Making SBCL compliant with the XDG Base Directory specification

The war on dotfiles continues, this time with SBCL. Let's see how we can make it comply with the XDG Base Directory specification.

SBCL has a global configuration file /etc/sbclrc which we can edit. It is just regular Common Lisp, so we can do whatever we want; we have access to ANSI Common Lisp and everything else that comes included with SBCL. Let's go ahead and add the following lines:

(require :asdf)
(let ((default-init-file (funcall sb-ext:*userinit-pathname-function*)))
   (unless (or (null default-init-file)
               (typep default-init-file 'stream)
               (uiop:file-exists-p default-init-file))
     (setf sb-ext:*userinit-pathname-function*
           (lambda () (uiop:xdg-config-home #P"sbcl/init.lisp")))))

There is a global function SB-EXT:*USERINIT-PATHNAME-FUNCTION* which takes zero arguments and returns either a pathname designator, a stream or NIL. Before we do anything we check whether any of the following conditions are met:

• Loading a configuration file has been disabled (return value is NIL)

• The configuration is loaded from a stream

• The configuration is loaded from a file that does exist

If any of those conditions is met we do nothing because the user expects the default behaviour. Otherwise we replace the function with one that returns the path to $XDG_CONFIG_HOME/sbcl/init.lisp. As we can see, the UIOP package, which is part of ASDF and comes included with SBCL, already has functions for the XDG Base Directory specification, so most of the work has already been done for us.