Add "Cool new scratch buffers" archive post

posts/Cool_new_scratch_buffers.mdwn

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+I had a thought today: It would be nice if I could have more than one
+scratch buffer, and even nicer if they could easily have different
+major modes.
+
+So, I had this function in my emacs init file, `raise-scratch`:
+
+[[!format el """
+(defun raise-scratch ()
+  "Show the *scratch* buffer."
+  (interactive)
+  (switch-to-buffer "*scratch*"))
+"""]]
+
+Its nice, it allowed me to do exactly what I wanted to do, easily open
+my scratch buffer.  I bound this to `XF86HomePage`, which makes sense
+to me, since emacs always starts in the `*scratch*` buffer.
+
+Today, though, it didn't seem to be quite enough.  As I said, it would
+be nice to have the ability to have multiple scratch buffers with
+different major modes.  This is handy for messing around with some
+`sawfish-mode` code, or `python-mode`, for example.
+
+[[!format el """
+(defun raise-scratch (&optional mode)
+  "Show the *scratch* buffer. If called with a universal
+argument, ask the user which mode to use. If MODE is not nil,
+open a new buffer with the name *MODE-scratch* and load MODE as
+its major mode."
+  (interactive (list (if current-prefix-arg
+                         (read-string "Mode: ")
+                       nil)))
+  (let* ((bname (if mode
+                    (concat "*" mode "-scratch*")
+                  "*scratch*"))
+         (buffer (get-buffer bname))
+         (mode-sym (intern (concat mode "-mode"))))
+
+    (unless buffer
+      (setq buffer (generate-new-buffer bname))
+      (with-current-buffer buffer
+        (when (fboundp mode-sym)
+          (funcall mode-sym))))
+
+    (switch-to-buffer buffer)))
+"""]]
+
+This is quite a bit bigger.  It now takes an optional `mode` argument,
+when it is called interactively it will check if the universal prefix
+argument (`C-u`) was used and if so, asks for the value of `mode`.
+
+If mode has been specified it will create a new buffer with the name
+`*MODE-scratch*`.  It will then insert a file local property line
+specifying the major mode to use and then switches to it.
+
+It's the first time I'm using a list with the `interactive` command,
+it always seemed very alien to me, but it seems quite clear how it
+works now.
+
+It was a challenge to figure out how I wanted to do this.  My first
+idea was to ask for a file extension and match that to
+`auto-mode-alist`, but that has regexps for keys, so not easily
+matched. Then there is the problem of figuring out how to load the
+right major mode in another way, since adding such a file local
+property line happens *after* the buffer has been loaded, and thus has
+no effect on which major mode is chosen.
+
+Of course, this approach doesn't ensure the right major mode gets
+chosen, but that's really up to whomever uses it.
+
+It makes me very happy to use such an extensible editor.
+
+**Update:** fixed my flawed code.
+
+[[!meta date="2012-08-02 00:27:00"]]
+[[!tag emacs elisp coding]]