WDYT?

+(define (set-packages!)
+  (let ((count 0))
+    (set! packages
+          (fold-packages (lambda (pkg res)
+                           (set! count (+ 1 count))
+                           (vhash-consq (object-address pkg) pkg res))
+                         vlist-null))
+    (set! packages-table (make-hash-table count))
+    (vlist-for-each (lambda (elem)
+                      (let* ((pkg (cdr elem))
+                             (key (name+version->key
+                                   (package-name pkg)
+                                   (package-version pkg)))
+                             (ref (hash-ref packages-table key)))
+                        (hash-set! packages-table key
+                                   (if ref (cons pkg ref) (list pkg)))))
+                    packages)))
+
+(set-packages!)

Given that ‘set-packages!’ has only on call site, what about removing
it, and instead writing directly:

  (define %packages
    (fold-packages ... vlist-null))

  (define %package-count
    (length %packages))

  (define %package-table
    (vlist-fold ...))

It’s also best to prefix global variable names with ‘%’.

I should point out that we try to stick to functional style (in
host-side code at least.)  Thus any identifier with an exclamation mark
gives you a -1 during review.  ;-)  See “Coding Style” in HACKING.

>>   • Add a section in the manual, probably under “Package Management”,
>>     describing the interface (probably as a separate .texi file
>>     @included from the main one.)  It can come later.
>
> This is new subject for me as well; I'll write it as soon as possible.
>
> For now the most important part of the documentation is that after
> installing guix with emacs UI, it may be used by putting:
>
>   (require 'guix-init)
>
> into ".emacs".  After that autoloaded "guix-..." commands should be
> available.

Cool!

Thanks,
Ludo’.