debug_case_statment.ex

defmodule Debug do
  defmacro debug_case(ast) do
    ast
    |> Macro.postwalk(fn
      {:->, ctx, [[clause], expr]} ->
        debug = quote do
          IO.puts("Hit clause: #{unquote(Macro.to_string(clause))}")
        end
        {:->, ctx, [[clause], {:__block__, [], [debug, expr]}]}

      ast -> ast
    end)
  end
 end

usage.ex

defmodule MyCode do
  require Debug

  def some_function(input) do
   case {input, 0} do
     {:cat, _} -> "Meow"
     {:dog, 0} -> "Woof"
     _ -> "???"
   end
   |> Debug.debug_case
  end
end

iex(1)> MyCode.some_function(:cat)
Hit clause: {:cat, _}
"Meow"