--- guix-package-orig	2012-12-16 17:38:40.000000000 +0000
+++ guix-package	2012-12-21 22:28:08.000000000 +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.
 ;;;
@@ -47,7 +48,8 @@
   #:use-module (srfi srfi-37)
   #:use-module (distro)
   #:use-module (distro packages guile)
-  #:export (guix-package))
+  #:export (guix-package)
+  #:export (roll-back))
 
 (define %store
   (open-connection))
@@ -87,13 +89,13 @@
     (_
      (error "unsupported manifest format" manifest))))
 
+(define (profile-rx profile)
+  (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 +179,25 @@
                                      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
+          (string-append %current-profile "-"
+                         (number->string (- current-profile-number 1))
+                         "-link")))
+    (if (= current-profile-number 1)
+        (error "there are no other profiles.")
+        (delete-file %current-profile))
+
+    (symlink previous-profile %current-profile)))
+

 ;;;
 ;;; Command-line options.