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defmodule Access do
@moduledoc """
Key-based access to data structures.
Elixir supports three main key-value constructs: keywords,
maps, and structs. It also supports two mechanisms to access those keys:
by brackets (via `data[key]`) and by dot-syntax (via `data.field`).
In the next section we will briefly recap the key-value constructs and then
discuss the access mechanisms.
## Key-value constructs
Elixir provides three main key-value constructs, summarized below:
* keyword lists - they are lists of two-element tuples where
the first element is an atom. Commonly written in the
`[key: value]` syntax, they support only atom keys. Keyword
lists are used almost exclusively to pass options to functions
and macros. They keep the user ordering and allow duplicate
keys. See the `Keyword` module.
* maps - they are the "go to" key-value data structure in Elixir.
They are capable of supporting billions of keys of any type. They are
written using the `%{key => value}` syntax and also support the
`%{key: value}` syntax when the keys are atoms. They do not
have any specified ordering and do not allow duplicate keys.
See the `Map` module.
* structs - they are named maps with a pre-determined set of keys.
They are defined with `defstruct/1` and written using the
`%StructName{key: value}` syntax.
## Key-based accessors
Elixir provides two mechanisms to access data structures by key,
described next.
### Bracket-based access
The `data[key]` syntax is used to access data structures with a
dynamic number of keys, such as keywords and maps. The key can
be of any type. The bracket-based access syntax returns `nil`
if the key does not exist:
iex> keywords = [a: 1, b: 2]
iex> keywords[:a]
1
iex> keywords[:c]
nil
iex> map = %{a: 1, b: 2}
iex> map[:a]
1
iex> star_ratings = %{1.0 => "★", 1.5 => "★☆", 2.0 => "★★"}
iex> star_ratings[1.5]
"★☆"
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, the bracket-based access syntax transparently ignores
`nil` values. When trying to access anything on a `nil` value, `nil`
is returned:
iex> keywords = [a: 1, b: 2]
iex> keywords[:c][:unknown]
nil
iex> nil[:a]
nil
Internally, `data[key]` translates to `Access.get(term, key, nil)`.
Developers interested in implementing their own key-value data
structures can implement the `Access` behaviour to provide the
bracket-based access syntax. `Access` requires the key comparison
to be implemented using the `===/2` operator.
### Dot-based syntax
The `data.field` syntax is used exclusively to access atom fields
in maps and structs. If the accessed field does not exist, an error is
raised. This is a deliberate decision: since all of the
fields in a struct are pre-determined, structs support only the
dot-based syntax and not the access one.
Imagine a struct named `User` with a `:name` field. The following would raise:
user = %User{name: "John"}
user[:name]
# ** (UndefinedFunctionError) undefined function User.fetch/2 (User does not implement the Access behaviour)
Instead we should use the `user.name` syntax to access fields:
user.name
#=> "John"
Differently from `user[:name]`, `user.name` is not extensible via
a behaviour and is restricted only to structs and atom keys in maps.
### Summing up
The bracket-based syntax, `user[:name]`, is used by dynamic structures,
is extensible and returns nil on missing keys.
The dot-based syntax, `user.name`, is used exclusively to access atom
keys in maps and structs, and it raises on missing keys.
## Nested data structures
Both key-based access syntaxes can be used with the nested update
functions and macros in `Kernel`, such as `Kernel.get_in/2`, `Kernel.put_in/3`,
`Kernel.update_in/3`, `Kernel.pop_in/2`, and `Kernel.get_and_update_in/3`.
For example, to update a map inside another map:
iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
iex> put_in(users["john"].age, 28)
%{"john" => %{age: 28}, "meg" => %{age: 23}}
This module provides convenience functions for traversing other
structures, like tuples and lists. These functions can be used
in all the `Access`-related functions and macros in `Kernel`.
For instance, given a user map with the `:name` and `:languages` keys,
here is how to deeply traverse the map and convert all language names
to uppercase:
iex> languages = [
...> %{name: "elixir", type: :functional},
...> %{name: "c", type: :procedural}
...> ]
iex> user = %{name: "john", languages: languages}
iex> update_in(user, [:languages, Access.all(), :name], &String.upcase/1)
%{
name: "john",
languages: [
%{name: "ELIXIR", type: :functional},
%{name: "C", type: :procedural}
]
}
See the functions `key/1`, `key!/1`, `elem/1`, and `all/0` for some of the
available accessors.
"""
@type container :: keyword | struct | map
@type nil_container :: nil
@type any_container :: any
@type t :: container | nil_container | any_container
@type key :: any
@type value :: any
@type get_fun(data, get_value) ::
(:get, data, (term -> term) ->
{get_value, new_data :: container})
@type get_and_update_fun(data, get_value) ::
(:get_and_update, data, (term -> term) ->
{get_value, new_data :: container} | :pop)
@type access_fun(data, get_value) ::
get_fun(data, get_value) | get_and_update_fun(data, get_value)
@doc """
Invoked in order to access the value stored under `key` in the given term `term`.
This function should return `{:ok, value}` where `value` is the value under
`key` if the key exists in the term, or `:error` if the key does not exist in
the term.
Many of the functions defined in the `Access` module internally call this
function. This function is also used when the square-brackets access syntax
(`structure[key]`) is used: the `fetch/2` callback implemented by the module
that defines the `structure` struct is invoked and if it returns `{:ok,
value}` then `value` is returned, or if it returns `:error` then `nil` is
returned.
See the `Map.fetch/2` and `Keyword.fetch/2` implementations for examples of
how to implement this callback.
"""
@callback fetch(term :: t, key) :: {:ok, value} | :error
@doc """
Invoked in order to access the value under `key` and update it at the same time.
The implementation of this callback should invoke `fun` with the value under
`key` in the passed structure `data`, or with `nil` if `key` is not present in it.
This function must return either `{get_value, update_value}` or `:pop`.
If the passed function returns `{get_value, update_value}`,
the return value of this callback should be `{get_value, new_data}`, where:
* `get_value` is the retrieved value (which can be operated on before being returned)
* `update_value` is the new value to be stored under `key`
* `new_data` is `data` after updating the value of `key` with `update_value`.
If the passed function returns `:pop`, the return value of this callback
must be `{value, new_data}` where `value` is the value under `key`
(or `nil` if not present) and `new_data` is `data` without `key`.
See the implementations of `Map.get_and_update/3` or `Keyword.get_and_update/3`
for more examples.
"""
@callback get_and_update(data, key, (value -> {get_value, value} | :pop)) :: {get_value, data}
when get_value: var, data: container | any_container
@doc """
Invoked to "pop" the value under `key` out of the given data structure.
When `key` exists in the given structure `data`, the implementation should
return a `{value, new_data}` tuple where `value` is the value that was under
`key` and `new_data` is `term` without `key`.
When `key` is not present in the given structure, a tuple `{value, data}`
should be returned, where `value` is implementation-defined.
See the implementations for `Map.pop/3` or `Keyword.pop/3` for more examples.
"""
@callback pop(data, key) :: {value, data} when data: container | any_container
defmacrop raise_undefined_behaviour(exception, module, top) do
quote do
exception =
case __STACKTRACE__ do
[unquote(top) | _] ->
reason = "#{inspect(unquote(module))} does not implement the Access behaviour"
%{unquote(exception) | reason: reason}
_ ->
unquote(exception)
end
reraise exception, __STACKTRACE__
end
end
@doc """
Fetches the value for the given key in a container (a map, keyword
list, or struct that implements the `Access` behaviour).
Returns `{:ok, value}` where `value` is the value under `key` if there is such
a key, or `:error` if `key` is not found.
## Examples
iex> Access.fetch(%{name: "meg", age: 26}, :name)
{:ok, "meg"}
iex> Access.fetch([ordered: true, on_timeout: :exit], :timeout)
:error
"""
@spec fetch(container, term) :: {:ok, term} | :error
@spec fetch(nil_container, any) :: :error
def fetch(container, key)
def fetch(%module{} = container, key) do
module.fetch(container, key)
rescue
exception in UndefinedFunctionError ->
raise_undefined_behaviour(exception, module, {^module, :fetch, [^container, ^key], _})
end
def fetch(map, key) when is_map(map) do
case map do
%{^key => value} -> {:ok, value}
_ -> :error
end
end
def fetch(list, key) when is_list(list) and is_atom(key) do
case :lists.keyfind(key, 1, list) do
{_, value} -> {:ok, value}
false -> :error
end
end
def fetch(list, key) when is_list(list) do
raise ArgumentError,
"the Access calls for keywords expect the key to be an atom, got: " <> inspect(key)
end
def fetch(nil, _key) do
:error
end
@doc """
Gets the value for the given key in a container (a map, keyword
list, or struct that implements the `Access` behaviour).
Returns the value under `key` if there is such a key, or `default` if `key` is
not found.
## Examples
iex> Access.get(%{name: "john"}, :name, "default name")
"john"
iex> Access.get(%{name: "john"}, :age, 25)
25
iex> Access.get([ordered: true], :timeout)
nil
"""
@spec get(container, term, term) :: term
@spec get(nil_container, any, default) :: default when default: var
def get(container, key, default \\ nil)
# Reimplementing the same logic as Access.fetch/2 here is done for performance, since
# this is called a lot and calling fetch/2 means introducing some overhead (like
# building the "{:ok, _}" tuple and deconstructing it back right away).
def get(%module{} = container, key, default) do
try do
module.fetch(container, key)
rescue
exception in UndefinedFunctionError ->
raise_undefined_behaviour(exception, module, {^module, :fetch, [^container, ^key], _})
else
{:ok, value} -> value
:error -> default
end
end
def get(map, key, default) when is_map(map) do
case map do
%{^key => value} -> value
_ -> default
end
end
def get(list, key, default) when is_list(list) and is_atom(key) do
case :lists.keyfind(key, 1, list) do
{_, value} -> value
false -> default
end
end
def get(list, key, _default) when is_list(list) do
raise ArgumentError,
"the Access calls for keywords expect the key to be an atom, got: " <> inspect(key)
end
def get(nil, _key, default) do
default
end
@doc """
Gets and updates the given key in a `container` (a map, a keyword list,
a struct that implements the `Access` behaviour).
The `fun` argument receives the value of `key` (or `nil` if `key` is not
present in `container`) and must return a two-element tuple `{get_value, update_value}`:
the "get" value `get_value` (the retrieved value, which can be operated on before
being returned) and the new value to be stored under `key` (`update_value`).
`fun` may also return `:pop`, which means the current value
should be removed from the container and returned.
The returned value is a two-element tuple with the "get" value returned by
`fun` and a new container with the updated value under `key`.
"""
@spec get_and_update(data, key, (value -> {get_value, value} | :pop)) :: {get_value, data}
when get_value: var, data: container
def get_and_update(container, key, fun)
def get_and_update(%module{} = container, key, fun) do
module.get_and_update(container, key, fun)
rescue
exception in UndefinedFunctionError ->
raise_undefined_behaviour(
exception,
module,
{^module, :get_and_update, [^container, ^key, ^fun], _}
)
end
def get_and_update(map, key, fun) when is_map(map) do
Map.get_and_update(map, key, fun)
end
def get_and_update(list, key, fun) when is_list(list) do
Keyword.get_and_update(list, key, fun)
end
def get_and_update(nil, key, _fun) do
raise ArgumentError, "could not put/update key #{inspect(key)} on a nil value"
end
@doc """
Removes the entry with a given key from a container (a map, keyword
list, or struct that implements the `Access` behaviour).
Returns a tuple containing the value associated with the key and the
updated container. `nil` is returned for the value if the key isn't
in the container.
## Examples
With a map:
iex> Access.pop(%{name: "Elixir", creator: "Valim"}, :name)
{"Elixir", %{creator: "Valim"}}
A keyword list:
iex> Access.pop([name: "Elixir", creator: "Valim"], :name)
{"Elixir", [creator: "Valim"]}
An unknown key:
iex> Access.pop(%{name: "Elixir", creator: "Valim"}, :year)
{nil, %{creator: "Valim", name: "Elixir"}}
"""
@spec pop(data, key) :: {value, data} when data: container
def pop(%module{} = container, key) do
module.pop(container, key)
rescue
exception in UndefinedFunctionError ->
raise_undefined_behaviour(exception, module, {^module, :pop, [^container, ^key], _})
end
def pop(map, key) when is_map(map) do
Map.pop(map, key)
end
def pop(list, key) when is_list(list) do
Keyword.pop(list, key)
end
def pop(nil, key) do
raise ArgumentError, "could not pop key #{inspect(key)} on a nil value"
end
## Accessors
@doc """
Returns a function that accesses the given key in a map/struct.
The returned function is typically passed as an accessor to `Kernel.get_in/2`,
`Kernel.get_and_update_in/3`, and friends.
The returned function uses the default value if the key does not exist.
This can be used to specify defaults and safely traverse missing keys:
iex> get_in(%{}, [Access.key(:user, %{name: "meg"}), Access.key(:name)])
"meg"
Such is also useful when using update functions, allowing us to introduce
values as we traverse the data structure for updates:
iex> put_in(%{}, [Access.key(:user, %{}), Access.key(:name)], "Mary")
%{user: %{name: "Mary"}}
## Examples
iex> map = %{user: %{name: "john"}}
iex> get_in(map, [Access.key(:unknown, %{}), Access.key(:name, "john")])
"john"
iex> get_and_update_in(map, [Access.key(:user), Access.key(:name)], fn prev ->
...> {prev, String.upcase(prev)}
...> end)
{"john", %{user: %{name: "JOHN"}}}
iex> pop_in(map, [Access.key(:user), Access.key(:name)])
{"john", %{user: %{}}}
An error is raised if the accessed structure is not a map or a struct:
iex> get_in(nil, [Access.key(:foo)])
** (BadMapError) expected a map, got: nil
iex> get_in([], [Access.key(:foo)])
** (BadMapError) expected a map, got: []
"""
@spec key(key, term) :: access_fun(data :: struct | map, get_value :: term)
def key(key, default \\ nil) do
fn
:get, data, next ->
next.(Map.get(data, key, default))
:get_and_update, data, next ->
value = Map.get(data, key, default)
case next.(value) do
{get, update} -> {get, Map.put(data, key, update)}
:pop -> {value, Map.delete(data, key)}
end
end
end
@doc """
Returns a function that accesses the given key in a map/struct.
The returned function is typically passed as an accessor to `Kernel.get_in/2`,
`Kernel.get_and_update_in/3`, and friends.
Similar to `key/2`, but the returned function raises if the key does not exist.
## Examples
iex> map = %{user: %{name: "john"}}
iex> get_in(map, [Access.key!(:user), Access.key!(:name)])
"john"
iex> get_and_update_in(map, [Access.key!(:user), Access.key!(:name)], fn prev ->
...> {prev, String.upcase(prev)}
...> end)
{"john", %{user: %{name: "JOHN"}}}
iex> pop_in(map, [Access.key!(:user), Access.key!(:name)])
{"john", %{user: %{}}}
iex> get_in(map, [Access.key!(:user), Access.key!(:unknown)])
** (KeyError) key :unknown not found in: %{name: \"john\"}
An error is raised if the accessed structure is not a map/struct:
iex> get_in([], [Access.key!(:foo)])
** (RuntimeError) Access.key!/1 expected a map/struct, got: []
"""
@spec key!(key) :: access_fun(data :: struct | map, get_value :: term)
def key!(key) do
fn
:get, %{} = data, next ->
next.(Map.fetch!(data, key))
:get_and_update, %{} = data, next ->
value = Map.fetch!(data, key)
case next.(value) do
{get, update} -> {get, Map.put(data, key, update)}
:pop -> {value, Map.delete(data, key)}
end
_op, data, _next ->
raise "Access.key!/1 expected a map/struct, got: #{inspect(data)}"
end
end
@doc ~S"""
Returns a function that accesses the element at the given index in a tuple.
The returned function is typically passed as an accessor to `Kernel.get_in/2`,
`Kernel.get_and_update_in/3`, and friends.
The returned function raises if `index` is out of bounds.
Note that popping elements out of tuples is not possible and raises an
error.
## Examples
iex> map = %{user: {"john", 27}}
iex> get_in(map, [:user, Access.elem(0)])
"john"
iex> get_and_update_in(map, [:user, Access.elem(0)], fn prev ->
...> {prev, String.upcase(prev)}
...> end)
{"john", %{user: {"JOHN", 27}}}
iex> pop_in(map, [:user, Access.elem(0)])
** (RuntimeError) cannot pop data from a tuple
An error is raised if the accessed structure is not a tuple:
iex> get_in(%{}, [Access.elem(0)])
** (RuntimeError) Access.elem/1 expected a tuple, got: %{}
"""
@spec elem(non_neg_integer) :: access_fun(data :: tuple, get_value :: term)
def elem(index) when is_integer(index) and index >= 0 do
pos = index + 1
fn
:get, data, next when is_tuple(data) ->
next.(:erlang.element(pos, data))
:get_and_update, data, next when is_tuple(data) ->
value = :erlang.element(pos, data)
case next.(value) do
{get, update} -> {get, :erlang.setelement(pos, data, update)}
:pop -> raise "cannot pop data from a tuple"
end
_op, data, _next ->
raise "Access.elem/1 expected a tuple, got: #{inspect(data)}"
end
end
@doc ~S"""
Returns a function that accesses all the elements in a list.
The returned function is typically passed as an accessor to `Kernel.get_in/2`,
`Kernel.get_and_update_in/3`, and friends.
## Examples
iex> list = [%{name: "john"}, %{name: "mary"}]
iex> get_in(list, [Access.all(), :name])
["john", "mary"]
iex> get_and_update_in(list, [Access.all(), :name], fn prev ->
...> {prev, String.upcase(prev)}
...> end)
{["john", "mary"], [%{name: "JOHN"}, %{name: "MARY"}]}
iex> pop_in(list, [Access.all(), :name])
{["john", "mary"], [%{}, %{}]}
Here is an example that traverses the list dropping even
numbers and multiplying odd numbers by 2:
iex> require Integer
iex> get_and_update_in([1, 2, 3, 4, 5], [Access.all()], fn num ->
...> if Integer.is_even(num), do: :pop, else: {num, num * 2}
...> end)
{[1, 2, 3, 4, 5], [2, 6, 10]}
An error is raised if the accessed structure is not a list:
iex> get_in(%{}, [Access.all()])
** (RuntimeError) Access.all/0 expected a list, got: %{}
"""
@spec all() :: access_fun(data :: list, get_value :: list)
def all() do
&all/3
end
defp all(:get, data, next) when is_list(data) do
Enum.map(data, next)
end
defp all(:get_and_update, data, next) when is_list(data) do
all(data, next, _gets = [], _updates = [])
end
defp all(_op, data, _next) do
raise "Access.all/0 expected a list, got: #{inspect(data)}"
end
defp all([head | rest], next, gets, updates) do
case next.(head) do
{get, update} -> all(rest, next, [get | gets], [update | updates])
:pop -> all(rest, next, [head | gets], updates)
end
end
defp all([], _next, gets, updates) do
{:lists.reverse(gets), :lists.reverse(updates)}
end
@doc ~S"""
Returns a function that accesses the element at `index` (zero based) of a list.
The returned function is typically passed as an accessor to `Kernel.get_in/2`,
`Kernel.get_and_update_in/3`, and friends.
## Examples
iex> list = [%{name: "john"}, %{name: "mary"}]
iex> get_in(list, [Access.at(1), :name])
"mary"
iex> get_and_update_in(list, [Access.at(0), :name], fn prev ->
...> {prev, String.upcase(prev)}
...> end)
{"john", [%{name: "JOHN"}, %{name: "mary"}]}
`at/1` can also be used to pop elements out of a list or
a key inside of a list:
iex> list = [%{name: "john"}, %{name: "mary"}]
iex> pop_in(list, [Access.at(0)])
{%{name: "john"}, [%{name: "mary"}]}
iex> pop_in(list, [Access.at(0), :name])
{"john", [%{}, %{name: "mary"}]}
When the index is out of bounds, `nil` is returned and the update function is never called:
iex> list = [%{name: "john"}, %{name: "mary"}]
iex> get_in(list, [Access.at(10), :name])
nil
iex> get_and_update_in(list, [Access.at(10), :name], fn prev ->
...> {prev, String.upcase(prev)}
...> end)
{nil, [%{name: "john"}, %{name: "mary"}]}
An error is raised for negative indexes:
iex> get_in([], [Access.at(-1)])
** (FunctionClauseError) no function clause matching in Access.at/1
An error is raised if the accessed structure is not a list:
iex> get_in(%{}, [Access.at(1)])
** (RuntimeError) Access.at/1 expected a list, got: %{}
"""
@spec at(non_neg_integer) :: access_fun(data :: list, get_value :: term)
def at(index) when is_integer(index) and index >= 0 do
fn op, data, next -> at(op, data, index, next) end
end
defp at(:get, data, index, next) when is_list(data) do
data |> Enum.at(index) |> next.()
end
defp at(:get_and_update, data, index, next) when is_list(data) do
get_and_update_at(data, index, next, [])
end
defp at(_op, data, _index, _next) do
raise "Access.at/1 expected a list, got: #{inspect(data)}"
end
defp get_and_update_at([head | rest], 0, next, updates) do
case next.(head) do
{get, update} -> {get, :lists.reverse([update | updates], rest)}
:pop -> {head, :lists.reverse(updates, rest)}
end
end
defp get_and_update_at([head | rest], index, next, updates) do
get_and_update_at(rest, index - 1, next, [head | updates])
end
defp get_and_update_at([], _index, _next, updates) do
{nil, :lists.reverse(updates)}
end
@doc ~S"""
Returns a function that accesses all elements of a list that match the provided predicate.
The returned function is typically passed as an accessor to `Kernel.get_in/2`,
`Kernel.get_and_update_in/3`, and friends.
## Examples
iex> list = [%{name: "john", salary: 10}, %{name: "francine", salary: 30}]
iex> get_in(list, [Access.filter(&(&1.salary > 20)), :name])
["francine"]
iex> get_and_update_in(list, [Access.filter(&(&1.salary <= 20)), :name], fn prev ->
...> {prev, String.upcase(prev)}
...> end)
{["john"], [%{name: "JOHN", salary: 10}, %{name: "francine", salary: 30}]}
`filter/1` can also be used to pop elements out of a list or
a key inside of a list:
iex> list = [%{name: "john", salary: 10}, %{name: "francine", salary: 30}]
iex> pop_in(list, [Access.filter(&(&1.salary >= 20))])
{[%{name: "francine", salary: 30}], [%{name: "john", salary: 10}]}
iex> pop_in(list, [Access.filter(&(&1.salary >= 20)), :name])
{["francine"], [%{name: "john", salary: 10}, %{salary: 30}]}
When no match is found, an empty list is returned and the update function is never called
iex> list = [%{name: "john", salary: 10}, %{name: "francine", salary: 30}]
iex> get_in(list, [Access.filter(&(&1.salary >= 50)), :name])
[]
iex> get_and_update_in(list, [Access.filter(&(&1.salary >= 50)), :name], fn prev ->
...> {prev, String.upcase(prev)}
...> end)
{[], [%{name: "john", salary: 10}, %{name: "francine", salary: 30}]}
An error is raised if the predicate is not a function or is of the incorrect arity:
iex> get_in([], [Access.filter(5)])
** (FunctionClauseError) no function clause matching in Access.filter/1
An error is raised if the accessed structure is not a list:
iex> get_in(%{}, [Access.filter(fn a -> a == 10 end)])
** (RuntimeError) Access.filter/1 expected a list, got: %{}
"""
@doc since: "1.6.0"
@spec filter((term -> boolean)) :: access_fun(data :: list, get_value :: list)
def filter(func) when is_function(func) do
fn op, data, next -> filter(op, data, func, next) end
end
defp filter(:get, data, func, next) when is_list(data) do
data |> Enum.filter(func) |> Enum.map(next)
end
defp filter(:get_and_update, data, func, next) when is_list(data) do
get_and_update_filter(data, func, next, [], [])
end
defp filter(_op, data, _func, _next) do
raise "Access.filter/1 expected a list, got: #{inspect(data)}"
end
defp get_and_update_filter([head | rest], func, next, updates, gets) do
if func.(head) do
case next.(head) do
{get, update} ->
get_and_update_filter(rest, func, next, [update | updates], [get | gets])
:pop ->
get_and_update_filter(rest, func, next, updates, [head | gets])
end
else
get_and_update_filter(rest, func, next, [head | updates], gets)
end
end
defp get_and_update_filter([], _func, _next, updates, gets) do
{:lists.reverse(gets), :lists.reverse(updates)}
end
end