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Archive for January 22nd, 2008

Emacs auto-save

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[The code you’re looking for is at the end of the post :)]
Following up on my previous post on auto-save…

GNU Emacs is one of the oldest programs existing, and it has had an auto-save-mode for god knows how many decades now, but it doesn’t quite do what I want. Like this other user, I was annoyed that it saves to a different file (/dir/filename is saved to /dir/#filename# while the file is being edited), and still requires you to save the file when you quit Emacs or close the buffer, thus making it not auto-save at all, but merely a backup facility guarding against Emacs crashes.

Someone at Emacswiki’s AutoSave page suggested M-x run-with-idle-timer RET SECONDS RET save-buffer RET, but I tried that for a while and it is really not a good idea; it tries to save every buffer, even those not associated with files, like completion buffers. I considered trying to figure out how to make it run on only buffers associated with files, but then I realised that it would probably be better to modify the built-in auto-save mechanism do what I want, instead of rolling my own new one, because that was smart about things like this.

C-h f auto-save-mode is actually quite sparse compared to the usual Emacs documentation. Anyway, I figured out with a bit of looking in the source files that I could edit make-auto-save-file-name function to modify what filename it used for auto-saves. On a bit more thought, I realised that it is usually never necessary to edit Emacs functions; you can always use hooks (or, in rare cases, advice). So this works: (add-hook 'auto-save-hook 'save-buffer). Yeah, it will still be saving the file in the #filename#, but it will be saving it to your file too. Actually I then realised I don’t need that either. Doing M-x apropos-variable RET auto-save RET gives several variables including:

  Variable: Transforms to apply to buffer file name before making auto-save file name.
  Variable: *Number of seconds idle time before auto-save.
  Variable: Non-nil says auto-save a buffer in the file it is visiting, when practical.

and setting auto-save-visited-file-name to t seemed to do what I want. Until I actually tried to use it, and discovered that whoever added that feature probably hasn’t actually used it at all — it asks a “File has changed on buffer, really edit it? (y/n/r/h)” each time it auto-saves and you try to continue editing. You could turn on auto-revert-mode to make that problem go away, but I’m unsure if that’s a good idea: sometimes I overwrite files by mistake (gcc hello.c -o hello.c) and having Emacs not auto-revert is very helpful in those cases. So I’ll stick with my earlier idea, which works perfectly, after being fixed by Bryan Murdock:

(defun save-buffer-if-visiting-file (&optional args)
"Save the current buffer only if it is visiting a file"
(if (buffer-file-name)
(save-buffer args)))
(add-hook 'auto-save-hook 'save-buffer-if-visiting-file)

Bryan Murdock also posted it.


Written by S

Tue, 2008-01-22 at 22:05:17

Posted in compknow

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with one comment

Good software does what you want. Preferably without your having to tell it to. And it matches your mental model of what you expect things to work like.

We have come a long way since the days personal computers were severely constrained in their resources, but some traditions have not changed. The humanized weblog looks at the save icon, but I want to point out that the “saving” “feature” is itself an anachronism. There is no analogue of the concept of “saving” a file in the real world; when you write on a sheet of paper the change is permanent. Why, then, does most software require you to explicitly “save” something if you want to leave it permanent? The answer, I guess, is that a long time ago, a “save” was an expensive action (I remember seeing “Saving…” progress bars), so you wanted users to be in control of when it happened, so as to not annoy them.

Today, personal computers have enough resources that in many applications (such as text editing), there is really no reason for software to insist that you remember to “save”, so as to not lose your work. Still, programs continue behaving this way, partly out of tradition, and partly because no one gives a thought to usability.

Fortunately, the rise of applications on the web brought with it an “everything old is new again” phenomenon and programmers began to take a fresh (and naive) look at everything, which, while often causing them to stupidly repeat the mistakes of desktop programs from decades ago and generally be inconsistent, has also allowed them to throw away meaningless traditions in situations where they don’t matter.

I believe it was Gmail which started this, and now Gmail, Blogger, WordPress, Google Docs, and any number of online text editing applications now “automatically save” your work for you every few seconds, and this idea is finally (slowly) taking hold in desktop applications as well.

Written by S

Tue, 2008-01-22 at 21:42:25

Posted in Uncategorized

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