How to check if a Map is also a Struct?

In general to check if map is a struct:

Map.has_key?(struct, :__struct__)

For different method declarations (more general method is second):

defmodule DifferentThings do
  def do_something(%{__struct__: _} = arg) do
    # ...
  end

  def do_something(arg) when is_map(arg) do
    # ...
  end
end

It is possible to pattern match between struct and map like below

defmodule DifferentThings do
  def do_something(arg = %_x{}) do
    IO.puts "This is a struct"
  end

  def do_something(arg = %{}) do
    IO.puts "This is a map"
  end
end

You can't have separate function heads for Map vs Struct with a guard, but you can do it with pattern matching.

defmodule Guard do

  def foo(%{:__struct__ => x })  do
    Struct
  end

  def foo(x) when is_map x do
    Map
  end

end