Pattern match function against empty map

It works this way by design, but admittedly it can be a bit confusing at first glance. This feature allows you to destructure maps using pattern matching, without having to specify all keys. For example:

iex> %{b: value} = %{a: 1, b: 2, c: 3}
%{a: 1, b: 2, c: 3}
    
iex> value
2

Consequently, %{} will match any map. If you want to match an empty map in a function, you have to use a guard clause:

defmodule PatternMatch do
  def modify(map) when map == %{} do
    %{}
  end

  def modify(map) do
    # ...
  end
end