Add ";;guix pin" magic.

* guix-jupyter-kernel.scm (channels->shtml, reply-for-channels)
(reply-for-channel-failure): New procedures.
(reply-execute-request): Handle ";;guix pin".
(reply-complete-request): Complete ";;guix pin".
* guix-kernel-demo.ipynb: Add ";;guix pin" example.

guix-jupyter-kernel.scm

@@ -18,6 +18,7 @@
 
 (use-modules (json)
              (simple-zmq)
+             (git)                                ;for 'git-error-message'
 	     (srfi srfi-1)
              (srfi srfi-13)
              (srfi srfi-19)
@@ -33,6 +34,7 @@
              (guix build utils)
              (guix gexp)
              (guix store)
+             (guix channels)
              (guix derivations)
              (guix inferior)
              (guix monads)
@@ -164,6 +166,34 @@ NAME with MANIFEST."
                           ("execution_count" . ,count))))
                 #:kernel-socket kernel-iopub))
 
+(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 ,(or (channel-commit channel)
+                                     (channel-branch channel))))))
+               channels)))))
+
+(define* (reply-for-channels kernel message channels
+                             #:key (count 0))
+  "Reply to MESSAGE, which comes from KERNEL, that we successfully switch to
+CHANNELS."
+  (reply-html kernel message
+              (sxml->html-string (channels->shtml channels))
+              count))
+
+(define* (reply-for-channel-failure kernel message channels error
+                                    #:key (count 0))
+  "Send KERNEL a reply saying we failed to switch to CHANNELS."
+  (reply-html kernel message
+              (string-append "Failed to switch to channels: "
+                             (git-error-message error) ".")
+              count))
+
 (define* (create-environment name specs state
                              #:key kernel message)
   "Spawn a new execution environment called NAME and containing SPECS, a list
@@ -286,6 +316,29 @@ stripped."
           (send-message proxy (execute-request-sans-magic message))
           (let ((state (increment-execution-count state)))
             (set-proxy-state-default-environment state name)))))
+      ((";;guix" "pin" commit)
+       (let ((count    (proxy-state-execution-count state))
+             (channels (map (lambda (ch)
+                              (if (guix-channel? ch)
+                                  (channel (inherit ch)
+                                           (commit commit))
+                                  ch))
+                            %default-channels)))
+         (format/log "pinning to these channels: ~s~%" channels)
+         ;; TODO: Send a message showing that something's happening.
+         (catch 'git-error
+           (lambda ()
+             (let ((inferior (inferior-for-channels channels)))
+               ;; TODO: Obtain the <channel-instance> records so the message
+               ;; can provide more info.
+               (reply-for-channels kernel message channels
+                                   #:count count)
+               (set-proxy-state-inferior (increment-execution-count state)
+                                         inferior)))
+           (lambda (key error . rest)
+             (reply-for-channel-failure kernel message channels error
+                                        #:count count)
+             state))))
       ((";;guix" _ ...)
        (reply-html kernel message
                    (sxml->html-string
@@ -349,7 +402,7 @@ Return STATE."
         ;; This is a completion request on a ";;guix" magic.
         (match (string-split (string-take first cursor) #\space)
           ((";;guix" command)
-           (send-completion-reply '("environment")
+           (send-completion-reply '("environment" "pin")
                                   (- cursor (string-length command))
                                   cursor)
            state)

guix-kernel-demo.ipynb

@@ -27,6 +27,15 @@
     "Create an environment called `guile-kernel` containing the Guile kernel for Jupyter:"
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    ";;guix pin 36cc160e721a764c16f53c6f7fbd9d09c581717e"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
@@ -212,7 +221,7 @@
     "x = random.sample(range(1, 5000), 1000)\n",
     "num_bins = 100\n",
     "n, bins, patches = plt.hist(x, num_bins, facecolor='green', alpha=0.5)\n",
-    " \n",
+    "\n",
     "plt.title('Histogram Example')\n",
     "plt.xlabel('Values')\n",
     "plt.xlabel('Counts')\n",