I think there should be a way to specify, in a source file for a CL macro, how SLIME should indent the macro. There is already a robust and capable, if under-documented, way to specify the indentation for arbitrary forms in cl-indent.el, and so it would be nice to wrap this up in a nice way and give it some SLIME support.

My idea is to have SWANK look for a property on the CL symbols, custom-indent-spec, that, if present, has a list (or perhaps a string that can be read as a list in Emacs) that can be assigned to the common-lisp-indent-function property on the Emacs symbol to modify the indentation. It'd be nice to have some documentation for the format of this list, too.

Anyway, here's the sort of thing I think would work.

(setf (get 'defclass-ext 'custom-indent-spec) '((6 4 defclass-slots-options-indent) (defun defclass-slots-options-indent (path state indent-point sexp-column normal-indent) (if (> (length path) 1) (1+ sexp-column) 2))))

The above form is Common Lisp and contained in the source file, either after the macro definition or somewhere else. The quoted list (caddr of the setf form) is written in Emacs lisp. (Symbol case issues? Should everything in the Emacs-destined form be downcased by SLIME or something?)

The first form in that list is the value the common-lisp-indent-function property on the Emacs symbol gets set to. The subsequent forms are function definitions that can be used in the first. My example would allow the following indentation:

(defclass-ext some-class () (slot-a :a :ia slot-b :a (:if 2)))

There should be some attention to name collision across packages--Emacs lisp symbols don't have packages, so two different CL authors with a macro with the same name and custom indentation could potentially collide in unpleasant ways. I think what has to happen is that SLIME adds and removes the common-lisp-indent-function property on Emacs symbols every time a package is switched, based on what is set up for the inferior lisp's symbols in the new package. Is this last bit feasible? Perhaps SLIME could check to make sure the Emacs symbols have been set up with the correct plists before every indentation, resetting them all if the package has changed since then. Other ideas?

Chris Capel