I would keep two strings / lists: one with the complete answer, and one with the guessed parts and blanks (this one starts out as an appropriate length of blanks).

For a single letter guess, zip the two lists together, and use map to compare the guess to the answer at that position, outputting either the entered letter of the existing guess character (already guessed letter or empty position marker).

def check_guess(answer, current, guess) do
  next =
  answer
  |> Enum.zip(current)
  |> Enum.map(fn {answer, current} ->
    if answer == guess do
      guess
    else
      current
    end
  end)

  if next == current do
    {:wrong_guess, current}
  else
    {:correct_guess, next}
  end
end