Shorter syntax for common key-value definition idiom.

It expands lists to keyword lists

iex> name = "Synex"
iex> version = 1
iex> keys([name, version, test: true])
[name: "Synex", version: 1, test: true]

iex> keys([a,b,_]) = [a: 1, b: 2, c: 3]
[a: 1, b: 2, c: 3]
iex> {a, b}
{1, 2}

iex> range = fn keys([from, to, by]) ->
...>   from
...>   |> Stream.iterate(& &1 + by)
...>   |> Enum.take_while(& &1 <= to)
...> end
iex> range.(from: 10, to: 30, by: 5)
[10, 15, 20, 25, 30]

And expand maps

iex> name = "Jack"
iex> age = 28
iex> keys(%{name, age})
%{age: 28, name: "Jack"}

iex> keys(%{x, z}) = %{x: 10, y: 100, z: 1000}
%{x: 10, y: 100, z: 1000}
iex> {x, z}
{10, 1000}

keys also expand structs

iex> keys(%Person{name, age}) = %Person{name: "John", age: 30}
%Person{age: 30, languages: [], name: "John"}
iex> age
30
iex> languages = [:elixir]
iex> keys(%Person{name, languages, age: 29})
%Person{age: 29, languages: [:elixir], name: "John"}

can be used in update syntax

iex> map = %{a: 1, b: 2, c: 3, d: 4}
iex> {a, b} = {10, 20}
iex> keys(%{map | a, b, c: 100, d: 200})
%{a: 10, b: 20, c: 100, d: 200}

you can pin values:

iex> {a, b} = {1, 2}
iex> keys(%{^a, ^b}) = %{a: 1, b: 2, c: 3}
%{a: 1, b: 2, c: 3}
iex> c = 10
iex> keys(%{^a, ^c}) = %{a: 1, b: 2, c: 3}
** (MatchError) no match of right hand side value: %{a: 1, b: 2, c: 3}