guix package: Add '--export-manifest'.

* guix/scripts/package.scm (export-manifest): New procedure.
(show-help, %options): Add '--export-manifest'.
(process-query): Honor it.
* guix/build/profiles.scm (build-profile): Mention it.
* tests/guix-package.sh: Test it.
* doc/guix.texi (Invoking guix package): Document it.

4 files changed, 90 insertions, 4 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 7393cc8ecd..e5872b5f24 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -3294,6 +3294,9 @@ objects, like this:
  '("emacs" "guile@@2.2" "guile@@2.2:debug"))
 @end lisp
 
+@xref{export-manifest, @option{--export-manifest}}, to learn how to
+obtain a manifest file from an existing profile.
+
 @item --roll-back
 @cindex rolling back
 @cindex undoing transactions
@@ -3596,6 +3599,24 @@ zeroth generation is never deleted.
 Note that deleting generations prevents rolling back to them.
 Consequently, this command must be used with care.
 
+@cindex manifest, exporting
+@anchor{export-manifest}
+@item --export-manifest
+Write to standard output a manifest suitable for @option{--manifest}
+corresponding to the chosen profile(s).
+
+This option is meant to help you migrate from the ``imperative''
+operating mode---running @command{guix install}, @command{guix upgrade},
+etc.---to the declarative mode that @option{--manifest} offers.
+
+Be aware that the resulting manifest @emph{approximates} what your
+profile actually contains; for instance, depending on how your profile
+was created, it can refer to packages or package versions that are not
+exactly what you specified.
+
+Keep in mind that a manifest is purely symbolic: it only contains
+package names and possibly versions, and their meaning varies over time.
+
 @end table
 
 Finally, since @command{guix package} may actually start build
diff --git a/guix/build/profiles.scm b/guix/build/profiles.scm
index 67ee9b665a..b42f498a80 100644
--- a/guix/build/profiles.scm
+++ b/guix/build/profiles.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2015, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2015, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -169,7 +169,9 @@ SEARCH-PATHS."
     (lambda (p)
       (display "\
 ;; This file was automatically generated and is for internal use only.
-;; It cannot be passed to the '--manifest' option.\n\n"
+;; It cannot be passed to the '--manifest' option.
+;; Run 'guix package --export-manifest' if to export a file suitable
+;; for '--manifest'.\n\n"
                p)
       (pretty-print manifest p)))
 
diff --git a/guix/scripts/package.scm b/guix/scripts/package.scm
index 6faf2adb7a..2b52016c67 100644
--- a/guix/scripts/package.scm
+++ b/guix/scripts/package.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
 ;;; Copyright © 2013, 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2014, 2016 Alex Kost <alezost@gmail.com>
@@ -48,6 +48,7 @@
                 #:select (directory-exists? mkdir-p))
   #:use-module (ice-9 format)
   #:use-module (ice-9 match)
+  #:autoload   (ice-9 pretty-print) (pretty-print)
   #:use-module (ice-9 regex)
   #:use-module (ice-9 vlist)
   #:use-module (srfi srfi-1)
@@ -322,6 +323,48 @@ Alternately, see @command{guix package --search-paths -p ~s}.")
 
 
 ;;;
+;;; Export a manifest.
+;;;
+
+(define* (export-manifest manifest
+                          #:optional (port (current-output-port)))
+  "Write to PORT a manifest corresponding to MANIFEST."
+  (define (version-spec entry)
+    (let ((name (manifest-entry-name entry)))
+      (match (map package-version (find-packages-by-name name))
+        ((_)
+         ;; A single version of NAME is available, so do not specify the
+         ;; version number, even if the available version doesn't match ENTRY.
+         "")
+        (versions
+         ;; If ENTRY uses the latest version, don't specify any version.
+         ;; Otherwise return the shortest unique version prefix.  Note that
+         ;; this is based on the currently available packages, which could
+         ;; differ from the packages available in the revision that was used
+         ;; to build MANIFEST.
+         (let ((current (manifest-entry-version entry)))
+           (if (every (cut version>? current <>)
+                      (delete current versions))
+               ""
+               (version-unique-prefix (manifest-entry-version entry)
+                                      versions)))))))
+
+  (match (manifest->code manifest
+                         #:entry-package-version version-spec)
+    (('begin exp ...)
+     (format port (G_ "\
+;; This \"manifest\" file can be passed to 'guix package -m' to reproduce
+;; the content of your profile.  This is \"symbolic\": it only specifies
+;; package names.  To reproduce the exact same profile, you also need to
+;; capture the channels being used, as returned by \"guix describe\".
+;; See the \"Replicating Guix\" section in the manual.\n"))
+     (for-each (lambda (exp)
+                 (newline port)
+                 (pretty-print exp port))
+               exp))))
+
+
+;;;
 ;;; Command-line options.
 ;;;
 
@@ -374,6 +417,8 @@ Install, remove, or upgrade packages in a single transaction.\n"))
   -S, --switch-generation=PATTERN
                          switch to a generation matching PATTERN"))
   (display (G_ "
+      --export-manifest  print a manifest for the chosen profile"))
+  (display (G_ "
   -p, --profile=PROFILE  use PROFILE instead of the user's default profile"))
   (display (G_ "
       --list-profiles    list the user's profiles"))
@@ -507,6 +552,10 @@ kind of search path~%")
                      (values (cons `(query search-paths ,kind)
                                    result)
                              #f))))
+         (option '("export-manifest") #f #f
+                 (lambda (opt name arg result arg-handler)
+                   (values (cons `(query export-manifest) result)
+                           #f)))
          (option '(#\p "profile") #t #f
                  (lambda (opt name arg result arg-handler)
                    (values (alist-cons 'profile (canonicalize-profile arg)
@@ -827,6 +876,12 @@ processed, #f otherwise."
          (format #t "~{~a~%~}" settings)
          #t))
 
+      (('export-manifest)
+       (let* ((manifest (concatenate-manifests
+                         (map profile-manifest profiles))))
+         (export-manifest manifest (current-output-port))
+         #t))
+
       (_ #f))))
 
 
diff --git a/tests/guix-package.sh b/tests/guix-package.sh
index 3e5fa71d20..7eaad6823f 100644
--- a/tests/guix-package.sh
+++ b/tests/guix-package.sh
@@ -1,5 +1,5 @@
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 # Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
 #
 # This file is part of GNU Guix.
@@ -395,6 +395,14 @@ EOF
 guix package --bootstrap -m "$module_dir/manifest.scm"
 guix package -I | grep guile
 test `guix package -I | wc -l` -eq 1
+
+# Export a manifest, instantiate it, and make sure we get the same profile.
+profile_directory="$(readlink -f "$default_profile")"
+guix package --export-manifest > "$tmpfile"
+guix package --rollback --bootstrap
+guix package --bootstrap -m "$tmpfile"
+test "$(readlink -f "$default_profile")" = "$profile_directory"
+
 guix package --rollback --bootstrap
 
 # Applying two manifests.