Shorter syntax for common map key-value definition idiom.

iex> {name, age} = {"Jack", 26}
iex> params(%{name, age, "test" => true})
%{"name" => "Jack", "age" => 26, "test" => true}

iex> person = %{"name" => "Jack", "age" => 26}
iex> name = "John"
iex> params(%{person | name, "age" => 28})
%{"name" => "John", "age" => 28}

iex> params(%{a, b}) = %{"a" => 1, "b" => 2, "c" => 3}
iex> {a, b}
{1, 2}

iex> {a, b} = {10, 20}
iex> params(%{^a, ^b, c}) = %{"a" => 10, "b" => 20, "c" => 30}
iex> c
30
iex> params(%{^a}) = %{"a" => "bad"}
** (MatchError) no match of right hand side value: %{"a" => "bad"}

It expands nested maps:

iex> params(%{a => %{b, c}}) = %{"a" => %{"b" => 2, "c" => 3}}
iex> {b, c}
{2, 3}
iex> a
%{"b" => 2, "c" => 3}