Example, I eval this in the scratch buffer (with lexical-binding set true):

    (read (let ((str "TEST")
                (pos 0)
                (prev nil))
            (lambda (ch)
              (cond
                (ch (push ch prev))
                (prev (pop prev))
                ((< pos (length str))
                 (prog1 (aref str pos)
                   (setq pos (1+ pos))))))))