refresh: Add '--type' option.

* guix/scripts/refresh.scm (%options, show-help): Add --type.
  (lookup-updater): New procedure.
  (update-package): Add 'updaters' parameter and honor it.
  (guix-refresh)[options->updaters]: New procedure.
  Use it, and honor --type.

doc/guix.texi

@@ -4211,8 +4211,12 @@ gnu/packages/glib.scm:77:12: glib would be upgraded from 2.34.3 to 2.37.0
 @end example
 
 It does so by browsing each package's FTP directory and determining the
-highest version number of the source tarballs
-therein@footnote{Currently, this only works for GNU packages.}.
+highest version number of the source tarballs therein.  The command
+knows how to update specific types of packages: GNU packages, ELPA
+packages, etc.---see the documentation for @option{--type} below.  The
+are many packages, though, for which it lacks a method to determine
+whether a new upstream release is available.  However, the mechanism is
+extensible, so feel free to get in touch with us to add a new method!
 
 When passed @code{--update}, it modifies distribution source files to
 update the version numbers and source tarball hashes of those packages'
@@ -4257,6 +4261,26 @@ The @code{non-core} subset refers to the remaining packages.  It is
 typically useful in cases where an update of the core packages would be
 inconvenient.
 
+@item --type=@var{updater}
+@itemx -t @var{updater}
+Select only packages handled by @var{updater}.  Currently, @var{updater}
+may be one of:
+
+@table @code
+@item gnu
+the updater for GNU packages;
+@item elpa
+the updater for @uref{http://elpa.gnu.org/, ELPA} packages.
+@end table
+
+For instance, the following commands only checks for updates of Emacs
+packages hosted at @code{elpa.gnu.org}:
+
+@example
+$ guix refresh -t elpa
+gnu/packages/emacs.scm:856:13: emacs-auctex would be upgraded from 11.88.6 to 11.88.9
+@end example
+
 @end table
 
 In addition, @command{guix refresh} can be passed one or more package

guix/scripts/refresh.scm

@@ -65,6 +65,9 @@
                     (x
                      (leave (_ "~a: invalid selection; expected `core' or `non-core'~%")
                             arg)))))
+        (option '(#\t "type") #t #f
+                (lambda (opt name arg result)
+                  (alist-cons 'updater (string->symbol arg) result)))
         (option '(#\l "list-dependent") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'list-dependent? #t result)))
@@ -106,6 +109,8 @@ specified with `--select'.\n"))
   -s, --select=SUBSET    select all the packages in SUBSET, one of
                          `core' or `non-core'"))
   (display (_ "
+  -t, --type=UPDATER     restrict to updates from UPDATER--e.g., 'gnu'"))
+  (display (_ "
   -l, --list-dependent   list top-level dependent packages that would need to
                          be rebuilt as a result of upgrading PACKAGE..."))
   (newline)
@@ -136,14 +141,21 @@ specified with `--select'.\n"))
   (list %gnu-updater
         %elpa-updater))
 
-(define* (update-package store package #:key (key-download 'interactive))
+(define (lookup-updater name)
+  "Return the updater called NAME."
+  (find (lambda (updater)
+          (eq? name (upstream-updater-name updater)))
+        %updaters))
+
+(define* (update-package store package updaters
+                         #:key (key-download 'interactive))
   "Update the source file that defines PACKAGE with the new version.
 KEY-DOWNLOAD specifies a download policy for missing OpenPGP keys; allowed
 values: 'interactive' (default), 'always', and 'never'."
   (let-values (((version tarball)
                 (catch #t
                   (lambda ()
-                    (package-update store package %updaters
+                    (package-update store package updaters
                                     #:key-download key-download))
                   (lambda _
                     (values #f #f))))
@@ -180,6 +192,19 @@ downloaded and authenticated; not updating~%")
                   (alist-cons 'argument arg result))
                 %default-options))
 
+  (define (options->updaters opts)
+    ;; Return the list of updaters to use.
+    (match (filter-map (match-lambda
+                         (('updater . name)
+                          (lookup-updater name))
+                         (_ #f))
+                       opts)
+      (()
+       ;; Use the default updaters.
+       %updaters)
+      (lst
+       lst)))
+
   (define (keep-newest package lst)
     ;; If a newer version of PACKAGE is already in LST, return LST; otherwise
     ;; return LST minus the other version of PACKAGE in it, plus PACKAGE.
@@ -216,6 +241,7 @@ update would trigger a complete rebuild."
 
   (let* ((opts            (parse-options))
          (update?         (assoc-ref opts 'update?))
+         (updaters        (options->updaters opts))
          (list-dependent? (assoc-ref opts 'list-dependent?))
          (key-download    (assoc-ref opts 'key-download))
          (packages
@@ -269,11 +295,12 @@ dependent packages are rebuilt: ~{~a~^ ~}~%"
                           (or (assoc-ref opts 'gpg-command)
                               (%gpg-command))))
             (for-each
-             (cut update-package store <> #:key-download key-download)
+             (cut update-package store <> updaters
+                  #:key-download key-download)
              packages))))
        (else
         (for-each (lambda (package)
-                    (match (package-update-path package %updaters)
+                    (match (package-update-path package updaters)
                       ((? upstream-source? source)
                        (let ((loc (or (package-field-location package 'version)
                                       (package-location package))))