peteg's blog 2007-11-27-XSLT-Turing-Complete.autumn

XSLT is Turing Complete. So What?

There are several proofs of this fact, such as this one from 2006. So, why would IBM's DeveloperWorks publish an an article that apparently says otherwise [*]?

Finally, XSLT is the familiar technique that, in a sense, best matches the structure of XML. Perhaps reflecting this match, XSL documents are themselves XML document instances. XSLT is a special-purpose functional programming language that allows you to specify transformations of XML documents into other things (especially, but not only, into other XML documents). Aside from the somewhat annoying verboseness of XSLT, it is limited in its expressiveness — the things you can say are expressed rather clearly (and functionally, not procedurally), but you quickly bump up against all the things that you simply cannot say in XSLT.

OK, so there is an appeal to a Turing tarpit argument, but how about that last phrase, the worrying ... all the things that you simply cannot say in XSLT? Let's keep reading:

The problem comes as soon as you want to filter or compute something for the output — something that is not included in the few comparisons available to XSLT. For example, maybe you want (in a numerological spirit) to display only the even-numbered hexagrams, or only the prime ones. With XSLT, you are out of luck for something this simple.

So, what we have here is something like control completeness — it has enough in the way of control flow constructs — but data-incompleteness — you can't munge your data in all the ways you'd like to. This has always bothered me: how do you prove that you have provided enough operations for your datatype? I'm sure there are people studying algebraic data types and algebraic specification who have some answers for that.

[*] As this article should make clear, the claim of Turing Completeness is weaker and slipperier than most people seem to think.

The Mac OS X 10.4.11 Upgrade Killed My MacBook.

That's a first, a Mac OS X update that screwed things up so badly the MacBook ceased to function. Oh well, I now know where an Apple store is in Hồ Chí Minh City; I went to:

Thuan My Co. Ltd - Apple Authorised Reseller
98 Nguyễn Công Trứ, District 1, Hồ Chí Minh City.
Tel: 84 8 8218936, 8218937
Fax: 84 8 8218937
Email: thuanmy-sales@hcm.fpt.vn

and tried to buy a copy, nay a licence, of Leopard. I'll spare you that story. The "update" function failed to work any magic (or didn't like the cafés I went to), but the "archive and install" thing did the trick. I get the impression that some database in my old 10.4 installation got trashed.

Here are some fix-ups for Leopard from around the net (sorry for the lack of attribution). Let's fix the Dock (make it look more like Tiger's):


defaults write com.apple.dock no-glass -boolean YES
killall Dock

and the transparent menubar (cut and paste this line, then — eek! — reboot):


sudo defaults write /System/Library/LaunchDaemons/com.apple.WindowServer 'EnvironmentVariables' -dict 'CI_NO_BACKGROUND_IMAGE' 0.63

Time machine claims to have done something but I haven't tried to use it yet. Spaces is clunkier than I'd expect; using an app that sprays windows around like Finder and expecting some kind of mid-90s "raise" functionality is apparently asking too much. The wifi widget on the menubar finally works like what every user of open networks wants it to. Worth the money? Probably not, but heh, anything to get the MacBook back on its feet. That's the last time I travel without a Mac OS X DVD.

Skype and PayPal.

I don't know why people hate PayPal; perhaps they were evil in the past. (Actually, reading that website makes me wish Visa et al. got properly into this game.) Anyway, as Vietnam doesn't seem to use phone cards, I wanted to put some cash into Skype so I could call my parents. The payment options are specific to the country where they guess your IP is, and in Vietnam one cannot use PayPal to pay Skype, though PayPal is happy for you to manipulate it from within Vietnam. The only option usable to me — Moneybookers — asks for a mobile phone number in Australia, which may or may not actually need to be valid.

What to do? Well, fortunately the Skype website is accessible, and the server hosting peteg.org is in Australia, so I just used links to punt some cash over. This setup is so stupid — how many ex-pats want to do the same thing? — though I can understand that Skype figures it's better to be safe than useful.

(Thanks André, thanks Adelaide.)

Die, X11, die...

This whole Unicode fiasco has finally killed X11 as a viable option for me. I wouldn't have thought it was so very hard to provide a complete set of easily-usable Unicode fonts, but there it is.

So, on André's advice, I've switched to:

Aquamacs, GNU Emacs with a shiny-happy Mac OS X face. Apart from a lot of minor irritations that come with losing about a decade's worth of XEmacs configuration, it seems quite slick. I tried Carbon XEmacs but it doesn't support Unicode out of the box, and I refuse to spend (more) hours fiddling with it.
Terminator, an xterm-alike written in Java, is possibly the best thing ever to run on the JVM.
A new bash from MacPorts that speaks Unicode better than the crusty old one that comes with Mac OS X 10.4.x.

Of course I'll still need X11 for sundry old-school things like Isabelle, but there the pain is much less.

So, why Aquamacs rather than a fancy closed-source editor? Well, TextMate crashed on me after about twenty minutes of use — I tried to open a file while saving-as another one, and was madly switching programs trying to navigate the directory tree — and so I recall the cardinal rule of editors: anything less than twenty years old hasn't been tested enough. Whether the (relatively shallow) differences that Aquamacs has to GNU Emacs matter is something I will soon discover.

iPhoto Script Export

This thing is magic: one can readily write a little script to (in my case) copy images from iPhoto into a Blosxom-friendly location, and output the requisite tags.

Accessing the Mac OS X paste buffer from the command line.

If you have ever wanted to ship data to or from your X11 environment, check out pbcopy and pbpaste. Now, if only X had a sane cut-and-paste model, we'd be home...

FireFox 1.5.0.11 Mac-optimised builds.

I'm not a huge fan of the new FireFox (v2.x) for reasons that slip my mind now. Fortunately a kind soul is still cranking out optimised builds for the Mac, sans fancy widgets.

xkobo rides again.

A vanilla xkobo source tree failed to compile on the MacBook, apparently due to some changes in GCC's C++ support over the last fifteen years. Who'd have thunk it. Fortunately the industrious whitehats at OpenBSD have produced some g++-placating patches that one can dredge from their port system. Joy.

A shiny new MacBook.

The storm this morning brought two things: mrak's much-talked-about postcards, one from Qatar, the other from Vietnam, both posted in the last week of his travels; and a man from TNT Express Worldwide bearing a MacBook.

Apple has a strange way of managing expectations; the Apple Store website told me it was late in shipping, but then the shipping itself took one day instead of two or more. Anyway, they managed to suck me into buying an iPod with a two hundred dollar discount so I guess they've earnt the right to laugh at me.

I spent most of the day trying to reinstall everything, after I asked Migration Assistant to ship all the crap in my home directory over. Surprisingly Isabelle installed with little hassle, and Apple's new(er) X11 works pretty well. I was shocked to find how easy it was to install Debian these days, even under (a trial version of) Parallels.

In short: it's bigger, heavier and much, much faster than the old iBook, and the fan comes on pretty much as soon as you do anything serious. The glossy screen seems to be OK; it's all about getting the angle right, like MySpace. The keyboard is actually quite fine too, despite appearances. No regrets so far.

André on Mac OS X essentials.

André's been procrastinating about something, and so we all benefit from his list of tried-and-true Mac apps. Hmm... XEmacs-style Meta-/ for Cocoa apps? It's all my Christmases come at once.