kernel: Handle ";;guix describe" magic.

* guix-jupyter-kernel.scm (channels->shtml): Move heading to...
(reply-for-channels): ... here.
(reply-channel-description): New procedure.
(reply-execute-request): Handle ";;guix describe".
(%magic-commands): Add "describe".

guix-jupyter-kernel.scm

@@ -162,16 +162,14 @@ NAME with MANIFEST."
 
 (define (channels->shtml channels)
   "Return SHTML representing CHANNELS."
-  `(div
-    (bold "Switched to these Guix channels:")
-    (p (table
-        ,@(map (lambda (channel)
-                 `(tr (tc (a (@ (href ,(channel-url channel)))
-                             (code ,(channel-name channel))))
-                      (tc (code ,(if (channel-commit channel)
-                                     (channel-commit-hyperlink channel)
-                                     (channel-branch channel))))))
-               channels)))))
+  `(p (table
+       ,@(map (lambda (channel)
+                `(tr (tc (a (@ (href ,(channel-url channel)))
+                            (code ,(channel-name channel))))
+                     (tc (code ,(if (channel-commit channel)
+                                    (channel-commit-hyperlink channel)
+                                    (channel-branch channel))))))
+              channels))))
 
 (define* (reply-for-channels kernel message channels
                              #:key profile (count 0))
@@ -183,7 +181,26 @@ to CHANNELS in PROFILE."
                          (lst lst))
                        channels)))
     (reply-html kernel message
-                (sxml->html-string (channels->shtml instances))
+                (sxml->html-string
+                 `(div
+                   (bold "Switched to these Guix channels:")
+                   ,(channels->shtml instances)))
+                count)))
+
+(define* (reply-channel-description kernel message inferior
+                                    #:key (count 0))
+  "Reply to MESSAGE, which comes from KERNEL, with a description of the
+channels currently used by INFERIOR."
+  (let* ((profile  (inferior-eval '(begin
+                                     (use-modules (guix describe))
+                                     (current-profile))
+                                  inferior))
+         (channels (profile-channels profile)))
+    (reply-html kernel message
+                (sxml->html-string
+                 `(div
+                   (bold "Using these Guix channels:")
+                   ,(channels->shtml channels)))
                 count)))
 
 (define* (reply-for-channel-failure kernel message channels error
@@ -402,6 +419,13 @@ stripped."
           (send-message proxy (execute-request-sans-magic message))
           (let ((state (increment-execution-count state)))
             (set-proxy-state-default-environment state name)))))
+      ((";;guix" "describe")
+       (let* ((state    (ensure-proxy-state-inferior state))
+              (inferior (proxy-state-inferior state)))
+         (reply-channel-description kernel message
+                                    (proxy-state-inferior state)
+                                    #:count count)
+         (increment-execution-count state)))
       ((";;guix" "pin" commit)
        (let ((count    (proxy-state-execution-count state))
              (channels (map (lambda (ch)
@@ -471,7 +495,7 @@ python-ipykernel"))
 
 (define %magic-commands
   ;; The ";;guix" magic commands.
-  '("download" "environment" "pin"))
+  '("describe" "download" "environment" "pin"))
 
 (define (reply-complete-request kernel kind message state)
   "Reply to a \"complete_request\" message--i.e., a completion request.