Read-line from socket-stream in lisp

Question

I am working on a TCP client in lisp, and am having difficulties reading/printing the socket-stream correctly. When the client sends data to the server it should receive a response, and I need to write this to stdout. This works correctly, except I am having an issue where the response is printing twice. If I have multiple commands being sent, only the last one will be printed twice. Here is my code:

(defun socket-print (string socket)
    (write-line string (usocket:socket-stream socket))
    (force-output (usocket:socket-stream socket)))


(defun socket-read (socket)
    (write-line (read-line (usocket:socket-stream socket) nil)))


(defun mytest (command)
    (let ((socket (usocket:socket-connect _IP_ _PORT_)))
        (socket-print command socket)
        (socket-read socket)))

Answer 1

I don't think it's printing twice. I think it's printing (via write-line) once, and then write-line is returning the line, and the REPL is printing the return value. E.g., if you modify your mytest to return nil, I think you'll see the output that you're expecting:

(defun mytest (command)
    (let ((socket (usocket:socket-connect _IP_ _PORT_)))
        (socket-print command socket)
        (socket-read socket)
        nil))

This should print the line with write-line, and then return nil, and the REPL will print nil. Compare this with:

CL-USER> (defun echo (line)
           (write-line line))
ECHO
CL-USER> (echo "hello")
hello
"hello"
CL-USER> (progn
           (echo "hello")
           (echo "world"))
hello
world
"world"
CL-USER> (progn
           (echo "hello")
           (echo "world")
           nil)
hello
world
NIL
CL-USER> (progn
           (echo "hello")
           (echo "world")
           (values))
hello
world
; No value

Also see:

• How can I avoid the nil printed in the end?
• prevent terminal output in LISP
• When you type "hello, world" in Clojure REPL, why does it say 'nil'? (not Common Lisp, but same concept)