Add "Cool new scratch buffers" post

blog/blog-2012-08-02-0027.org

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+#+TITLE: Cool new scratch buffers
+
+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=:
+
+#+begin_src emacs-lisp
+  (defun raise-scratch ()
+    "Show the *scratch* buffer."
+    (interactive)
+    (switch-to-buffer "*scratch*"))
+#+end_src
+
+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.
+
+#+begin_src emacs-lisp
+  (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)))
+
+      (unless buffer
+        (setq buffer (generate-new-buffer bname))
+        (with-current-buffer buffer
+          (add-file-local-variable-prop-line 'mode (make-symbol mode))
+          (set-auto-mode t)
+          (goto-char (point-max))
+          (newline)))
+
+      (switch-to-buffer buffer)))
+#+end_src
+
+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.