Often there are multiple ways to solve one problem. You need to give an example of what you want to do including example input, output and what you tried so far, before anybody would give you a satisfying solution.

Your Elixir code looks good to me, what’s your issue with it?

One thing I’d change is make it more obvious what part of a tuple you’re getting at the end e.g. not use elem but the then construct – just to make it little more readable.

But other than that the code looks good IMO.

Having 2-element Tuple as acc is not a shame. Many reduce-based solutions written by senior developers are using it. Your code is really good enough.

However there is other way to write same code. You can use a simple pattern matching solution, for example:

defmodule Example do
  # function header with default arguments
  def sample(files, bookmarks \\ "", counter \\ 1)

  # when done return only bookmarks string
  def sample([], bookmarks, _counter), do: bookmarks

  # tail recursion
  def sample([file | files], acc_bookmarks, acc_counter) do
  	# your logic goes here, for example
  	{file_bookmarks, file_counter} = process_file(file)
  	# recursively call this for rest files with updated bookmarks and counter
  	sample(files, acc_bookmarks <> file_bookmarks, acc_counter + file_counter)
  end
end

Personally I like separating accumulation and process logic, as in example above, which makes the code more clear.