, b: 2, c: 3, d: 4}, fn {_k, v} -> rem(v, 2) == 0 end)
    {%{b: 2, d: 4}, %{a: 1, c: 3}}

    iex> Map.split_with(%{a: 1, b: -2, c: 1, d: -3}, fn {k, _v} -> k in [:b, :d] end)
    {%{b: -2, d: -3}, %{a: 1, c: 1}}

    iex> Map.split_with(%{a: 1, b: -2, c: 1, d: -3}, fn {_k, v} -> v > 50 end)
    {%{}, %{a: 1, b: -2, c: 1, d: -3}}

    iex> Map.split_with(%{}, fn {_k, v} -> v > 50 end)
    {%{}, %{}}

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# take(map, keys)

View Source

    @spec take(map(), [key()]) :: map()

Returns a new map with all the key-value pairs in `map` where the key is in `keys`.

If `keys` contains keys that are not in `map`, they're simply ignored.

## Examples

    iex> Map.take(%{a: 1, b: 2, c: 3}, [:a, :c, :e])
    %{a: 1, c: 3}

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# to_list(map)

View Source

    @spec to_list(map()) :: [{term(), term()}]

Converts `map` to a list.

Each key-value pair in the map is converted to a two-element tuple `{key, value}` in the resulting list.

Inlined by the compiler.

## Examples

    iex> Map.to_list(%{a: 1})
    [a: 1]
    iex> Map.to_list(%{1 => 2})
    [{1, 2}]

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# update(map, key, default, fun)

View Source

    @spec update(
      map(),
      key(),
      default :: value(),
      (existing_value :: value() -> new_value :: value())
    ) ::
      map()

Updates the `key` in `map` with the given function.

If `key` is present in `map` then the existing value is passed to `fun` and its result is used as the updated value of `key`. If `key` is not present in `map`, `default` is inserted as the value of `key`. The default value will not be passed through the update function.

## Examples

    iex> Map.update(%{a: 1}, :a, 13, fn existing_value -> existing_value * 2 end)
    %{a: 2}
    iex> Map.update(%{a: 1}, :b, 11, fn existing_value -> existing_value * 2 end)
    %{a: 1, b: 11}

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# update!(map, key, fun)

View Source

    @spec update!(map(), key(), (existing_value :: value() -> new_value :: value())) ::
      map()

Updates `key` with the given function.

If `key` is present in `map` then the existing value is passed to `fun` and its result is used as the updated value of `key`. If `key` is not present in `map`, a `KeyError` exception is raised.

## Examples

    iex> Map.update!(%{a: 1}, :a, &(&1 * 2))
    %{a: 2}

    iex> Map.update!(%{a: 1}, :b, &(&1 * 2))
    ** (KeyError) key :b not found in: %{a: 1}

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# values(map)

View Source

    @spec values(map()) :: [value()]

Returns all values from `map`.

Inlined by the compiler.

## Examples

    iex> Map.values(%{a: 1, b: 2})
    [1, 2]