title: How can I have an autocompletable input field in emacs lisp?
body: <p>I want to make a function that will ask the user for the path to a file, and then do some stuff depending if the file exists or not. I know about the <code>(interactive "s")</code>, and I tried using it, but, pressing TAB doesn't suggest possible path names. 
  How can I prompt for a path, and store it in a variable?</p>