[bug#33286] Add 'list-formats' to 'guix pack'

[Efraim Flashner <efraim@flashner.co.il>]

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I was looking at the help menu for 'guix pack' and I realized that I
didn't know what formats were available and there wasn't a flag to show
them.

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Efraim Flashner   <efraim@flashner.co.il>   אפרים פלשנר
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From af9a132a662f1d703df1c32278a45d2adca146ed Mon Sep 17 00:00:00 2001
From: Efraim Flashner <efraim@flashner.co.il>
Date: Tue, 6 Nov 2018 11:50:48 +0200
Subject: [PATCH] pack: List the available formats.

* guix/scripts/pack.scm (show-formats): New variable.
(%options, show-help): Add 'list-formats' option.
---
 guix/scripts/pack.scm | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/guix/scripts/pack.scm b/guix/scripts/pack.scm
index 83bfa4ce0..9056ada6d 100644
--- a/guix/scripts/pack.scm
+++ b/guix/scripts/pack.scm
@@ -3,6 +3,7 @@
 ;;; Copyright © 2017, 2018 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2018 Konrad Hinsen <konrad.hinsen@fastmail.net>
 ;;; Copyright © 2018 Chris Marusich <cmmarusich@gmail.com>
+;;; Copyright © 2018 Efraim Flashner <efraim@flashner.co.il>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -551,6 +552,18 @@ please email '~a'~%")
     (squashfs . ,squashfs-image)
     (docker  . ,docker-image)))
 
+(define (show-formats)
+  ;; Print the supported pack formats.
+  (display (G_ "The supported formats for 'guix pack' are:"))
+  (newline)
+  (display (G_ "
+  tarball       A self-contained tarball, ready to run on another machine."))
+  (display (G_ "
+  squashfs      A squashfs image, ready for 'cluster engines' and the like."))
+  (display (G_ "
+  docker        A tarball ready for 'docker load'."))
+  (newline))
+
 (define %options
   ;; Specifications of the command-line options.
   (cons* (option '(#\h "help") #f #f
@@ -567,6 +580,10 @@ please email '~a'~%")
          (option '(#\f "format") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'format (string->symbol arg) result)))
+         (option '("list-formats") #f #f
+                 (lambda args
+                   (show-formats)
+                   (exit 0)))
          (option '(#\R "relocatable") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'relocatable? #t result)))
@@ -621,6 +638,8 @@ Create a bundle of PACKAGE.\n"))
   (newline)
   (display (G_ "
   -f, --format=FORMAT    build a pack in the given FORMAT"))
+  (display (G_ "
+      --list-formats     list the formats available"))
   (display (G_ "
   -R, --relocatable      produce relocatable executables"))
   (display (G_ "
-- 
2.19.1


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