scripts: home: Wire and document --expression flag.

* guix/scripts/home.scm (show-help): Add --expression option.
(%options): Likewise.
* doc/guix.texi (Invoking guix home): Document it.

doc/guix.texi

@@ -79,7 +79,7 @@ Copyright @copyright{} 2020 Jack Hill@*
 Copyright @copyright{} 2020 Naga Malleswari@*
 Copyright @copyright{} 2020, 2021 Brice Waegeneire@*
 Copyright @copyright{} 2020 R Veera Kumar@*
-Copyright @copyright{} 2020 Pierre Langlois@*
+Copyright @copyright{} 2020, 2021 Pierre Langlois@*
 Copyright @copyright{} 2020 pinoaffe@*
 Copyright @copyright{} 2020 André Batista@*
 Copyright @copyright{} 2020, 2021 Alexandru-Sergiu Marton@*
@@ -36093,6 +36093,20 @@ $ guix home list-generations 10d
 
 @end table
 
+@var{options} can contain any of the common build options (@pxref{Common
+Build Options}).  In addition, @var{options} can contain one of the
+following:
+
+@table @option
+
+@item --expression=@var{expr}
+@itemx -e @var{expr}
+Consider the home-environment @var{expr} evaluates to.
+This is an alternative to specifying a file which evaluates to a home
+environment.
+
+@end table
+
 @node Documentation
 @chapter Documentation
 

guix/scripts/home.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2021 Andrew Tropin <andrew@trop.in>
 ;;; Copyright © 2021 Xinglu Chen <public@yoctocell.xyz>
+;;; Copyright © 2021 Pierre Langlois <pierre.langlois@gmx.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -86,6 +87,9 @@ Some ACTIONS support additional ARGS.\n"))
 
   (show-build-options-help)
   (display (G_ "
+  -e, --expression=EXPR  consider the home-environment EXPR evaluates to
+                         instead of reading FILE, when applicable"))
+  (display (G_ "
   -v, --verbosity=LEVEL  use the given verbosity LEVEL"))
   (newline)
   (display (G_ "
@@ -115,6 +119,9 @@ Some ACTIONS support additional ARGS.\n"))
                    (let ((level (string->number* arg)))
                      (alist-cons 'verbosity level
                                  (alist-delete 'verbosity result)))))
+         (option '(#\e "expression") #t #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'expression arg result)))
          %standard-build-options))
 
 (define %default-options