Calling Lua from Common Lisp (with eev and CFFI - 2025)

1. For the impatient

In the video we are going to learn how to do this in the SBCL REPL and in the Sly REPL, with a test block:

This is the log of the part in blue in the screenshots, in the Sly REPL:

CL-USER> (cffi:load-foreign-library "./2025-cffi-0.so")
#<CFFI:FOREIGN-LIBRARY 2025-CFFI-0.SO-523 "2025-cffi-0.so">
CL-USER> (defun lua-runstring (str)
           (cffi:foreign-funcall "lua_runstring" :string str :string))
LUA-RUNSTRING
CL-USER> (lua-runstring "return 'ab'..'cd'")
"abcd"
CL-USER>

The strings in this color are both Lua strings and Common Lisp strings; technically they are "foreign strings" (at some point).

2. Introduction

When I tried to learn how to use the CFFI I looked for examples, and the first page that I found that had an example that was easy to run was this one:

Test Common Lisp+C with CFFI
https://gist.github.com/lagagain/df1b97eb08e57c2759c950efc3a2c7dd

After understanding it I rewrote it, kept only its most basic parts, and put its three parts - the parts in C, shell, and in Common Lisp, rewritten - into a single file: hello2.c. I got this,

that was neat, but not interesting enough.

And then I tried to use CFFI to call Lua. I wanted to start with a prototype with no error checking at all, like in my Emacs<->Lua bridge, in which the Emacs module in C sends to Lua a string that is a Lua program, and that must return a string when executed... like this:

• (eepitch-lua53)
• (eepitch-kill)
• (eepitch-lua53)
str1 = "return 'a'..'b'"
str2 = load(str1)()
= str2

  lua_runstring = function (str1) return load(str1)() end
= lua_runstring "return 'a'..'b'"

in which the output of the last line is the string "ab" - in Lua `..' is the concatenation operator.

3. Trying it

If you have all the dependencies installed then you can try

•• (find-angg "LISP/2025-lua-runstring-0.c")
• (eepitch-shell)
• (eepitch-kill)
• (eepitch-shell)
cd /tmp/
wget -N http://anggtwu.net/LISP/2025-lua-runstring-0.c
# (find-fline "/tmp/2025-lua-runstring-0.c")

(find-windows-beginner-intro "4. Use eepitch to install Debian packages")
(find-try-sly-intro "2. Install some elisp packages")
(find-try-sly-intro "4. Download quicklisp.lisp")
(find-try-sly-intro "5. Install Quicklisp and Slynk")

4. The video

In this page I will explain a way to try CFFI that should be even trivial to run if you