Lisp Tlen -

That's it. 15 lines of Lisp, and you have a protocol server. You might think: "A loop that reads and writes? Python can do that."

Next time you need to debug an SMTP server or test a custom TCP service, skip nc (netcat) for an hour. Fire up a Lisp REPL, open a socket, and talk to the machine directly. You'll never look at curl the same way again. If you landed here searching for "Lisp CLOS" (Common Lisp Object System) or "Lisp TCO" (Tail Call Optimization), drop a comment below. I've got drafts on both. But if you really meant tlen as some obscure library... well, now you know how to roll your own. Happy hacking, parentheses and all. lisp tlen

;;; tlen.lisp - A minimalist Telnet echo server (require :usocket) ; A portable socket library (defun handle-client (stream) "Echo back whatever the client sends, but shout it in uppercase." (loop :for line = (read-line stream nil) :while line :do (write-line (string-upcase line) stream) (force-output stream))) That's it

Note: "Tlen" is not a standard term in mainstream Lisp literature (Clojure, Common Lisp, Racket, etc.). It is most likely a typo or autocorrect error. Based on common search patterns, I have assumed you meant one of three things: (Common Lisp Object System), "TCO" (Tail Call Optimization), or "TELNET" (network protocols). Python can do that

I recently spent a weekend revisiting Telnet, not as a sysadmin, but as a Lisp programmer. Why? Because stripping away TLS, JSON, and REST frameworks reveals something beautiful:

For a Lisp REPL, this is home turf. Lisp doesn't care if you're crunching matrices, parsing XML, or listening on port 23. The code looks the same. Let's build a toy Telnet server in Common Lisp. We'll call it tlen.lisp (see what I did there?).

If you came of age in the modern cloud era (Post-2010), Telnet is that "insecure thing" you disable on routers. But for those of us who cut our teeth on BBSes, mainframes, or early Unix hacking, —a raw, text-based window into another machine.