My first Lisp program

timer.lisp

(defun pluralize (amount word)
  (if (= amount 1) word (format NIL "~ds" word))
)

(defun main ()
  (format t "~%Set a timer (in seconds): ")

  (let
    ((idx (read)))
    (loop
      (format t "~s ~d left~%" idx (pluralize idx "second"))
      (setf idx (- idx 1))
      (sleep 1)
      (when (<= idx 0)
        (format t "~%Time's up!~%")
        (return)
      )
    )
  )
)

(main)

Usage:

1. Make sure you have installed sbcl.
2. Run $ sbcl --script timer.lisp.