guix package: add a "show" option.

* doc/guix.texi: Update the documentation.
* guix/scripts/package.scm: Add a "show" option.
* tests/guix-package.sh: Add a test for the "show" option.

doc/guix.texi

@@ -844,6 +844,30 @@ name: gmp
 @dots{}
 @end example
 
+@item --show=@var{package}
+Show details about @var{package}, taken from the list of available packages, in
+@code{recutils} format (@pxref{Top, GNU recutils databases,, recutils, GNU
+recutils manual}).
+
+@example
+$ guix package --show=python | recsel -p name,version
+name: python
+version: 2.7.6
+
+name: python
+version: 3.3.5
+@end example
+
+You may also specify the full name of a package to only get details about a
+specific version of it:
+@example
+$ guix package --show=python-3.3.5 | recsel -p name,version
+name: python
+version: 3.3.5
+@end example
+
+
+
 @item --list-installed[=@var{regexp}]
 @itemx -I [@var{regexp}]
 List the currently installed packages in the specified profile, with the

guix/scripts/package.scm

@@ -517,6 +517,8 @@ (define (show-help)
   (display (_ "
   -A, --list-available[=REGEXP]
                          list available packages matching REGEXP"))
+  (display (_ "
+  --show=PACKAGE         show details about PACKAGE"))
   (newline)
   (show-build-options-help)
   (newline)
@@ -615,6 +617,11 @@ (define %options
                    (values (cons `(query list-available ,(or arg ""))
                                  result)
                            #f)))
+         (option '("show") #t #t
+                 (lambda (opt name arg result arg-handler)
+                   (values (cons `(query show ,arg)
+                                 result)
+                           #f)))
 
          %standard-build-options))
 
@@ -1042,6 +1049,14 @@ (define (list-generation number)
                       (find-packages-by-description regexp)))
            #t))
 
+        (('show requested-name)
+         (let-values (((name version)
+                       (package-name->name+version requested-name)))
+           (leave-on-EPIPE
+            (for-each (cute package->recutils <> (current-output-port))
+                      (find-packages-by-name name version)))
+           #t))
+
         (('search-paths)
          (let* ((manifest (profile-manifest profile))
                 (entries  (manifest-entries manifest))

tests/guix-package.sh

@@ -176,6 +176,9 @@ then false; else true; fi
 # Check whether `--list-available' returns something sensible.
 guix package -p "$profile" -A 'gui.*e' | grep guile
 
+# Check whether `--show' returns something sensible.
+guix package --show=guile | grep "^Package: guile"
+
 # There's no generation older than 12 months, so the following command should
 # have no effect.
 generation="`readlink_base "$profile"`"