pack: Add '--manifest'.

* guix/scripts/pack.scm (%options, show-help): Add --manifest.
(guix-pack)[manifest-from-args]: New procedure.
Use it.
* doc/guix.texi (Invoking guix pack): Document --manifest.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -2841,6 +2841,19 @@ This has the same purpose as the same-named option in @command{guix
 build} (@pxref{Additional Build Options, @code{--expression} in
 @command{guix build}}).
+@item --manifest=@var{file}
+@itemx -m @var{file}
+Use the packages contained in the manifest object returned by the Scheme
+code in @var{file}.
+
+This has a similar purpose as the same-named option in @command{guix
+package} (@pxref{profile-manifest, @option{--manifest}}) and uses the
+same manifest files.  It allows you to define a collection of packages
+once and use it both for creating profiles and for creating archives
+for use on machines that do not have Guix installed.  Note that you can
+specify @emph{either} a manifest file @emph{or} a list of packages,
+but not both.
+
 @item --system=@var{system}
 @itemx -s @var{system}
 Attempt to build for @var{system}---e.g., @code{i686-linux}---instead of

guix/scripts/pack.scm

@@ -2,6 +2,7 @@
 ;;; Copyright © 2015, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2017 Efraim Flashner <efraim@flashner.co.il>
 ;;; Copyright © 2017 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2018 Konrad Hinsen <konrad.hinsen@fastmail.net>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -292,6 +293,9 @@ (define %options
          (option '(#\e "expression") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'expression arg result)))
+         (option '(#\m "manifest") #t #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'manifest arg result)))
          (option '(#\s "system") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'system arg
@@ -344,6 +348,9 @@ (define (show-help)
   -C, --compression=TOOL compress using TOOL--e.g., \"lzip\""))
   (display (G_ "
   -S, --symlink=SPEC     create symlinks to the profile according to SPEC"))
+  (display (G_ "
+  -m, --manifest=FILE    create a new profile generation with the manifest
+                         from FILE"))
   (display (G_ "
       --localstatedir    include /var/guix in the resulting pack"))
   (newline)
@@ -375,10 +382,21 @@ (define maybe-package-argument
        (read/eval-package-expression exp))
       (x #f)))
 
+  (define (manifest-from-args opts)
+    (let ((packages      (filter-map maybe-package-argument opts))
+          (manifest-file (assoc-ref opts 'manifest)))
+      (cond
+       ((and manifest-file (not (null? packages)))
+        (leave (G_ "both a manifest and a package list were given~%")))
+       (manifest-file
+        (let ((user-module (make-user-module '((guix profiles) (gnu)))))
+          (load* manifest-file user-module)))
+       (else (packages->manifest packages)))))
+
   (with-error-handling
     (parameterize ((%graft? (assoc-ref opts 'graft?)))
       (let* ((dry-run?    (assoc-ref opts 'dry-run?))
-             (packages    (filter-map maybe-package-argument opts))
+             (manifest    (manifest-from-args opts))
              (pack-format (assoc-ref opts 'format))
              (name        (string-append (symbol->string pack-format)
                                          "-pack"))
@@ -397,7 +415,7 @@ (define maybe-package-argument
 
           (run-with-store store
             (mlet* %store-monad ((profile (profile-derivation
-                                           (packages->manifest packages)
+                                           manifest
                                            #:target target))
                                  (drv (build-image name profile
                                                    #:target