--- guix-package-orig	2012-12-29 22:19:46.000000000 +0000
+++ guix-package	2012-12-29 22:42:41.000000000 +0000
@@ -13,6 +13,7 @@
 !#
 ;;; Guix --- Nix package management from Guile.         -*- coding: utf-8 -*-
 ;;; Copyright (C) 2012 Ludovic Courtès <address@hidden>
+;;; Copyright (C) 2012 Nikita Karetnikov <address@hidden>
 ;;;
 ;;; This file is part of Guix.
 ;;;
@@ -87,13 +88,14 @@
     (_
      (error "unsupported manifest format" manifest))))
 
+(define (profile-rx profile)
+  "Return a regular expression that matches PROFILE's name and number."
+  (make-regexp (string-append "^" (regexp-quote (basename profile))
+                              "-([0-9]+)")))
+
 (define (latest-profile-number profile)
   "Return the identifying number of the latest generation of PROFILE.
 PROFILE is the name of the symlink to the current generation."
-  (define %profile-rx
-    (make-regexp (string-append "^" (regexp-quote (basename profile))
-                                "-([0-9]+)")))
-
   (define* (scandir name #:optional (select? (const #t))
                     (entry<? (@ (ice-9 i18n) string-locale<?)))
     ;; XXX: Bug-fix version introduced in Guile v2.0.6-62-g139ce19.
@@ -129,7 +131,7 @@
              (sort files entry<?))))
 
   (match (scandir (dirname profile)
-                  (cut regexp-exec %profile-rx <>))
+                  (cut regexp-exec (profile-rx profile) <>))
     (#f                                         ; no profile directory
      0)
     (()                                         ; no profiles
@@ -137,7 +139,7 @@
     ((profiles ...)                             ; former profiles around
      (let ((numbers (map (compose string->number
                                   (cut match:substring <> 1)
-                                  (cut regexp-exec %profile-rx <>))
+                                  (cut regexp-exec (profile-rx profile) <>))
                          profiles)))
        (fold (lambda (number highest)
                (if (> number highest)
@@ -177,6 +179,31 @@
                                      packages)
                                 #:modules '((guix build union))))
 
+(define (profile-number profile)
+  "Return PROFILE's number. PROFILE should be an absolute filename."
+  (match:substring (regexp-exec (profile-rx profile)
+                                (basename (readlink profile))) 1))
+
+(define (roll-back)
+  "Roll back to the previous profile."
+  (let* ((current-profile-number
+          (string->number (profile-number %current-profile)))
+         (previous-profile-number (number->string (1- current-profile-number)))
+         (previous-profile
+          (string-append %current-profile "-"
+                         previous-profile-number "-link")))
+
+    (define (switch)
+      "Switch to the previous generation."
+      (simple-format #t "guix-package: switching from generation ~a to ~a~%"
+                     current-profile-number previous-profile-number)
+      (delete-file %current-profile)
+      (symlink previous-profile %current-profile))
+
+    (if (= current-profile-number 1)
+        (error "there are no other profiles.") ; XXX: handle this error
+        (switch))))
+

 ;;;
 ;;; Command-line options.
@@ -201,6 +228,8 @@
   (display (_ "
   -n, --dry-run          show what would be done without actually doing it"))
   (display (_ "
+      --roll-back        roll back to the previous generation"))
+  (display (_ "
   -b, --bootstrap        use the bootstrap Guile to build the profile"))
   (display (_ "
       --verbose          produce verbose output"))
@@ -236,6 +265,10 @@
         (option '(#\r "remove") #t #f
                 (lambda (opt name arg result)
                   (alist-cons 'remove arg result)))
+        (option '("roll-back") #f #f
+                (lambda args
+                  (roll-back)
+                  (exit 0)))
         (option '(#\p "profile") #t #f
                 (lambda (opt name arg result)
                   (alist-cons 'profile arg