Interact with a locally long-running Common Lisp image, possibly daemonized, from the command line

Question

How could one interact with a locally long-running Common Lisp image, possibly daemonized, from the command line?

I know it is possible to run a Common Lisp function fro

Answer 1

It certainly is operating system specific, because you want some inter-process communication, and they are provided by the OS.

Let's assume you have a POSIX like OS, e.g. Linux.

Then you could set up a socket(7) or fifo(7) to which you send s-exprs to be evaluated. Of course you need to adapt the Common Lisp program to add such a REPL.

SBCL has some networking facilities, you could build on them.

Of course, you should understand first how IPC work on your OS. If it is Linux, you could read Advanced Linux Programming (it is centered on C programming, which is the low-level way of using OS services on POSIX, but you can adapt what you have learned to SBCL). And indeed, the Common Lisp standard might not have an official POSIX interface, so you need to dive into implementation specific details.

Perhaps you should learn more about BSD sockets. There are tons of tutorials on them. Then you could use TCP sockets (see tcp(7)) or Unix ones (see unix(7)). Advanced users could use the unsafe telnet command. Or you might make your software use SSL, or perhaps some libssh e.g. use ssh as their client.

You could decide and document that the protocol between user apps and your program is : send some-sexpr (on a documented socket) from user-app code to your server which is terminated by a double newline or by a form feed, and have your server eval it and send back the result or some error message. I did similar things in MELT and it is not a big deal. Be careful about buffering.

^{I guess that you assume that you have responsible and competent users (so don't open such a remote REPL to the wild Internet!). If you care about malicious or stupid use of a remote REPL, it is becoming complex.}

Alternatively, make your server a web application (by using some HTTP server library inside it), and ask users to use their browser or some HTTP client program (curl) or library (libcurl) to interact with it.