You're seeing just the function change_key, go back to PassiveSupport.Map module for more information.
Link to this function

change_key(map, key, fun)


change_key(map(), key(), key() | (key() -> key()) | (key(), map() -> key())) ::

Returns a new map with key replaced by new_key or the return of fun

If key is not found within map, returns the map unaltered.

Useful for when you're shuffling values around inside of a map, or, I dunno, going through your music collection and you discover you accidentally attributed an entire Beatles album to the Monkees.

Although how you did that is beyond me. You monster.


iex> change_key(%{dog: "rusty"}, :dog, :cat)
%{cat: "rusty"}

iex> change_key(%{dog: "rusty"}, :dog, &(:"best_#{&1}"))
%{best_dog: "rusty"}

iex> change_key(%{1 => "foo", 2 => "bar", 5 => "idklol"}, 1, fn _key, map -> Enum.max(Map.keys(map)) + 1 end)
%{2 => "bar", 5 => "idklol", 6 => "foo"}

iex> change_key(%{cake_boss: "you blew it"}, :no_key_here, :oops)
%{cake_boss: "you blew it"}