Matching *within* function parameter declaration

The below contains a "matching" expression within the function declaration:

iex > defmodule Hulk do
... > def print({1, a} = tt) do
... > IO.inspect(a)
... > IO.puts("**********")
... > IO.inspect(tt)
... > end
... > end

This allows the function access to the variable that was bound while matching and the full value passed in to the function. It returns:

iex > Hulk.print({1, 3})
3
**********
{1, 3}

This is particularly useful for Maps which is the only datatype to support partial matching (eg %{a: a} = %{a: 1, b: 2}) so that the unknown and unmatched portion of the map will be available.

This is a common idiom in pheonix while declaring actions in controllers:

def show(conn, %{"messenger" => messenger} = params) do

The above is from the pheonix docs