This one is really simple. Most people are familiar with REPLs for Python, JavaScript, or even repl.it but the term comes from Lisp.

REPL stands for Read-Eval-Print-Loop, and here's how you make one in Common Lisp:

(loop(print(eval(read))))

It's pretty easy to tell what it does; it just reads from stdin, evals the input, prints the evaluation, and loops infinitely. It's a pretty ugly REPL though since it doesn't add a newline after the output:

~ Δ clisp repl.cl
(+ 3 5)

8 (* (+ 3 5) 12)

96

It can be made a lot nicer with a little work:

(defun println (s)
  (format T "~s~%" s))

(defun readprompt (p)
  (format T p)
  (read))

(loop(println(eval(readprompt "> "))))

~ Δ clisp repl.cl
> (+ 5 3)
8
> (* 3 (+ 42 (* 3 2)))
144
>

For reference, here's a simple REPL in Python:

while True: print(eval(input()))

~ Δ python repl.py
5 + 5 * 3
20
[x**2 for x in range(10)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

And to make it print a prompt it's pretty simple:

while True: 
  print("> ", end="")
  print(eval(input()))

~ Δ python repl.py
> 5 + 3 * 2
11
> [x**3 for x in range(15) if x % 2 == 0]
[0, 8, 64, 216, 512, 1000, 1728, 2744]
>