scripts: package: Add --install-from-file option.

* guix/scripts/package.scm (show-help): Add help text for --install-from-file
  option.
  (%options): Add --install-from-file option.
* tests/guix-package.sh: Test it.
* doc/guix.texi ("invoking guix package"): Document it.
* doc/package-hello.scm: New file.
* doc.am (EXTRA_DIST): Add it.

doc.am

@@ -26,7 +26,8 @@ EXTRA_DIST +=					\
   doc/images/bootstrap-graph.eps		\
   doc/images/bootstrap-graph.pdf		\
   doc/images/coreutils-size-map.eps		\
-  doc/environment-gdb.scm
+  doc/environment-gdb.scm			\
+  doc/package-hello.scm
 
 OS_CONFIG_EXAMPLES_TEXI =			\
   doc/os-config-bare-bones.texi			\

doc/guix.texi

@@ -1167,6 +1167,22 @@ Note that this option installs the first output of the specified
 package, which may be insufficient when needing a specific output of a
 multiple-output package.
 
+@item --install-from-file=@var{file}
+@itemx -f @var{file}
+Install the package that the code within @var{file} evaluates to.
+
+As an example, @var{file} might contain a definition like this
+(@pxref{Defining Packages}):
+
+@example
+@verbatiminclude package-hello.scm
+@end example
+
+Developers may find it useful to include such a @file{package.scm} file
+in the root of their project's source tree that can be used to test
+development snapshots and create reproducible development environments
+(@pxref{Invoking guix environment}).
+
 @item --remove=@var{package} @dots{}
 @itemx -r @var{package} @dots{}
 Remove the specified @var{package}s.

doc/package-hello.scm

@@ -0,0 +1,18 @@
+(use-modules (guix)
+             (guix build-system gnu)
+             (guix licenses))
+
+(package
+  (name "hello")
+  (version "2.8")
+  (source (origin
+            (method url-fetch)
+            (uri (string-append "mirror://gnu/hello/hello-" version
+                                ".tar.gz"))
+            (sha256
+             (base32 "0wqd8sjmxfskrflaxywc7gqw7sfawrfvdxd9skxawzfgyy0pzdz6"))))
+  (build-system gnu-build-system)
+  (synopsis "Hello, GNU world: An example GNU package")
+  (description "Guess what GNU Hello prints!")
+  (home-page "http://www.gnu.org/software/hello/")
+  (license gpl3+))

guix/scripts/package.scm

@@ -381,6 +381,10 @@ (define (show-help)
   -e, --install-from-expression=EXP
                          install the package EXP evaluates to"))
   (display (_ "
+  -f, --install-from-file=FILE
+                         install the package that the code within FILE
+                         evaluates to"))
+  (display (_ "
   -r, --remove PACKAGE ...
                          remove PACKAGEs"))
   (display (_ "
@@ -454,6 +458,12 @@ (define %options
                    (values (alist-cons 'install (read/eval-package-expression arg)
                                        result)
                            #f)))
+         (option '(#\f "install-from-file") #t #f
+                 (lambda (opt name arg result arg-handler)
+                   (values (alist-cons 'install
+                                       (load* arg (make-user-module '()))
+                                       result)
+                           #f)))
          (option '(#\r "remove") #f #t
                  (lambda (opt name arg result arg-handler)
                    (let arg-handler ((arg arg) (result result))

tests/guix-package.sh

@@ -225,6 +225,15 @@ cat > "$module_dir/foo.scm"<<EOF
 EOF
 guix package -i emacs-foo-bar-patched -n
 
+# Make sure installing from a file works.
+cat > "$module_dir/package.scm"<<EOF
+(use-modules (gnu))
+(use-package-modules bootstrap)
+
+%bootstrap-guile
+EOF
+guix package --bootstrap --install-from-file="$module_dir/package.scm"
+
 # This one should not show up in searches since it's no supported on the
 # current system.
 test "`guix package -A super-non-portable-emacs`" = ""