2 files changed, 21 insertions(+), 6 deletions(-)

M guix/profiles.scm
M guix/scripts/package.scm

@@ 5,6 5,7 @@
 ;;; Copyright © 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2015 Sou Bunnbu <iyzsong@gmail.com>
 ;;; Copyright © 2016 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2016 Chris Marusich <cmmarusich@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;


@@ 97,6 98,7 @@
             generation-number
             generation-numbers
             profile-generations
+            relative-generation-spec->number
             relative-generation
             previous-generation-number
             generation-time


@@ 1039,6 1041,23 @@ former profiles were found."
         '()
         generations)))
 
+(define (relative-generation-spec->number profile spec)
+  "Return PROFILE's generation specified by SPEC, which is a string.  The SPEC
+may be a N, -N, or +N, where N is a number.  If the spec is N, then the number
+returned is N.  If it is -N, then the number returned is the profile's current
+generation number minus N.  If it is +N, then the number returned is the
+profile's current generation number plus N.  Return #f if there is no such
+generation."
+  (let ((number (string->number spec)))
+    (and number
+         (case (string-ref spec 0)
+           ((#\+ #\-)
+            (relative-generation profile number))
+           (else (if (memv number (profile-generations profile))
+                     number
+                     #f))))))
+
+
 (define* (relative-generation profile shift #:optional
                               (current (generation-number profile)))
   "Return PROFILE's generation shifted from the CURRENT generation by SHIFT.

@@ 5,6 5,7 @@
 ;;; Copyright © 2014, 2016 Alex Kost <alezost@gmail.com>
 ;;; Copyright © 2016 Roel Janssen <roel@gnu.org>
 ;;; Copyright © 2016 Benz Schenk <benz.schenk@uzh.ch>
+;;; Copyright © 2016 Chris Marusich <cmmarusich@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;


@@ 782,12 783,7 @@ processed, #f otherwise."
                                    #:key dry-run?)
   "Switch PROFILE to the generation specified by SPEC."
   (unless dry-run?
-    (let* ((number (string->number spec))
-           (number (and number
-                        (case (string-ref spec 0)
-                          ((#\+ #\-)
-                           (relative-generation profile number))
-                          (else number)))))
+    (let ((number (relative-generation-spec->number profile spec)))
       (if number
           (switch-to-generation* profile number)
           (leave (_ "cannot switch to generation '~a'~%") spec)))))